Exposure Risks from Microbiological Hazards in Buildings and Their Control—A Rapid Review of the Evidence

Abstract

A rapid review was undertaken to consider the evidence for human exposure to harmful microorganisms from indoor air and surfaces. Published information about these contaminants, as well as measures to control them, including building design and energy conservation, were included in this review. Information on domestic dwellings, office environments, and other non-industrial settings was assessed to determine the reported prevalence, persistence, and transmission of microorganisms in these settings. Environmental factors that influence indoor microbiological colonization were also included. The evidence strongly indicates that ventilation is the primary factor for controlling indoor dampness, helping to mitigate indoor mold colonization and the accumulation of other indoor contaminants, including infectious microorganisms. Although modern building airtightness, including retrofits of older builds, contributes to thermal comfort and building energy efficiency, this may also limit a building’s ventilation capacity. This in turn can potentially allow biological pollutants to accumulate, increasing the likelihood of harmful exposures and ill-health effects for building occupants. Effective building design and maintenance, which promote appropriate levels of air exchange for indoor spaces, are therefore important for the control of indoor moisture and microbiological contamination.

1. Introduction and Background

In recent years, researchers have regularly described how many people now spend between 80% and 90% of their time indoors and may therefore be exposed to harmful indoor contaminants [1,2,3,4,5]. Just as external environments can influence our well-being, it is clear that the indoor environments we rely on and regularly interact with can also influence our health [6,7].

The negative influences of indoor living on our health can differ, with causation and health outcomes often complex to properly investigate. Health problems associated with poor indoor air quality (IAQ) and surface contamination by microorganisms include wheeze, rhinitis, inflammation and asthma [6,7,8,9,10], toxicity [10], and infections [5]. Factors influencing these effects include indoor temperature and moisture levels [11]; the effectiveness of mechanical or natural ventilation [11,12]; draught-proofing provision; and physical design features, such as insulation levels and internal material choices [13]. The numbers and behaviors of building occupants also influence the indoor environment [13].

In modern buildings, airborne pathogens such as Influenza virus, SARS-CoV-2, and other respiratory viruses can be transmitted from person to person. This can cause severe, widespread illness [14,15], which may in turn pressurize national economies and healthcare systems [13]. Indoor environments can also harbor other microorganisms that elicit human allergic responses, either by exposure to the organism itself or harmful microbiological byproducts [16,17]. These can cause illnesses that include acute and chronic respiratory disease [17,18,19]. Collectively, these microorganisms and others linked to building water supply and wastewater systems highlight the potential health impact the indoor microbiome may have on occupant health. These hazards exist, despite our increasing awareness of them and the mitigations available for their control.

In this rapid review, we consider these hazards and how they may occur due to the design, maintenance, structural condition, and general management of the buildings, homes, and workplaces. This study aims to identify evidence about harmful microorganisms in the domestic and workplace settings and to consider factors that may influence their presence inside buildings. The measures available to control indoor contaminants and the harm they can cause were also considered. This review was undertaken to provide empirical evidence that could be used by others (our building safety colleagues) to develop evidence-based changes. As such, this review is not intended to specify changes to building regulation or policy, but to inform those in a position to make those decisions.

An expert panel formulated the following research questions to serve as the focus of this review:

• Within residential and non-residential buildings *, what evidence is there for the presence of bacteria, viruses, and fungi, in the air and on fomites (surfaces), which may be harmful to occupants’ health?
• Do control measures such as enhanced ventilation or enhanced sanitation measures reduce occupant exposure to harmful bacteria, viruses, and fungi in the indoor environment?
• Could the information from Qs 1 and 2 reliably inform changes to national standards required when a new building is erected or building work is done to an existing structure?
• Is there published evidence to indicate that any mitigations proposed or introduced will influence economic or net-zero outcomes?

* Includes domestic dwellings, office environments, and other non-industrial indoor workspaces, but excludes spaces used solely as health or education facilities

2. Methods

2.1. Information Searches, Paper Selection, and Data Extraction

Detailed review methods are described in the Supplementary Materials. Briefly, search terms were developed by subject experts. These terms and the themed topics for this review are described in Supplementary Table S1. The literature search criteria and the search tools used are described in the Supplementary Materials, along with details of the reference databases searched. Citations were exported to Endnote^TM reference manager (V.20.4.1), and duplicate papers were removed using the software’s integrated software function. The paper selection and sifting process is summarized in Supplementary Figure S1. Remaining records were independently double-screened by title, abstract, and full-text against eligibility criteria. An extraction form was used to collect technical information from each fully reviewed paper and to assess study quality (Supplementary Table S2). Data were assessed by two reviewers, and any discrepancies were resolved by discussion between authors. In total, 51 of 328 reviewed papers were identified as having high relevance and high quality, based on the criteria described in the Supplementary Materials.

2.2. Presentation of the Evidence

Take-home messages were formulated for each of the research questions based upon the published evidence reviewed. Confidence statements were assigned to each take-home message; the term “high confidence”, ascribed to the evidence statements, was based on the observation that three or more relevant, high-quality papers, with additional studies of high relevance and medium quality, reported consistent results and conclusions. Medium-confidence statements reflected more limited high-quality evidence and/or a consensus body of still-relevant medium-quality evidence.

The take-home messages and evidence statements are summarized in the text boxes within the Results section below, informed by peer-reviewed studies and other reliable information sources.

3. Results

3.1. Review Question 1

Within residential and non-residential buildings, what evidence is there for the presence of bacteria, viruses, and fungi, in the air and on fomites (surfaces), which may be harmful to occupants’ health?

Evidence and confidence statements—take-home messages:

• The presence of airborne allergenic fungi in indoor air is well documented in published evidence reviews of indoor air quality (high confidence).
• Damp conditions increase the mold colonization and moldy odors of indoor environments (high confidence).
• Dampness and detectable or visible mold in indoor settings are associated with respiratory ill-health effects, including asthma, rhinitis, and wheezing (high confidence).
• Penicillium, Aspergillus, and Cladosporium species are commonly detected in damp buildings and implicated in respiratory ill-health effects for occupants (high confidence).
• Infectious viruses and bacteria have been detected in the air and on surfaces in indoor environments, with evidence of airborne viral transmission between occupants (high confidence).
• Levels of microbiological exposure associated with ill health are variable and environmentally dependent for both allergenic and infectious microorganisms (medium confidence).

3.1.1. Harmful Microorganisms in Indoor Air

Airborne Fungi and Other Airborne Microbiological Allergens

Eight evidence reviews described microbiological air quality linked to fungal or other microbial allergens, including possible mechanisms for fungal contamination [1,4,6,8,16,17,18,19]. When these organisms are airborne, human exposure risk usually occurs via inhalation and airway deposition (

Table 1. Evidence-based reviews describing hazardous fungi and other allergens in indoor air, excluding healthcare and school environments—in chronological order.

Author(s) and Year	Review Aims/Description	Relevant Evidence Presented/Author Conclusions
Krieger et al., 2010 [16]	A systematic review that considered interventions for reducing exposure to indoor biological agents that can cause asthma or exacerbate symptoms amongst existing sufferers.	Interventions to reduce airborne allergens included eliminating moisture intrusion and leaks, and removing of moldy items from homes. In-home moisture reduction was linked with a reduction in mold contamination, improving ill-health respiratory symptoms in residents. Mold and endotoxin levels in homes can rise during renovation work, e.g., following flood damage, but generally decrease once work is completed. The effectiveness of interventions can vary by region due to climate variability and housing types. Tailoring to local conditions may be important.
Mendell et al., 2011 [8]	A rapid review that examined studies on dampness, microbiologic agents, and respiratory or allergic effects.	Building dampness or airborne mold was consistently associated with multiple allergic and respiratory effects. Microbiological agents detected in dust provided no clear trend. Evidence for positive and negative associations for some agents was identified. Prevention and remediation of indoor dampness and mold are likely to reduce health risks, but the evidence did not support measuring specific indoor microbiologic factors to guide health-protective actions.
Jaakola et al., 2013 [17]	A systematic review and meta-analysis looking at the relationship between indoor dampness and mold and the risk of different types of rhinitis. The types of exposure were also considered.	Dampness and mold exposures in the home are determinants of allergic rhinitis and rhino-conjunctivitis. The highest risk was usually related to mold odor, suggesting that microbial exposures play a role in rhinitis outcomes. The evidence supported the prevention and remediation of indoor dampness and mold problems, potentially lowering the incidence of rhinitis.
Kanchongkittiphon et al., 2015 [1]	A rapid review that revised an earlier Institute of Occupational Medicine review about specific indoor exposures and the exacerbation of asthma. Indoor dampness, fungal exposures, and the presence of bacterial endotoxin were considered.	There is evidence of a causal association between dampness or dampness-related agents and exacerbation of asthma in children, and of an association in adults. This relationship may not be restricted to those sensitized to fungi or dust mites. There is limited or suggestive evidence of an association between indoor culturable Penicillium fungal exposure and exacerbation in asthmatic children with specific sensitization, fungal sensitization, or unspecified sensitization. There is limited or suggestive evidence of an indoor association between total culturable fungal exposure and asthma exacerbations in children sensitized to fungi. There is evidence of an association between indoor exposure to endotoxin and asthma exacerbation.
Sharpe et al., 2015 [6]	A systematic review assessing the role of indoor fungal diversity and any evidence linked to asthma symptoms in infants, children, and adults.	Exposure to Aspergillus, Penicillium, Cladosporium, Acremonium, Epicoccum, Alternaria, and other fungal species may present a respiratory health risk to asthmatic patients living in homes with increased fungal concentrations. Longitudinal studies assessed increased exposure to indoor fungi before the onset of asthma symptom, indicating that Penicillium, Aspergillus, and Cladosporium spp. pose a respiratory health risk in susceptible populations. The presence of Cladosporium, Alternaria, Aspergillus, and Penicillium species increased the exacerbation of current asthma symptoms by 36–48% compared with those exposed to lower concentrations of these fungi.
Du et al., 2021 [18]	A rapid review of indoor mold growth characteristics, the main species found in homes and their sources. Also discusses the influences of existing building designs and standards on indoor mold-exposure risks.	The occurrence of molds is strongly favored by damp homes and exacerbated by warmer temperatures and high humidity/moisture. Molds such as Cladosporium, Aspergillus, and Penicillium spp. are commonly found in households. The mechanism of childhood asthma onset and exacerbation is inferred by the sensitization and activation of immune responses through inhaling these types of airborne mold spores. Well-insulated and airtight homes can improve the indoor thermal environment and reduce condensation risk. However, water leakage in damaged buildings and inadequate ventilation contributed to elevated indoor moisture levels, increasing mold contamination risk in buildings.
Fakunle et al., 2021 [4]	A systematic review and meta-analysis designed to assess whether exposure to indoor microbial aerosols is associated with respiratory symptoms among children under 5 years of age.	There was an increased risk of wheezing in 67% of children under 5 exposed to a combination of indoor Aspergillus and Penicillium spp. This was reduced in the presence of Cladosporium species, suggesting that microbial interaction may play a role in the respiratory health of children under 5. The association of indoor microbial exposure with allergic rhinitis was not found to be significant. Exposure to endotoxin was protective against asthma, although the estimate was from only two studies. The pooled risk estimate from a random effect model showed a significant association between microbial exposure and respiratory ill-health symptoms.
Groot et al., 2023 [19]	A systematic review and meta-analyses relating to exposure to residential mold and dampness and associations with respiratory tract infections (RTIs), and symptoms in children in high-income countries.	Early life exposure to residential mold and dampness indoors had a small-to-moderate effect on respiratory tract infections (RTIs) and symptoms compatible with RTIs. Overall findings suggested a detrimental effect of mold and dampness on the risk of RTIs in children in high-income countries. The effect estimates were generally larger for associations between mold or mold and dampness combined and respiratory symptoms. The authors concluded that interventions to reduce mold and dampness in homes can reduce symptom prevalence.

). Seven of the studies linked indoor bioaerosols to respiratory ill health, including asthma [1], rhinitis [17], wheeze [4,18], and infection [19]. One review [6] reported increased exposure to indoor fungi prior to development of asthma symptoms, implicating species of Penicillium, Aspergillus, and Cladosporium in respiratory health risks for susceptible populations. Others [1,8,17] presented associations between building dampness, airborne mold, and the consistent relationship with these factors and multiple allergic and respiratory effects.

In addition to the research summarized in Table 1, several individual studies provided further evidence for airborne bacteria and fungi indoors (Supplementary Table S3). The health impact of poor indoor microbiological air quality and its relationship to low-quality building conditions was commonly reported [20,21,22,23,24,25,26,27,28,29]. A commonly identified theme is the connection between residential indoor dampness, regardless of source, and mold growth [21,22,23,25,26]. This is particularly well documented for airborne Alternaria [20,25,27], Aspergillus [21,29], Penicillium [21,25,27], and Cladosporium [24,25,27] species. These fungi were frequently identified by a large number of studies of damp indoor environments [4,6,16,18,20,25], where associated bioaerosols were linked to an increased risk of allergic conditions and the exacerbation of existing medical conditions.

Not all studies confirmed a clear association between indoor bioaerosols and respiratory ill health. For example, one failed to find a significant association between indoor microbial exposure and allergic rhinitis in under-5-year-olds [4], and microbial agents such as bacterial endotoxin were sometimes found to be protective against asthma, although the finding was based on evidence from only two studies. Despite the prevalence of Penicillium and Aspergillus in damp buildings, some research found that these fungi could not serve as a reliable marker for damp housing conditions [21], though they were present at lower levels in buildings unaffected by moisture.

3.1.2. Airborne Infectious Microorganisms

Four evidence reviews considered the health impact of airborne respiratory pathogens in indoor environments [13,14,15,30] (

Table 2. Evidence-based reviews describing infectious agents in indoor air, excluding healthcare * and school environments.

Author(s) and Year	Review Aims/Description	Relevant Evidence Presented/Author Conclusions
Li et al., 2007 [14]	A systematic review of published information about ventilation, airflow, and transmission of viruses indoors.	There was an association between ventilation, airflow patterns indoors, and the transmission of infectious diseases, including measles, TB, chickenpox, influenza, smallpox, and SARS-CoV-1. There were insufficient data to specify minimum ventilation requirements in hospitals, schools, offices, homes, and isolation rooms to mitigate airborne infection spread. High-occupancy buildings would benefit from increased ventilation rates during influenza peak seasons Further studies were needed to determine ventilation requirements in non-hospital settings, e.g., offices and homes.
Dinoi et al., 2022 [15]	A systematic review of current knowledge for identifying and quantifying SARS-CoV-2 RNA in air samples.	Indoor contamination from SARS-CoV-2 RNA was more common on surfaces than in air samples, but viral positivity rates were lower on surfaces compared to air samples. Of 68 identified studies, 10 were for community indoor spaces—the remaining 58 were healthcare focused, so data were limited for non-healthcare settings. Concentrations of SARS-CoV-2 RNA in the air were variable and lower outdoors than indoors. Viral concentrations in community buildings were lower than in hospitals and other healthcare settings. Indoor room volumes and ventilation rates in public areas strongly influenced the presence of and concentrations of SARS-CoV-2 RNA and the associated risk of indoor airborne transmission.
Salman et al., 2022 [30]	A systematic review provided an overview of building systems and technologies that can be used to mitigate the transmission of airborne viruses.	There was US-CDC evidence for SARS-CoV-2 transmission via the airborne route and the inhalation of droplets or viral particles that remained in the air for long periods. Advice from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) stated that airborne pathogens can be transferred from primary to secondary host as bioaerosols. To limit airborne transmission, engineering controls must be used to protect building occupants.
Zhang et al., 2022 [13]	A systematic review that considered Facilities Management (FM) interventions for indoor infection control.	Evidence showed that most airborne infection studies (60) focused on SARS-CoV-2 (2020–2021), followed by influenza (9), SARS (8), and TB (3). Six studies did not specify the respiratory pathogen type. HVAC interventions were found to reduce airborne transmission by lowering pathogen concentrations in the air through mechanisms of air dilution, air filtration, air purification, and air pressure controls.

* Pathogen studies were often hospital-based, but some described assessments in homes and public spaces, alongside healthcare settings. Where integral to the study, collective information is presented in the table above.

). Airborne and surface-contact routes were inter-linked with systemic infections [13,15,30]. One review [14] reported the airborne spread of measles, TB, chickenpox, influenza, smallpox, and SARS-CoV-1, describing associations between ventilation and the indoor transmission of these infectious diseases. However, more studies were recommended to fully characterize ventilation interventions. Two reviews [15,30] presented evidence for SARS-CoV-2 airborne transmission, now acknowledged as the main transmission route for this respiratory pathogen, and a third [13] considered SARS-CoV-2, along with tuberculosis and influenza virus transmission. These studies confirmed that these microorganisms can be transmitted via indoor air, also describing control measures (see section “How Effective Are Different Specific Control Measures (e.g., Ventilation Rates, Air Filtration, and Other Methods) Against Different Microorganisms?”).

Additional evidence for infectious-agent transmission was obtained from individual studies of airborne microorganisms [25,31,32,33,34,35,36] (Supplementary Table S4). These reported the detection of potentially infectious microorganisms in offices and meeting rooms [35], restaurants [31], residential air [32,33,36] and included a case where nationally set airborne microbiological thresholds were repeatedly exceeded [25]. Here, indoor carbon dioxide (CO₂) levels were associated with the bacterial concentration, probably due to occupancy and insufficient ventilation. Air change rates were also implicated in modelled aerosol dissemination and deposition, potentially contributing to the spread of indoor airborne diseases [34]. Other supporting information focused on the detection of the SARS-CoV-2 virus, including a study of transmission in homes [32]. The airborne SARS-CoV-2 strains detected were identical to those in occupiers’ saliva, supporting the link between infectious bioaerosols and the human source. Further evidence confirmed that ventilation and thermal plumes within buildings contributed to airborne virus dissemination [32,33,34,35,36].

3.1.3. Harmful Microorganisms on Surfaces

Allergenic Microorganisms on Surfaces

Two literature reviews provided evidence of allergenic microorganisms on surfaces, typically fungi detected in surface dust [17,18]. Penicillium, Aspergillus, Cladosporium, and Alternaria species were those most commonly detected in dust and on surfaces in residential environments (Table 3). Indoor conditions encouraging fungal growth, such as high humidity and indoor temperatures or reduced ventilation, promoted colonization and were associated with adverse health effects such as allergic rhinitis. Where fungal colonization of indoor surfaces occurs, the natural dissemination mechanism of fungi by sporulation can result in spores being released into the air and thus being inhaled by residents [18]. Some evidence was provided linking sealed modern building envelopes and indoor damp problems, should moisture ingress occur (see Section 3.3.2).

Individual studies provided further evidence for allergens on surfaces and in surface dust residues (Supplementary Table S5). Common fungi identified in residences, offices, and other indoor environments included Penicillium, Aspergillus, Alternaria, and Cladosporium species, with most studies reporting the detection of multiple fungal types with allergenic potential [9,20,21,29,37,38,39,40,41,42,43]. A range of detection and/or quantification techniques were described for use in these investigations, possibly accounting for the different results reported. For example, in contrast to the more commonly cultured fungi described previously, some studies used a combination of culture, DNA-based methods, and fungal antibody detection techniques to identify less common sporulating fungi and yeasts, including species of Leptosphaerulina, Aureobasidium, Thekopsora, Phaeococcomyces, Macrophoma, Ulocladium, Stachybotrys, and Epicoccum [9,21,38,40,43]. Fungal residues persisted in organic dust on hard floors; carpets; and damp surfaces, such as plaster (gypsum) board, chipboard, and solid wood (Supplementary Table S5).

Table 3. Evidence-based reviews describing allergens and infectious agents on surfaces, excluding healthcare * and school environments.

Author(s) and Year	Review Aims/Description	Relevant Evidence Presented/Author Conclusions
Allergens on surfaces
Jaakkola et al., 2013 [17]	A systematic review and meta-analysis examining the relationship between indoor dampness, visible mold, odor, and the risk of rhinitis. The study considered whether these relations differ according to the type of exposure.	The risk of developing rhinitis is significantly increased in relation to home dampness and mold exposures, with the largest risk associated with indoor mold odor (rhinitis, 2.18; and allergic rhinitis (AR), 1.87). The risks for rhinitis linked to visible mold conditions were consistently raised (rhinitis, 1.82; AR, 1.51; and rhino-conjunctivitis, 1.66), with comparable effect estimates. Mold species detected in dust analysis might help to identify hidden mold problems undetected by visual inspection. Dust measurements are limited because they represent only a single time point; occupant reports may better capture long-term exposures in relation to health effects.
Du et al., 2021 [18]	A rapid review of the scientific literature on indoor mold occurrence, to assess the growth characteristics, main species found in homes and their sources. Additionally, the influences of existing building designs and standards on indoor mold-exposure risks are discussed.	Penicillium, Aspergillus, Cladosporium, Alternaria were among fungal genera most commonly found in residential dust. However, different dust types, collection methods, types of room sampled, testing times, and seasons, mean that dominant genera vary between different studies. Well-insulated building envelopes and increased airtightness can benefit the indoor thermal environment and reduce the risk of condensation, but water ingress via building envelopes, alongside insufficient ventilation and air change rate, can increase moisture and the risks of mold growth in buildings. The incidence of mold growth on indoor materials is increased by warm temperatures and high humidity, resulting in airborne spore release from Cladosporium, Aspergillus, and Penicillium spp.
Infectious agents on surfaces
Salman et al., 2022 [30]	This systematic review summarizes building systems and technologies that can mitigate the spread of airborne viruses, but included some information on contact transmission.	Limited information was identified for contact transmission, but this was recognized as a significant infection route. There is a need for hand hygiene to interrupt the chain of infection, as recommended by the US Centers for Disease Control (CDC). New and future building designs could reduce the need for residents to touch surfaces, e.g., using touchless technologies for opening doors and for handwashing facilities. The focus of the review was mainly on respiratory viruses, in particular, SARS-CoV-2.
Vardoulakis et al., 2022 [44]	A systematic review assessing the risk of transmission of viral or bacterial infections through inhalation, surface contact, and fecal–oral routes in public washrooms. A short list of 38 studies formed the focus of the review.	Bacterial aerosolization and deposition on inanimate surfaces; contact with contaminated surfaces and contaminated water; and wet, contaminated hands were all identified as surface transmission pathways. Open-lid toilet flushing, ineffective handwashing or hand drying, inadequate surface cleaning, blocked drains, and exposed refuse bins can cause bacterial and/or viral contamination in washrooms. Heavy use, infrequent cleaning, or poor maintenance increased the contamination of public washroom surface contamination with bacteria and viruses. Many of the bacteria identified in the review were commensal or environmentally ubiquitous species, with the risk of infection low for healthy individuals. Good hand-hygiene practices reduce pathogen transmission, with effective ventilation and frequent cleaning of surfaces supporting this outcome.
Zhang et al., 2022 [13]	This systematic review considered academic studies on interventions for infection control, from the perspective of a Facilities Manager (FM).	Respiratory viruses can be transmitted in droplets of infected human saliva via shared indoor surfaces and can spread to others in contact with the same surfaces. Hand hygiene and surface cleaning are key measures for removing such pathogens. The likelihood of respiratory virus transmission from indoor surfaces correlates with the frequency of occupant hand to surface contact. This is influenced by the number of occupants and their interaction with the building. Minimizing accessible surfaces in rooms and limiting room occupancy are effective techniques for reducing outbreak risks via fomites. Faulty floor drains and toilet flushing may facilitate the transmission of airborne pathogens via the building’s drainage system.

* Pathogen studies were often hospital-based, but some described assessments in both homes and healthcare settings. Where integral to the study, collective information is presented in the table above.

Table 3. Evidence-based reviews describing allergens and infectious agents on surfaces, excluding healthcare * and school environments.

Author(s) and Year	Review Aims/Description	Relevant Evidence Presented/Author Conclusions
Allergens on surfaces
Jaakkola et al., 2013 [17]	A systematic review and meta-analysis examining the relationship between indoor dampness, visible mold, odor, and the risk of rhinitis. The study considered whether these relations differ according to the type of exposure.	The risk of developing rhinitis is significantly increased in relation to home dampness and mold exposures, with the largest risk associated with indoor mold odor (rhinitis, 2.18; and allergic rhinitis (AR), 1.87). The risks for rhinitis linked to visible mold conditions were consistently raised (rhinitis, 1.82; AR, 1.51; and rhino-conjunctivitis, 1.66), with comparable effect estimates. Mold species detected in dust analysis might help to identify hidden mold problems undetected by visual inspection. Dust measurements are limited because they represent only a single time point; occupant reports may better capture long-term exposures in relation to health effects.
Du et al., 2021 [18]	A rapid review of the scientific literature on indoor mold occurrence, to assess the growth characteristics, main species found in homes and their sources. Additionally, the influences of existing building designs and standards on indoor mold-exposure risks are discussed.	Penicillium, Aspergillus, Cladosporium, Alternaria were among fungal genera most commonly found in residential dust. However, different dust types, collection methods, types of room sampled, testing times, and seasons, mean that dominant genera vary between different studies. Well-insulated building envelopes and increased airtightness can benefit the indoor thermal environment and reduce the risk of condensation, but water ingress via building envelopes, alongside insufficient ventilation and air change rate, can increase moisture and the risks of mold growth in buildings. The incidence of mold growth on indoor materials is increased by warm temperatures and high humidity, resulting in airborne spore release from Cladosporium, Aspergillus, and Penicillium spp.
Infectious agents on surfaces
Salman et al., 2022 [30]	This systematic review summarizes building systems and technologies that can mitigate the spread of airborne viruses, but included some information on contact transmission.	Limited information was identified for contact transmission, but this was recognized as a significant infection route. There is a need for hand hygiene to interrupt the chain of infection, as recommended by the US Centers for Disease Control (CDC). New and future building designs could reduce the need for residents to touch surfaces, e.g., using touchless technologies for opening doors and for handwashing facilities. The focus of the review was mainly on respiratory viruses, in particular, SARS-CoV-2.
Vardoulakis et al., 2022 [44]	A systematic review assessing the risk of transmission of viral or bacterial infections through inhalation, surface contact, and fecal–oral routes in public washrooms. A short list of 38 studies formed the focus of the review.	Bacterial aerosolization and deposition on inanimate surfaces; contact with contaminated surfaces and contaminated water; and wet, contaminated hands were all identified as surface transmission pathways. Open-lid toilet flushing, ineffective handwashing or hand drying, inadequate surface cleaning, blocked drains, and exposed refuse bins can cause bacterial and/or viral contamination in washrooms. Heavy use, infrequent cleaning, or poor maintenance increased the contamination of public washroom surface contamination with bacteria and viruses. Many of the bacteria identified in the review were commensal or environmentally ubiquitous species, with the risk of infection low for healthy individuals. Good hand-hygiene practices reduce pathogen transmission, with effective ventilation and frequent cleaning of surfaces supporting this outcome.
Zhang et al., 2022 [13]	This systematic review considered academic studies on interventions for infection control, from the perspective of a Facilities Manager (FM).	Respiratory viruses can be transmitted in droplets of infected human saliva via shared indoor surfaces and can spread to others in contact with the same surfaces. Hand hygiene and surface cleaning are key measures for removing such pathogens. The likelihood of respiratory virus transmission from indoor surfaces correlates with the frequency of occupant hand to surface contact. This is influenced by the number of occupants and their interaction with the building. Minimizing accessible surfaces in rooms and limiting room occupancy are effective techniques for reducing outbreak risks via fomites. Faulty floor drains and toilet flushing may facilitate the transmission of airborne pathogens via the building’s drainage system.

* Pathogen studies were often hospital-based, but some described assessments in both homes and healthcare settings. Where integral to the study, collective information is presented in the table above.

3.1.4. Infectious Microorganisms on Surfaces

The evidence supporting the transmission of infectious microorganisms via surfaces was described in three well-conducted evidence reviews [13,30,44], including the pathogens’ ability to persist on, but also to be spread via, touched objects or fomites (Table 3). These reviews included information about control strategies (see Section 3.2) and the importance of hand-hygiene and surface-cleaning measures to mitigate the spread of surface pathogens on contact areas. The dissemination of saliva droplets from infected individuals was also described, indicating that even where airborne transmission is a recognized primary infection route, surface contamination is also well documented for respiratory viruses such as SARS-CoV-2 [13,44]. One study of SARS-CoV-2 transmission [44] reported other indoor sources of surface contaminants, including splash back from handwash basins and droplet generation from toilet flushing. Both studies concluded that high-frequency use of public washroom/toilet increased the risk of surface contamination and pathogen transfer, particularly when hygiene measures were inadequate [13,44].

Several individual studies provided evidence for the presence of infectious agents on surfaces (Supplementary Table S6). Four of these described the detection of SARS-CoV-2 or virus simulants on different surfaces. Although airborne transmission is now regarded as the primary route for SARS-CoV-2 infection, there is potential for contact transmission of this and other respiratory viruses via droplet deposition on items such as computer keyboards and mobile phones [33,36,45,46]. A residential study [47] reported the presence of the opportunistic pathogen Staphylococcus aureus in most surface dust samples taken from 24 homes. As well as causing infections, this bacterium can induce allergic inflammatory responses via Staphylococcal Enterotoxin (SE) superantigens. Although evidence for these toxins was detected in homes using PCR-based methods, not all detected S. aureus carried these inflammatory markers. Another study [31] confirmed the presence of bacterial contamination across surfaces sampled in public areas. The levels detected did not allow any conclusions to be drawn about the risks posed by the bacteria to public health. However, the presence of contaminants on high-touch surfaces demonstrated the potential for microbiological contact transmission between individuals in public places.

3.1.5. Supporting Evidence for the Presence of Harmful Microorganisms in the Indoor Setting and Related Transmission Routes

In addition to the papers summarized above and in Table 1, Table 2 and Table 3, and Supplementary Tables S3–S6, several other papers offered supporting evidence for indoor microbiological contamination and its sources. These provided further evidence for the presence of potentially harmful microorganisms within indoor settings [2,12,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67].

Commonly reported fungal contaminants from these studies were similar to those described earlier. They included Penicillium, Aspergillus, Cladosporium, Trichoderma, and Alternaria species, with their presence in the air or on surfaces repeatedly linked to damp housing conditions [48,51,52,57,61,63,66]. If contaminating indoor bacteria and fungi are present, their cellular residues, including toxins may be present too. The adverse health impact of mycotoxins, endotoxins, glucans, and other microbiological metabolites on human health was described [48]. Evidence for the presence of microbiological residues indoors included a study where the levels of bacterial endotoxin in residential settings were described [67]. Average endotoxin loads in floor dust varied between 660 EU/m² of floorspace (sampled in the Netherlands) and 107,000 EU/m² (US). Endotoxin has been linked to respiratory ill-health outcomes by other studies. This includes an association with wheeze, chronic bronchitis, and emphysema, with ill-health effects dependent on sub-group vulnerabilities [68,69,70]. Numerous factors influenced reported ill-health outcomes, including the age of houses, cleaning regime, residential setting [68,69], floor coverings, occupants/pets numbers [68,70], and relative humidity (RH), with dust becoming problematic when aerosolized. The clustering of endotoxin, along with allergens in dust from homes with pets or in homes of low socioeconomic status, was associated with respiratory ill health [70].

Moisture can enter the home from outside and can also accumulate due to human activity such as showering, cooking, or drying laundry indoors. Indoor moisture is exacerbated by cold interior surfaces, which promote surface condensation and mold growth [61,63,66]. Not all studies quantify these microbiological contaminants, but one [52] reported a range of airborne fungal concentrations for indoor air, including commonly occurring fungi such as Aspergillus, Penicillium, and Cladosporium species. Levels ranged from tens or hundreds of colony forming units (CFU) per m³ of air in some residential dwellings and schools, and up to 17,000 CFU/m³ in a study of over 100 Polish apartments. The highest levels were detected when fungi were present alongside bacteria. Intermediate concentrations in the 1000–2000 CFU/m³ range were reported from other countries. A 2016 review [54] reported evidence for Aspergillus, Penicillium, Cladosporium, and Stachybotrys species in household dust, furniture, carpets, and ventilation systems. Fungal spore concentrations varied from undetectable to 1000 CFU/m³, with conditions such as wheeze, cough, and asthma linked to fungi and their residues in the indoor environment.

There is further evidence for indoor sources of the fungi Fusarium and Aspergillus, as well as Penicillium spp., Alternaria alternata, Cladosporium spp., and Acremonium spp. [53]. Poorly maintained heating, ventilation, and air-conditioning (HVAC) systems supported abundant growth of Penicillium spp. and resulted in 50-to-80-times-higher concentrations of airborne fungi in an affected office than in an unaffected one. Water-damaged building materials were associated with a 50% increase in total viable fungi in dust, with total viable fungi concentrations of 2.55 × 10⁵ CFU/g reported in dust when moldy odor was present. When mold and water damage were reported, Aspergillus and Penicillium levels were twice as high as when these conditions were absent.

Older properties are more likely to suffer from elevated levels of dampness and fungal contamination, especially Penicillium and Aspergillus spp., with homes designed for high energy efficiency but with low-socioeconomic tenants being at greatest risk of dampness due to poor ventilation and insufficient use of home heating [51]. Comparing microbiological assessments from various studies remains challenging. This is because the indoor microbiota can be altered by factors such as architectural design, humidity, and ventilation levels [57]. In all, 1 m³ of indoor air reportedly contains 10⁵ virus-like and bacteria-like particles, about a tenth of the concentrations found in outdoor air, which can act as a source [57]. Fungal spores are less abundant and vary in numbers from around 80 to 10⁴ CFU/m³. Humans can emit around 10⁷ copies of bacterial and fungal genomes per hour, and influenza virus can reach indoor concentrations of 10⁵/m³ of air. Influencing factors for indoor colonization were moisture, age of the home, and pet ownership, with fungi confirmed as a significant source of allergens and mycotoxins [51].

Outdoor fungal communities reportedly drive indoor fungal communities in buildings without moisture problems [2]. By contrast, damp buildings often have distinct fungal profiles and have increased production of allergens, toxins, and pathogenicity, with building design and operation greatly influencing microbial communities. In addition, certain taxa were found to be more prevalent when windows were used for ventilation rather than mechanical ventilation.

Several other papers, including reviews, provided supporting evidence for the presence of infectious microorganisms in indoor air and on surfaces. Some recent studies considered the transmission of SARS-CoV-1 and -2, including evidence of the airborne transmission of coronaviruses based on Asian outbreaks [55]. However, technological differences in HVAC systems’ designs prevented the meta-analysis of internationally reported results. The possibility of the SARS-CoV-2 transmission via air-conditioning systems raised safety concerns, but available evidence at the time was insufficient to prove this transmission link. Others have reported SARS-CoV-2 transmission via the airborne route, especially indoors and without sufficient ventilation [62]. Here, indoor sampling demonstrated infection transmission between people inhabiting common spaces in the absence of close or direct contact. This confirmed that multiple SARS-CoV-2 transmission routes were possible, involving contact, droplet, and aerosolized mechanisms.

A later study [65] concluded that airborne viruses remained infectious in the air and could persist for hours, with pollutants such as particulate matter (PM₁₀ and PM_2.5), volatile organic compounds (VOCs), and gaseous pollutants exacerbating the speed of transmission and mortality rate of COVID-19 disease. Indoor temperature and humidity also influenced transmissions, and the need for adequate IAQ to reduce the spread of the virus and other pollutants was emphasized. Asymptomatic infectious individuals vocalizing during light exercise created high SARS-CoV-2 emissions, whereas symptomatic SARS-CoV-2 carriers in a resting state could have low emissions. Others found that physical exertion, speech loudness and articulation, coughing, and singing generated more aerosol particles than tidal breathing [12]. This study also reported that viral aerosolization from wastewater vents or natural ventilation air ducts was implicated in the vertical spread of SARS-CoV-2 in high-rise apartment buildings. A later review [67] reported that SARS-CoV-2 aerosols remained infectious for several hours, increasing transmission risk, even when the source was no longer present. Poor ventilation was implicated, with infectious aerosols remaining longer in indoor air, increasing transmission potential.

Indoor exposure to respirable, non-biological particulates can impair human host defenses, leading to increased susceptibility to airborne infections such as SARS-CoV-2 [59]. Indoor particulate sources are primarily from human activities, compounding the indoor air infection risk from the SARS-CoV-2 virus. Inhaled particles increase the likelihood of pulmonary infection and remain suspended for minutes to hours in the air. Smaller infectious particles also have reportedly higher pathogen loads in the aerosol fraction (≤5 µm in diameter) compared with larger aerosols. This is not just the case for SARS-CoV-2; Influenza A and B virus quantified from 142 symptomatic cases at a university campus had a geometric mean count three times higher in aerosols of ≤5 µm versus >5 µm [12].

The modes of indoor SARS-CoV-2 transmission have been related to how individuals move through the built environment [56]. Viral particles may be deposited or resuspended due to natural or mechanical airflow patterns, or as a result of other indoor turbulence such as walking and thermal plumes. When an individual contacts a contaminated surface, a virus can be transferred from the individual to the surface and vice versa, with infected individuals acting as part of a transmission cycle. This study [56] presented further evidence for fomite contamination with SARS-CoV-2 particles via human bodily secretions such as saliva and nasal fluid; on dirty hands; and disseminated in larger droplets via talking, sneezing, coughing, and vomiting.

Several additional reviews provided evidence for infectious microbiological sources and transmission potential indoors, including the following:

• The transmission of viruses such as influenza virus, rhinovirus, coronavirus, respiratory syncytial virus (RSV), and adenovirus inside buildings. These can be transmitted via aerosols, larger droplets, or contact with contaminated indoor surfaces. Supporting evidence included enterovirus detected at levels of 30,000 copies/m³ of air in a medical center and RSV shed at 10⁷ copies/mL of nasal discharge from infants [50].
• Human pathogen transmission, including norovirus, rotavirus, hantavirus, influenza virus, rhinovirus, coronavirus (SARS-CoV-1), rabies virus, chickenpox, and measles. Multiple exposure routes were described, including ingestion, direct exposure via droplet transmission, or exposure to inhaled particles [54].
• Pathogens indoor that originate from human carriers, whose actions spread microorganisms in droplets and via aerosols to the surroundings [67]. Multiple potential sources were described, including those resulting from inadequate toilet and shower hygiene, and poorly managed water storage and supply systems (leading to Legionella pneumophila and Pseudomonas aeruginosa growth).
• Heat-recovery units, also known as energy-recovery ventilators (ERVs), as a source of airborne pathogens [71]. These offer important energy saving whilst still providing ventilation control to a building. Their potential as a source of bioaerosol was often overlooked during the SARS-CoV-2 pandemic, but pathogen transfer in ERVs could occur due to carryover and leakage.
• The spread of fecal bacteria aerosols generated during toilet flushing —here, closing the toilet lid was ineffective at reducing bacterial air counts, and disinfectant use was the required intervention [58].
• Toilet flushing generating up to 145,000 aerosol particles per flush, most of which are less than 5 μm in diameter and can contain fecal microorganisms [53]. When gastro-intestinal infections occur, concentrations of between 10⁵ and 10⁹ bacteria and between 10⁸ and 10⁹ viral particles/g of feces were reported. This review also described how Legionella bacteria can be aerosolized when showering and using hot water at sinks, with between 10⁵ and 10⁶ cells/m³ of air detected. However, these data were from nursing and other healthcare facilities, not residential or business premises.
• The water pressure of toilet flush systems linked to levels of emitted bacteria, with higher pressure flushes corresponding to increased bacterial emissions [72]

3.1.6. The Broader Health Impact of Indoor Microbiological Exposures

The impact of indoor microorganisms on health was widely reported by numerous well-conducted studies describing hazardous microorganisms in the built environment. Most described the onset or exacerbation of allergic asthma, including ill health associated with poverty [6]. Other conditions, such as those associated with allergy and sensitization, such as wheeze and allergic rhinitis, were also reported [8], including seasonal influences on indoor fungal exposures [29,37]. For example, epidemiological evidence showing that indoor dampness or mold is consistently associated with asthma development and exacerbation of numerous other respiratory tract symptoms [8]. Other studies also identified dampness and mold exposures at home as determinants of conditions such as allergic rhinitis and rhino-conjunctivitis, including relationships between disease development and the presence of Penicillium, Aspergillus, and Cladosporium species [1,6,17].

Some effects were exacerbated or more evident in atopic (already sensitized) individuals [41], and elsewhere, seasonal effects were observed, such as high levels of Cladosporium (35,000 CFU/g dust) in winter-time household dust, tripling the risk of allergic sensitization in children [37]. In a study of microbial volatile organic compounds (MVOCs), a significant association was found between MVOCs 1-octen-3-ol and allergic rhinitis, though not between any of the MVOCs and asthma or atopic dermatitis [24].

Although most fungal-exposure studies were related to allergy, sensitization, and asthma, fungal impact on the risk of respiratory tract infections (RTIs) was considered [19]. This indicated that early life exposure to residential mold and indoor dampness had a small-to-moderate impact on the occurrence of RTIs or related symptoms. Interventions to reduce mold and dampness could reduce these symptoms. Earlier research identified that airborne Penicillium, dust-borne Cladosporium, Zygomycetes, and Alternaria were associated with lower RTIs, after controlling for other factors [73].

Some studies provided mixed results [39], such as where elevated levels of yeasts in bedroom floor dust were associated with reduced (i) wheezing at any age, (ii) fungal sensitization, and (iii) asthma development by age 13. However, Aspergillus exposure was associated with increased rhinitis and risk of fungal sensitization by age 12 and was exacerbated in children with a maternal history of fungal sensitization. These studies illustrate that for those exposed, there is complex interaction between exposure to these microorganisms and other environmental factors.

Research on infectious agents in built environments has often focused on detecting pathogens, rather than assessing their impact on health and disease. Some studies recognized the health implications of exposure but did not provide data on infection rates, whereas others acknowledged the health outcomes but did not provide supporting data on infection rates. Some findings investigated infection control outcomes but reported other adverse outcomes. For instance, adverse outcomes from increasing ventilation rates to manage airborne infection risk were reported, such as decreased thermal comfort due to draught and higher noise levels [13].

Another study investigated the exposure for adults and children due to the transmission of airborne pathogens and found few differences existed between the two groups [34]. Building structures were implicated, such as increased bioaerosol concentrations in the turbulent vortices and recirculation structures around room corners or close to leeward walls.

Those living with symptomatic COVID-19 cases risked exposure to airborne SARS-CoV-2, even when spatially separated from sick individuals. Simple measures such as opening windows or employing an air-cleaning device could mitigate this risk [36].

Infectious aerosols generated by toilet flushing can create a risk of infection for the immunologically vulnerable [72]. Evidence of an association between ventilation, the control of airflow directions in buildings, and the transmission and spread of multiple pathogens was identified. Infections were more likely to spread where inadequate ventilation was present.

Several reviews and individual studies provided supporting evidence for wider health impact. For example, household dampness, mold growth, and mold residues were commonly associated with asthma [74], rhinitis [75] eczema [76], and wheezing [77]. Several studies reported an increased risk of multiple ill-health outcomes associated with indoor dampness, mold, and poor IAQ [74,76,77,78,79,80,81]. A systematic review [49] concluded that visible mold was significantly associated with a higher risk in children of allergic respiratory disorders, including asthma, wheezing, and allergic rhinitis. Here, Alternaria, Aspergillus, and Aureobasidium spp. in dust were significantly and positively associated with diagnosed allergic rhinitis or hay fever at 5 years of age. Homes with visible indoor mold growth raised the risk of allergic respiratory health outcomes and childhood diseases.

A review by Wimalasena et al. [61] confirmed that household moisture and dampness was the main housing structural risk factor for respiratory health, reported in around 7% of the reviewed publications. The authors found positive correlations between measured area of mold colonization and respiratory conditions such as asthma, pneumonia, and upper respiratory disease. Poor ventilation, cold surfaces, and greater than 80% RH were all associated with mold growth.

Penicillium, Aspergillus, and Cladosporium spp. may increase the risk of developing asthma symptoms, and exposure to Penicillium, Aspergillus, Cladosporium, and Alternaria was associated with increased exacerbation of symptoms in individuals already diagnosed with asthma [64]. However, the impact of fungal exposure on health is nuanced, and the diversity of molds in the environment and the timing of the exposure may influence health outcomes, including some immune-protective effects.

Bioaerosols and biological material indoors affect health through irritation, toxicity, immunotoxicity, and other biological mechanisms [52]. For example, endotoxin in house dust aggregates with allergens from pets or mites, increasing asthma and wheezing symptoms, particularly in people with low socioeconomic status [70].

Some studies described the impact of the airborne concentrations of microorganisms on health risks. The concentrations of fungi that cause allergic reactions were correlated with disease exacerbations among patients with fungal allergy, but there was no correlation with total fungal spore concentration [78]. Here, the allergenic threshold levels of fungal concentration were relatively low and differed between species. For Cladosporium oxysporum, the threshold was 100 CFU/m³, and the threshold was 10 CFU/m³ for Aspergillus niger, Penicillium brevicompactum, and Penicillium oxalicum. Higher fungal concentrations led to acute exacerbations, indicating that specific allergenic spore concentrations may be the primary cause of worsening symptoms, rather than total fungal count. An important caveat for fungal exposures was that both dead and viable fungi are likely to cause allergic responses, and there may be circumstances when the CFU counts alone underestimate the immunological exposure load [57].

Increased health risks and their economic impact from indoor dampness and microbial contamination have been considered [48]. Aspergillus, Penicillium, Chaetomium, Epicoccum, and Fusarium spp., as well as Stachybotrys chartarum, were identified as more abundant indoors than outdoors. Promptly removing occupants from contaminated indoor environments was recommended, as was the need for more health-effect studies of bio-contaminants and their impact on humans. An assessment of the cost impact of illness described how asthma affects 8% of the US population, costing an estimated USD 56 billion per year, with all age groups affected, often from childhood [2]. The same study reported that dampness and mold issues in housing cost the US USD 3.5 billion per year. These reports are further supported by large-scale longitudinal studies that have considered respiratory health and its health and cost impacts on national populations, such as the European Community Respiratory Health Survey (ECRHS) [82] and the US NHANES [83] cohort studies.

3.2. Review Question 2

What is the evidence that control measures such as enhanced ventilation or enhanced sanitation measures can reduce occupant exposure to harmful bacteria, viruses, and fungi in the indoor environment?

Evidence and confidence statements—take-home messages:

• Ventilation is an important mechanism for diluting indoor air, either by mechanical or natural means, helping to reduce exposure to airborne microorganisms that may otherwise cause ill health (high confidence).
• Poorly managed mechanical ventilation may increase the likelihood of airborne pathogen spread indoors (medium confidence),
• Air-cleaning technologies that employ UV and/or high-efficiency air filtration have been evaluated with mixed outcomes. Still, most studies have reported a reduction in airborne microorganisms compared to untreated control areas (medium confidence).
• Introducing measures to control contaminated water splash and no-touch facility applications in building sanitary areas can reduce the contact transmission of indoor pathogens (medium confidence).

Reviewed papers provided evidence for controls that varied from enhanced handwashing and environmental hygiene regimes [13,44,45] to central ventilation controls [14,30] and retrofitted interventions [31,32] for reducing indoor exposure to microorganisms. Hand hygiene and surface cleaning are important for controlling indoor disease transmission. However, these are regarded as behavioral public health measures, rather than building safety-related measures, and they are not the focus of this review. In contrast, the safety features of a building and their health-related outcomes encompass ventilation rates, humidity control, choice of building material and finishes, design of water/wastewater systems, and overall building layout. These elements are covered in the existing Approved Documents that support the English Building Regulations [84]. A summary of reported interventions from well-conducted studies and their impact is given in

Table 4. Collective evidence for control/mitigation measures for microorganisms in air and on surfaces—in chronological order.

Author(s) and Year	Review Aims/Description	Relevant Evidence Presented/Author Conclusions
Li et al., 2007 [14]	This systematic review sought published information about ventilation, airflow, and transmission of viruses indoors. Searching literature databases between 1960 and 2005, 40 original studies were chosen for review, based on a set of criteria.	Airflow direction and thermal plumes created by building design features such as radiator placement were implicated in the spread of airborne pathogens in multiple case studies. Conclusive data from viral infection outbreaks were only available for healthcare settings. These indicated the spread of diseases due to the dispersion of bioaerosols from an index patient’s location to other rooms or other parts of the same room. There were insufficient data to support minimum ventilation requirements to mitigate airborne infection spread in schools, offices, and other non-hospital settings. Infectious-source strength is important because, for some viruses, even a small infectious dose, e.g., 1–100 infectious microorganisms, can cause infection.
Krieger et al., 2010 [16]	A systematic review was conducted to identify housing-related interventions to improve resident health.	3 of 11 reported interventions were sufficiently effective to be implemented for asthma reduction. The interventions were reducing cockroach allergen, eliminating moisture intrusion and leaks, and removing moldy items. These reduced the prevalence of respiratory symptoms. Concerns were raised about portable air cleaners generating unwanted ozone during use.
Tang et al., 2020 [84]	This study examined whether confined spaces in an unoccupied, three-bedroom/two-bath test house harbored potential contaminants (particle/gases), e.g., within cupboards and other confined spaces. It also considered the impact of air conditioning on the dispersal and buoyancy of aerosols.	Natural air circulation dispersed a test aerosol throughout all rooms of a building, becoming well mixed, due to temperature differences between rooms. Momentum-driven airflow (e.g., due to an air-conditioning fan) had little effect on air circulation in confined spaces. Sensitized individuals should consider solid floors over carpet since dust removal is easier, and dust resuspension rates are lower for hard floors. This is important in high-moisture areas, such as bathrooms. Future home design and furnishing choices could potentially improve human health.
Lee et al., 2022 [31]	This study examined whether fixed germicidal UV room air cleaners, installed in high-occupancy rooms could significantly reduce airborne and surface microbial contamination in occupied commercial indoor environments.	Fixed in-room UVGI air cleaners in commercial buildings significantly reduced airborne and surface-borne bacterial contamination. Total airborne reductions by UV at six separate commercial sites averaged 73% (p < 0.0001), with a 71–88% range. The mean value of indoor airborne bacteria was 320 CFU/m3 before intervention and 76 CFU/m3 after, with reductions statistically significant. Rooms installed with fixed in-room UVGI air cleaners showed significant CFU reductions for local surface contamination.
Myers et al., 2022 [32]	A blinded, randomized crossover trial using air cleaners as the intervention in the homes of COVID-19 sufferers. The presence of airborne SARS-CoV-2 RNA was investigated in total suspended particles in a primary “self-isolation room” used by the infected participant and in a secondary room in the residence.	With air cleaners absent, 43.8% of the samples (7/16 residences) tested positive for SARS-CoV-2 RNA. Only 25% of the samples (4/16 residences) tested positive for SARS-CoV-2 RNA during the test “filtration” period. Despite evidence for a reduction in SARS-CoV-2 aerosol exposure using portable air cleaners, the data were not statistically significant. Similar proportions of SARS-CoV-2-positive aerosol samples were detected in homes with central HVAC (primary room, 50%, 5/10 residences; and secondary room, 44.4%, 4/9 residences) when compared to non-central HVAC (primary room, 33.3%, 2/6 residences; and secondary room, 50%, 3/6 residences).
Salman et al., 2022 [30]	This systematic review summarized studies of building systems and technologies that mitigate the spread of airborne viruses.	HVAC systems in most buildings were not equipped to limit the spread of SARS-CoV-2 virus. HEPA filters catch the smallest virus particles and are therefore recommended. Natural ventilation is the most cost-effective method for introducing fresh air and diluting pollutants, but it is necessary on every floor of a building. This results in costly heat loss, making geographical and seasonal influences relevant. Reducing recirculated indoor air and increasing incoming fresh air can improve IAQ. However, this increases costs from heat loss. CFD modeling studies can indicate the flow of air/airborne viruses within a building’s HVAC system. Botanical air-filtration system and indoor gardens can improve IAQ, with air quality monitored using real-time sensors. The installation of touchless technologies in buildings can mitigate the contact transmission of SARS-CoV-2 and other infectious agents.
Vardoulakis et al., 2022 [44]	A systematic review assessing the risk of transmission of viral or bacterial infections through inhalation, surface contact, and fecal–oral routes in public washrooms. A short list of 38 studies formed the focus of the review.	Washroom design can reduce the potential for infectious disease transmission. Adequate ventilation, including mechanical ventilation with air filtration, can help mitigate airborne disease transmission. Splash—a contributor to surface contamination—can be reduced using toilet bowls that have a low volume and flush force. Sink designs and U-bends in plumbing systems also reduce droplet splash. Avoiding warm-water bidet toilets also reduces the splashing of contaminated water. Non-touch flush buttons and other sensor-operated fittings for hand dryers, and soap and paper-towel dispensers can reduce the contamination of touched surfaces.
Zhang et al., 2022 [13]	This systematic review considered existing academic studies on Facilities Management (FM) interventions for infection control, from the perspective of a Facilities Manager.	Current ventilation standards, such as the minimum fresh airflow requirements in various building codes, may be insufficient to prevent airborne transmission, particularly in high-occupancy buildings such as school classrooms, restaurants, and hotels Adverse outcomes resulting from increasing ventilation rate include (1) draught and increased noise levels, (2) higher energy use and costs, and (3) higher CO2 emissions. Ventilation rates in different rooms within a central ventilated facility often vary depending on their location and the design layout of the ventilation system Ventilation alone cannot prevent indoor airborne transmission, though multiple studies described ventilation rate as a critical factor for pathogen removal. Any single structural or environmental solution is insufficient to prevent indoor respiratory virus transmission. An integrated approach is required, but few studies identified combined intervention effects on different disease transmission channels.
Liu et al., 2023a [34]	This study systematically assessed the probability of infection in adults and children, accounting for different ventilation conditions and respiratory pathogens.	By increasing ACH from 6 h−1 to 18 h−1, the area-averaged air age in the infusion room was reduced by 66 percent, indicating that aerosols can be removed rapidly, and the short-term exposure of airborne aerosols can be significantly suppressed. Children did not differ much from their adults in exposure risk by airborne transmission, although they were assumed to have smaller tidal volume and lower breathing height.
Liu et al., 2023b [35]	This study assessed the airborne transmission risk of SARS-CoV-2 in different thermally stratified indoor environments using a previously developed airborne infection risk model.	Indoor temperature gradients cause peaks of the transmission risk for SARS-CoV-2 with distancing from source; in office, hospital ward, and classroom spaces, the second peak of the transmission risk exceeded that of larger spaces, such as coach stations and airports, which typically have ceiling heights of over 3 meters. In these larger spaces, consideration of the vertical temperature gradient is essential for accurate infection risk assessment. Instead of only increasing the distance from the source, it is advised to use physical interventions like partitions to block transmission pathways and staggered seating to lower inhalation exposure to reduce airborne SARS-CoV-2 transmission.

.

How Effective Are Different Specific Control Measures (e.g., Ventilation Rates, Air Filtration, and Other Methods) Against Different Microorganisms?

Several studies identified ventilation as essential for diluting indoor air, either by mechanical or natural means, therefore controlling exposure to airborne infectious microorganisms that might otherwise cause ill health [30,34,44]. The evidence supports vents or windows being open on every floor for the effective natural ventilation in larger buildings, but this is reliant on geographical and seasonal factors. Even effective ventilation could not prevent the penetration of a small percentage of indoor aerosols into enclosed indoor spaces of test homes [85]. Indoor heat loss—and, hence, the disadvantages of cost and thermal comfort—is a limitation of this otherwise low-cost approach [30]. By contrast, mechanical ventilation can conserve heat by recirculating air. However, this may be inadvisable where microbiological air quality is concerned, as it may reduce the dilution of bioaerosols and negatively impact IAQ [30].

Trebling ventilation flow rates from 6 to 18 air changes per hour (ACH) enabled short-term exposure to bioaerosol to be reduced [34]. However, using ventilation in this way can affect resident comfort in the treated areas and may achieve microbial removal only if high-efficiency filtration standards are introduced [13,30]. Additional evidence suggests that ventilation alone cannot control or reduce bioaerosols, despite the benefits [13,30]. Some review data implicated ventilation in the spread of airborne infection [14]. However, the evidence was restricted to healthcare disease outbreaks, with limited data for non-healthcare settings, and minimum airflow rates required to mitigate transmission in other indoor settings were not provided.

Considering interventions and controls more broadly, published evidence supported three approaches that might immediately improve asthma and allergic conditions [16]:

-

Elimination of moisture intrusion and leaks;

-

Complete removal of any visibly mold-damaged materials;

-

Removal of any insect pests (and their residues).

This study [16] acknowledged the limitations of these actions, as effectiveness varied by region due to climate variation and housing types, requiring approaches tailored to local conditions.

Retro-fitting buildings with either integrated or mobile ventilation systems may reduce contamination on surfaces and can complement routine hygiene controls. Systems that employ UV or air filtration have been evaluated, with mixed outcomes [30,31,32]. One study reported average airborne bioaerosol reductions of 73% (range: 71 to 88%) after fixed UV system installation at six commercial sites [31]. Others identified measurable reductions in SARS-CoV-2 aerosol levels in 16 residences when portable air cleaners were used, but the findings were not statistically significant [32]. A review synthesizing papers predominantly published during the COVID-19 pandemic acknowledged the potential benefits of UV treatment for eradicating airborne SARS-CoV-2 in ventilation ducts [30]. However, the authors were concerned about toxic by-products, such as aldehydes.

The risk of contact transmission via indoor surfaces can be influenced by building interior design. Optimizing toilet and basin design and adjusting the basin water pressure can reduce splash and droplet production in toilet areas [44]. Non-touch mechanisms for toilet flushing or opening doors, particularly in intensively used public areas, can also reduce the transmission of infectious agents via these surfaces.

In addition to the information in Table 4, several other publications provided supporting evidence for control strategies related to indoor microorganisms and their potential health impact.

Effective ventilation can mitigate indoor mold contamination; reduce the risk of airborne pathogen transmission; and dilute indoor chemical contaminants, including MVOCs [24]. Suggested interventions included opening windows to dilute indoor air, or using air cleaning interventions, in order to promote good IAQ for building occupants [77,86,87]. However, ineffective ventilation may contribute to indoor microbiological contamination [88]. For older properties lacking mechanical ventilation, air exchange can occur via small openings in the building envelope, or by simply opening windows and doors. Reducing any of these ventilation routes increases resident exposure to indoor air pollutants [66,80] and may result in raised indoor moisture, mold growth, and exacerbation of residents’ asthma symptoms and other respiratory diseases [51,61,80,89]. In mechanically ventilated homes, only high-efficiency filters can reliably prevent the ingress and recirculation of outdoor air pollutants, including microorganisms [66].

In a pre-COVID study, a link was identified between a rise in infectious illnesses and a decline in ventilation rates, but the impact of mechanical ventilation on the airborne transmission of infectious agents was not determined [90]. The study emphasized that buildings do not experience constant ventilation rates because HVAC systems differ in their design, and/or because varying outdoor air temperature can influence indoor flow rates.

The importance of avoiding air recirculation to reduce pathogen transfer between rooms was reported [71], with increased air changes per hour reducing indoor infectious aerosol concentrations. Mechanical ventilation was regarded as an effective engineering control for this purpose, but improper ventilation was linked to airborne disease transmission. Others have described ventilation as a potential contributor to disease transmission, including HVAC systems suspected of spreading SARS-CoV-1 and MERS in hospital and community settings [55]. Prevention in workplaces must be personalized, with stringent ventilation measures applied when circumstances require it.

High-intensity outdoor air exchange rate can mitigate the transmission of the airborne SARS-CoV-2 virus by reducing indoor aerosol concentrations [65]. Natural and mechanical ventilation were beneficial, and high humidity and temperatures were shown to inhibit viral spread. The value of good ventilation for improved air quality, without compromising thermal comfort and energy efficiency standards, was emphasized [65]. The World Health Organization (WHO) recommended minimum ventilation rate of 10 l/s for offices and residential property was cited, with the ASHRAE (62.1) recommended minimum ventilation rate deemed insufficient to reduce the indoor SARS-CoV-2 transmission during a pandemic.

In a further review of ventilation controls, the need for increased outside air fractions and air exchange rates to dilute indoor air contaminants was again described [67]. This decreased the probability of infectious transmission, whereas recirculating indoor air increased exposure risk. Another study contrasted HVAC use in non-pandemic and pandemic times [62]. HVAC systems are designed to remove indoor heating/cooling load and contamination, e.g., CO₂ and respirable suspended particulates. During disease outbreaks, effective HVAC systems can reduce contaminated airflow to occupied areas by diluting viral concentration. A poorly designed air supply system can aggravate viral spread.

Controlling indoor viral transmission is often reliant on ventilation containing inline filtration media [56]. Effective residential and commercial systems typically require a minimum efficiency reporting value (MERV) of 8 (70 to 85% of particles captured; 3.0 to 10.0 µm range). Higher MERV ratings are required to filter supply air due to outdoor particulate levels, but these cannot be assumed to eliminate all airborne transmission risk.

A range of measures were considered for reducing exposure to pathogenic aerosols in buildings, including ventilation, filtration, germicidal UV, and airflow management systems [12]. These offered potential public health benefits, but real-world evidence was not always available. Engineering controls were regarded as having key advantages because of the following reasons:

• They do not rely on knowledge of occupant infectious status—important because asymptomatic cases can shed as much virus as symptomatic ones.
• They reduce bioaerosol exposure and can prevent infection.
• They can provide protection without incurring behavioral resistance to measures like mask use or vaccination.

This same review [12] also provided detailed information on various ventilation requirements from authoritative sources.

The impact of RH on airborne disease spread was investigated [91], providing evidence of an association between the control of ventilation-driven airflow in buildings and the transmission and spread of infectious diseases. The transmission efficiency of influenza A virus, for example, is dependent on RH, with transmission highly efficient at low RH (20% or 35%), less so at 65%, and with no transmission at 80% RH.

In public spaces, there was limited evidence that air-cleaning technologies reduced infection frequency [92], but these treatments could reduce surface contamination when germicidal UV and high-efficiency particulate air (HEPA) filtration was applied. More evidence was required to confirm the real-world effectiveness of these technologies and to confirm whether portable air cleaners could lower respiratory infection rates, including those from SARS-CoV-2. However, some evidence was found for them reducing airborne pathogenic bacteria [60]. An analysis of UV/HEPA air cleaners found they could eliminate > 99.9% of bacterial pathogens after 45 min of test-chamber application [7]. Over 80% of the reviewed studies found that air disinfection methods were effective over baseline conditions, but impact on reducing disease transmission remained unclear.

Reducing the levels of viable airborne pathogens remains an important intervention, whether achieved by integrated building ventilation, mobile air cleaners, or natural ventilation. Targeting bioaerosols provides the additional benefits of reducing their deposition on to indoor surfaces [54].

3.3. Review Question 3

Could information from this review inform changes to national Building Regulations and associated guidance, i.e., the standards that should be achieved when a new building is erected or building work is done to an existing building?

• English Building Regulations do not directly regulate microbiological air quality, but they promote building conditions that reduce microbiological contamination risks (high confidence).
• Design and construction compliance with relevant guidance supports safe IAQ (high confidence).
• Ventilation systems have received increased attention since COVID-19 for their role in limiting the spread of airborne pathogens (high confidence).
• Guidance and evidence from multiple sources emphasize improved air exchange rates and HEPA filtration in certain settings, particularly where occupants have increased health vulnerabilities (high confidence).

3.3.1. Regulatory Background and Context

Much of the presented information concerns IAQ and its impact on allergic and infectious diseases in the built environment (Table 1 and Table 2). Microorganisms can also contaminate surfaces, and combined with high indoor moisture levels, this can promote fungal growth (Table 3).

Despite these known mechanisms, England has no legally enforceable exposure threshold for indoor microorganisms, although certain types and levels of microorganisms are known to be hazardous to health (Table 1, Table 2 and Table 3). Exposure thresholds have probably not been set because airborne microorganisms are generally excluded from international IAQ guidelines [93] and reasons for the contamination of buildings can be diverse. Establishing a link between exposure and ill health can also be complex, so it is unsurprising that the English Building Regulations do not define thresholds for microbiological IAQ or surface bio-contaminants. These regulations are, however, designed to reduce the likelihood of exposure, for example, through effective ventilation, building moisture control, and measures such as appropriate building materials and insulation choices. Understanding these factors and their implications can help to identify areas that may deserve further attention.

A précis of the selected English Building Regulation Approved Documents that influence microbiological contamination is given below, along with relevant published research (Section 3.3.2). The Approved Documents provide technical guidance on common building situations and how to meet the Building Regulations requirements for England [84]. It is not within the scope of this review to list the full contents of the Approved Documents, but relevant content can be summarized as follows:

Approved Document F: Ventilation. Aims to prevent the buildup of harmful indoor contaminants, including microorganisms, by ensuring proper ventilation and airflow. Microbiological limits are not specified, but flow rates are indicated for various room and building types. Effective ventilation reduces conditions that promote indoor bioaerosols and surface contaminants.

Aspects of Approved Document F relevant to microbiological air quality and surface contamination:

• Ventilation rates: Effective airflow prevents the accumulation of humidity, which is a key factor in controlling microbial proliferation.
• Mechanical ventilation systems: These must include air filtration mechanisms and maintenance programs to prevent particulate dissemination—relevant for microbial contamination. HEPA filter standards and other measures, such as UVC treatment of air, are included here.
• Moisture control: Reducing condensation through appropriate extraction (e.g., in bathrooms and kitchens) helps prevent high indoor RH that can lead to mold growth.
• Minimum air change rates: Sufficient airflow through a property reduces the concentration of airborne pathogens and prevents stagnant air conditions—this includes the use of CO₂ monitoring as an additional indicator of air quality.

Approved Document C: Site Preparation and Resistance to Contaminants and Moisture. Addresses dampness and moisture control, the management of which is critical for preventing microbiological contamination, particularly indoor mold growth.

Aspects of Approved Document C relevant to microbiological air quality and surface contamination:

• Damp-proofing and drainage: Prevents water ingress from roofing and foundations, reducing the risk of interior dampness and subsequent mold development.
• Ventilation of voids: Ensures airflow in subfloors and cavities, reducing humidity that could otherwise encourage microbial growth.
• Radon protection: Includes ventilation and barrier membrane measures to prevent accumulation of radioactive particles, which may also promote microbiological control.

Approved Document H: Drainage and waste disposal systems to ensure buildings are safe, hygienic, and environmentally compliant. Includes all aspects of drainage systems, such as foul water drainage, other wastewater handling, and ventilation controls to prevent foul odors. Rainwater drainage, including runoff from buildings and the prevention of water pooling, is also addressed.

Aspects of Approved Document H relevant to microbiological air quality and surface contamination:

• Poorly designed or maintained drainage systems can lead to stagnant water, leaks, and backflow, increasing the risk of microbiological contamination.
• Proper venting and drainage capacity prevent anaerobic conditions, which favor microbial proliferation and the buildup of foul odors and noxious gases.
• Water ingress through poor rainwater management can result in damp conditions, which promote mold growth.
• Proper wastewater catchment and treatment reduce the risk of pathogenic organisms entering the water table or nearby soil.
• Blocked or poorly ventilated drainage systems can release aerosol contaminants (e.g., sewage gases), which may harbor pathogenic microorganisms.
• Inadequate solid waste management and related ventilation can lead to microbial growth, especially in warm, humid conditions, resulting in foul odors and toxic gas emissions.

Approved Document L: Conservation of Fuel and Power. Focusses on energy efficiency, providing guidance on reducing unwanted heat loss, by achieving optimum airtightness. This can impact building IAQ, and the importance of minimum ventilation requirements are emphasized, as are consideration of air movement within a building (links to Approved Document F). If not properly ventilated, airtight buildings may suffer from the following:

• Stale air conditions, which can foster microbial growth due to lack of dilution by outside air and progressive indoor accumulation of microorganisms due to human and/or pet activity.
• Accumulated condensation and dampness, which create environments conducive to mold proliferation.

To counteract these potential problems, effective vent installation in residential buildings and mechanical ventilation systems in larger developments may be required in more airtight buildings.

Health and Safety Executive (HSE) Information and guidance

HSE provides additional guidance on topics related to building biohazards:

• ** Legionella control: HSE’s L8 Approved Code of Practice (ACoP) requires regular maintenance of water systems to prevent Legionella bacteria growth, which can become aerosolized and impact human health in or around affected buildings [94].
• Ventilation and air-conditioning-system maintenance: Preventing fungal and bacterial buildup [95].

** The Building Regulations Approved Document G also addresses hot water safety and efficiency of water in buildings, as is relevant to Legionella control. However, the document only mentions this briefly, with L8 cited.

Relevant standards and guidance documents

The Building Regulations do not define specific microbiological air-quality levels, but some standards and guidance documents provide relevant benchmarks, including the following:

• BS EN 16798-3:2017—Energy and ventilation performance of buildings. Ventilation standards that include preventive measures relevant to microbiological contamination [96].
• CIBSE TM40—Health and well-being in building services. Describes best practices for IAQ management in non-domestic buildings, including minimizing microbial contamination through filtration and airflow management.
• BS 5250:2021—Managing moisture in buildings. Includes the control of condensation in buildings, helping to prevent microbial growth through moisture management [97].
• BS ISO 16000-36:2018—A standard method for assessing the reduction rate of culturable airborne bacteria by air purifiers using a test chamber [98].

3.3.2. The Evidence for Potential Changes to Regulations

Information from literature reviews typically includes international sources. As this review was intended to be relevant for English regulations, with those of other UK nations being devolved, the evidence was considered in terms of housing types, climatic challenges, and regulatory context that reflected the English situation.

English building design and functionality are comprehensively supported by Approved Documents that support Building Regulations [84], but adverse effects on residents’ health can still occur. The following studies have attempted to identify key areas of concern, but they do not necessarily reflect deficiencies in Building Regulations.

The impact of inadequate ventilation is widely reported, linked to increased levels of indoor moisture and microbial proliferation in homes, public buildings or workplaces. When moisture is present in conjunction with inadequate ventilation, the risk of mold growth and impact on health due to asthma and respiratory allergies is elevated [1,8,16,17,18]. In addition, a large systematic review and meta-analysis of indoor mold, dampness, and respiratory tract infections (RTIs) identified a weak-to-moderate effect on associated risk of RTIs in children in high-income countries [19]. Although building design and construction must follow ventilation guidelines, problems may develop as buildings age, such as declining ventilation controls for cooking and bathing. Older properties are also at increased risk from rising damp or rainwater intrusion, necessitating attention and repair. Extreme storms and flooding can hasten building damage, a problem increasingly common in the UK [99,100].

Integrating high standards such as Passivhaus low-energy design/retrofitting standards into English Building Regulations could contribute to healthier and more eco-friendly housing [66]. This rigorous energy efficiency standard for buildings is designed to promote ultra-low energy consumption through superior insulation, airtight construction, and high-performance windows. Passivhaus prioritizes good IAQ and comfort and can be applied to residential, commercial, and public buildings, promoting sustainable, energy-efficient living.

Current building designs and guidelines are based on various perspectives on building technology and indoor environment designs that have not been updated alongside progress in this field of research. Existing building standards were found to be insufficient by some to address the health consequences of poor IAQ, such as controlling excess moisture and mold in buildings [18]. A central tenet here was that adequate ventilation must control surface condensation in buildings, a conclusion also reported elsewhere [9]. One study [6] concluded that future housing interventions should aim to exceed a Standard Assessment Procedure (SAP) rating greater than 71 to lower the risk of mold contamination, with improved ventilation strategies required to increase airflow in energy-efficient homes. It was concluded that current heating and ventilation strategies were ineffective in lowering indoor exposures to mold associated with damp environments, especially in fuel-poor populations.

IAQ microbiological thresholds were investigated [27], and it was noted that the French National Agency for Environment Health had established an “abnormal” threshold based on cultured fungal colonies exceeding of 1000 CFU/m³, or a moldy surface area larger than 3 m². These values provided an action level for health authorities when considering rehousing residents. The WHO IAQ guidelines reportedly provided no threshold limit values for molds, and no US Environmental Protection Agency regulations or standards for airborne mold contaminants were identified. Finland and Belgium have tried to define guidelines to specify “inappropriate” fungal levels for dwellings using health data, but they struggled to identify microbiological threshold limits. Portuguese decree law 118/2013 (PDL118) was the most prescriptive, making a distinction between the various species and threshold levels, based on their potential impact on human health. These ranged from as few as >12 CFU/m³, classified as dangerous for nine toxic species to seven molds classified as not posing a health risk in dwellings at ≤500 CFU/m³, but with higher levels considered a health risk. Despite these reported initiatives, it was concluded that most international regulations established to control IAQ omit or only briefly consider threshold values for bioaerosol pollutants [93]. The main reasons for this were associated with the expense and complexity of formally identifying different bacterial, fungal, and viral contaminants.

Preventing the spread of infections, such as COVID-19, via air and contact surfaces, using building design, antimicrobial technologies, and cleaning and maintenance, has been described [67]. The following was concluded:

• Prevention measures should be implemented during the design, early construction phases, and throughout a building’s lifecycle.
• A hygiene expert should be nominated for each construction project to drive hygiene targets and monitor their fulfilment throughout a project.
• Guidelines for constructing hygienic indoor environments, set by authorities and with certification, would be necessary when integrating measures throughout a building’s lifecycle.

Integrating an indoor environmental quality (IEQ) assessment could be helpful before and after building improvements (retrofits) [101]. A pre-retrofit assessment provides valuable information to improve IEQ, e.g., adequate ventilation needed, whereas a post-retrofit assessment confirms whether IEQ has fulfilled building standards. Such renovation must include these assessments and should include an evaluation of potential risks associated with IEQ.

English Building Regulations prescribe adequate ventilation requirements and require a minimum whole dwelling ventilation rate of at least 0.3 L/s per square meter of internal floor area, corresponding to around 0.5 ACH [51]. Others concluded that poor ventilation increased pollutants inside houses and that airtightness driven by energy efficiency decreased passive ventilation [61]. This can lead to increased indoor pollutant concentrations and underlines the need for household energy efficiency standards and regulations that still deliver adequate ventilation and good IAQ. Recognition of indoor infection spread has enhanced infection control efforts, moving beyond the simple idea that “more ventilation is better” [12]. Increasing ventilation rate above 1 L/s/m² reduced airborne SARS-CoV-2 infection risk, and 4 L/s/m² was described as a lower boundary in office meeting rooms or classrooms (corresponding to 5ACH). The WHO has suggested 5– 6 ACH for public buildings for SARS-CoV-2 transmission control, with ventilation regarded as a helpful control alongside other layers of protection [12]. Upgrading HVAC filters to MERV13 or better, using portable air cleaners (with MERV13 filters or better), and using upper room germicidal UV (GUV) were all methods that aid infection control.

Current ventilation standards, such as the minimum fresh airflow requirements in various building codes, were reportedly insufficient to prevent airborne COVID-19 transmission, particularly in high-occupancy public buildings [13]. The effective ventilation rate for a gym required more than twice that stipulated in the Dutch Building Code (995 m³/h vs. 433 m³/h). The risk of transmitting respiratory viruses via a building’s drainage system was deemed low if systems were well maintained, with regular wastewater plumbing inspections and avoiding drain obstructions.

The design, policy, and enforcement oversight for the operation of HVAC systems was also investigated [62]. During normal use, demand-controlled ventilation (DCV) systems conserve energy by switching off ventilation systems in response to monitored CO₂ concentration, but this should be overridden during pandemics to prioritize IAQ over energy conservation. The importance of introducing outdoor air into indoor spaces was prioritized, minimizing air recirculation in HVAC systems and utilizing mechanical and natural ventilation to full capacity to improve IAQ. An ACH of 3 was deemed enough to reduce indoor virus levels by 95% in most cases.

3.4. Review Question 4

Is there evidence in the published literature to indicate that any mitigations proposed or introduced will influence economic or net-zero outcomes?

Evidence and confidence statements—take-home messages:

• Modern building airtightness contributes to energy efficiency, indoor thermal comfort, and winter energy savings but can limit the efficiency of naturally ventilated buildings, potentially reducing IAQ (high confidence).
• Increasing centrally controlled ventilation flow rates and outdoor air intake in-creases energy costs, e.g., during airborne disease outbreaks. Modern HVAC control and design solutions can off-set this (medium confidence).
• Supplementary IAQ controls, such as ceiling-mounted germicidal UV and portable air cleaners, may improve microbiological air quality, but these have initial and ongoing cost implications and must be well-designed and maintained to be effective (medium confidence).

Information was gathered on any implications for IAQ control, linked to energy conservation and green initiatives. This included increased energy consumption from mechanical ventilation, financial challenges, conflicts between structural design and safety, or regulatory conflicts.

Some research has argued for a greater understanding of energy-efficient home performance under climate change scenarios, including the quality of installed ventilation systems, and any impact on occupant health and well-being [44]. For example, Passivhaus design principles reduce energy use and associated carbon dioxide emissions in winter, but there is a risk of overheating in hot weather unless effective ventilation and active cooling systems are used. Generally, houses from 1980 onward were likely to be more affected by issues related to airtightness and overheating, compared to older housing stock.

Modern green building initiatives aim to achieve energy efficiency and low CO₂ emissions during construction, but few design and engineering measures address controlling infection transmissions in public buildings. This is despite a justifiable need to design, construct, and renovate healthier buildings to reduce infection transmissions in built environments [67]. Creating healthy and hygienic buildings was found to require technical knowledge, implemented at different stages of the building’s lifecycle, leading to improved indoor infection prevention. Key elements included thoughtful spatial planning (like smart ventilation choices), touchless surface options, efficient water management systems, regulated movement of occupants and delivery vehicles, designated clean and dirty zones, and easy access to well-placed cleaning and maintenance rooms spaces.

An earlier study concluded that infection prevention controls can also reduce energy costs and water use [102]. Several “factorial measures” contributed to infection risk reduction, increased resilience and improved public health, whilst maintaining sustainability. These included minimalist (Nordic-like) building design, textile-free floors, spacious entrance areas, increased natural sunlight, natural ventilation, and new HVAC systems in non-residential buildings.

Research has described a need for building factors such as ventilation, occupancy, and material choices to be used to promote a healthier indoor microbiome [2]. Describing the United States Green Building Council (USGBC) and its Leadership in Energy and Environmental Design (LEED) green certification program, it was noted that this program has not yet considered indoor microbial populations and their potential risks to human health. A suggested approach was to establish new standards to evaluate a building’s overall impact on human health to inform decisions by those involved in building design, construction, and management decisions.

One study examined 44 new and 105 retrofitted energy efficient homes in Switzerland [100], where a building certification scheme was implemented to demonstrate energy efficiency and its impact on occupant comfort. The study identified different fungal types on surfaces in naturally ventilated versus mechanically ventilated dwellings. Aspergillus sp. was often detected in naturally ventilated bedrooms, but not in those with mechanical ventilation. Urban and peri-urban locations were the most affected by molds, with mechanically ventilated dwellings more effective at preventing fungal particle infiltration, compared to naturally ventilated dwellings. This was regarded as important for ventilation systems in energy-efficient dwellings to control indoor fungal proliferation. Such factors are also considered in a wide-ranging and comprehensive review into respiratory allergen risk associated with mold growth indoors [18].

Buildings have become progressively more protected against ambient climate fluctuations to save on energy costs for heating and cooling [103], with increasing international energy prices an influencing factor. Decreased use of heating during colder months, coupled with airtight buildings aiding energy efficiency, can result in poor microbial IAQ and increased moisture and dampness [104]. There is a need for guidance in maintaining or improving microbial IAQ to avoid increased exposure to harmful microorganisms. Building upgrades and raising occupant awareness could significantly lower the allergen burden of these residences.

Home draught-proofing in US homes and its influence on indoor microbial exposure, indoor pollutants, and bacterial communities were assessed [7]. Outcomes were compared to non-weatherized homes and indicated an average reduction in building (air) leakage of 22% per home. Families were more thermally comfortable, but energy consumption increased in some homes after weatherization—a possible “rebound effect”. Bacterial levels remained unchanged and correlated with occupancy rate, temperature, and RH. There were no changes in respiratory symptom reporting or exacerbation of pre-existing symptoms after weatherization, but these observations were limited to a relatively short period post-intervention.

Others described how building practices are changing to build tighter homes with more efficient mechanical ventilation, such as using heat-recovery units, to save on energy losses [80]. Because these recover energy from the exhaust air to pre-condition incoming fresh air, they reduce energy consumption. Energy-efficiency retrofits can enhance the energy-efficiency of older buildings, including weather-proofing and sealing of doors and windows, and sealing cracks in the building envelope. These and other measures must be balanced with adequate airflow.

Several types of heat-recovery unit have been developed over the last decade to reduce HVAC energy consumption [71]. During the COVID-19 pandemic, some pandemic HVAC guidelines recommended limiting their use due to concerns of bioaerosol cross-contamination, based on operational principles rather than substantial published evidence. More research is therefore required to determine if heat-recovery units need to be operated with constraints during disease outbreaks. This is important because HVAC heat-recovery devices could maintain final energy consumption at normal operational levels during outbreaks, despite 40–50% increases in ventilation demand. These devices can of course also reduce energy consumption under normal operational conditions.

A cost–benefit analysis was undertaken for different interventions designed to improve IAQ, with a focus on disease outbreaks [105]. In order to achieve 99.9% removal of airborne particulates (including bioaerosols), the improvement cost was USD 280/HVAC filter replacement, with increased fan duty also required. Increasing outdoor air dilution rate was estimated to cost an extra USD 5/m² of floor-space/year due to extra energy consumption using centralized ventilation, or an extra USD 10/m²/year for non-centralized ventilation. Upper-room germicidal UV systems were valued at USD 40–90/m² floor area. Design-based interventions, such as improved natural sunlight (virucidal) and careful control of occupier levels, were cost free in ongoing terms, with indoor ventilation deemed crucial for controlling the risk of infection transmission.

Control measures such as modified ventilation rates increased HVAC system energy consumption during the pandemic [62], mainly due to incoming outdoor air increases using more energy. The outdoor air must be filtered, and thermally and RH-adjusted before being supplied to rooms, increasing energy demand. Increased ventilation rates may help to decrease cooling demand, but increased heating and power demand for driving ventilation fans eventually increases total energy requirements. US-EPA figures for increasing outdoor airflow rate from 2.5 L/(s/person) to 10 L/(s/person) indicate an increased operational HVAC cost of between 2% and 10%.

Portable air-cleaning devices are often seen as appealing interventional choices, as they incur moderate initial investment and offer lower energy demands compared to standard ventilation systems [13,18]. However, any initial cost savings and energy benefits should be considered alongside variable efficiency, ongoing maintenance expenses such as filter changes, and waste produced from filter disposal. With increasing awareness of energy efficiency it is important to achieve a balance between energy-efficient designs and high IAQ standards, thus providing comfortable indoor environments and promoting occupant well-being. Published literature has not thoroughly explored the cost-effectiveness of various integrated interventions and combinations [13].

Following a review of 28 indoor air-cleaning studies, few mentioned the cost of the technologies themselves or of their ongoing maintenance [92]. However, the installation costs of a germicidal UV system for an office building housing 1000 employees was estimated at USD 52,000 for installation and USD 14,000 in annual running costs (electricity and replacement UV lamps). This equated to an investment of approximately USD 52 and annual running costs of USD 14 per staff member employee. For HEPA filtration devices in private homes, installation of two portable units’ costs ~USD 900 each, with annual running costs of USD 500. For comparison, installation of similar air cleaners in a healthcare setting cost between USD 200 and USD 400 per installed unit. Some researchers assumed that HEPA device filters required changing only once a year, but others changed the filters during a 12-week study. The considerable investments, especially at office scale, underlined the importance of authors publishing data on the implementation, operational and energy efficiency costs of the systems described in their evaluations.

Ventilation standards have been described as prioritizing the energy consumption perspective, with interventions that improve the airtightness of dwellings also reducing uncontrolled ventilation [61]. This can increase indoor generated pollutant concentrations and household energy efficiency standards and regulations must therefore ensure adequate ventilation to avoid adverse IAQ impact. Hybrid ventilation systems combine natural and mechanical ventilation, providing indoor comfort, and can help to keep energy costs low.

A pre-pandemic study [51] discovered limited evidence regarding the effects of enhanced energy efficiency measures on IAQ and respiratory health, but noted that adequate ventilation for managing IAQ is governed by the Building Regulations Approved Document. A subsequent paper [6] discussed a related UK Policy aimed at reducing household carbon footprint and fuel poverty. This involved upgrading heating systems, insulation and reducing ventilation rates to prevent heat loss. The authors expressed concerns about how this might impact on occupants’ respiratory health due to increased indoor dampness, which they reported affected 16% of European dwellings.

A study of indoor room temperature (IRT) control and its impact on the health and performance of workers in office environments found that the optimal IRT range depends on the specific region, because people are acclimatized to different regional conditions [106]. A narrow optimum range of IRT in building thermal control and performance may therefore be required, often resulting in high energy consumption in buildings. Using ventilation effectively was found to enhance IAQ, as reported by the WHO 2010 guidance on IAQ. Maintaining a cost–benefit balance was regarded as crucial for effective management of such controls systems.

The application of an innovative internal insulation system using calcium silicate sheets was investigated [107]. This was to prevent water vapor condensation and to limit mold growth in the occupied basement of an historic (museum) building. Evaluations of retrofitted and non-retrofitted spaces focused on thermal comfort, IAQ, acoustic comfort, and aesthetics. Winter thermal comfort tests and a questionnaire indicated a 20% improved user satisfaction with treated spaces, compared to untreated ones. Treated areas exhibited lower surface RH, less mold growth (including in wall cavities) and diminished moldy odor.

4. Discussion

This review applied an evidence-based approach to assess published literature on the topic of indoor microbiological contamination. Well-conducted studies, including evidence reviews, were used where available to address four review questions, with further supporting evidence from other published studies and standards and regulatory sources. This was not a full systematic review, but the use of well-defined review questions, agreed search terms, standardized data extraction, and quality and relevance assessments, ensured the process was aligned to a systematic approach.

The findings underline the importance of ventilation for all aspects of IAQ. Whether mechanical or natural, ventilation allows air exchange that dilutes indoor air pollutants and reduces moisture levels. This can lower bioaerosol levels and related allergenic and infectious microorganisms and their toxins, in turn limiting the deposition of organic aerosol components and associated microorganisms. Sub-optimal ventilation in any building can potentially allow biological pollutants to accumulate, increasing the likelihood of harmful exposures and ill-health effects for occupants.

In the UK, it is less usual to have residential mechanical ventilation, but it is commonplace in larger public buildings and non-industrial workplaces. For these larger environments, the ventilation system design, air exchange rate and ongoing maintenance programs (including air filtration specification), are crucial in optimizing IAQ and providing clean air to building occupants. This is particularly important for complex HVAC systems, which can enhance IAQ, but may be detrimental if poorly managed. Good levels of UK domestic ventilation generally rely on effective building design to provide appropriate air exchange for areas of the home that generate the greatest moisture levels, such as bathroom, kitchen and utility areas.

The importance of designed-in ventilation for private dwellings, such as trickle ventilation in windows, extractor fans in bathrooms and passive air exchange mechanisms around the home, is particularly important in climates where colder weather may deter people from opening windows and doors to promote air exchange. Thermal comfort provision, balanced with building heating costs, must ensure that energy conservation does not negatively impact IAQ. A positive indicator is when those on limited budgets can afford well-insulated homes that are effectively heated and enjoy good IAQ, even in cooler months. Effective build design, including retro-fitted home improvements, is therefore essential and building occupants should understand how their behaviors can influence IAQ issues.

Regardless of geographical location, published evidence repeatedly demonstrates that inadequate ventilation contributes to elevated indoor moisture levels, increasing mold colonization risk. When poor ventilation combines with other factors, like water ingress, mold issues worsen and can adversely affect residents’ health.

Despite the benefits of effective ventilation and the importance of related factors that affect IAQ and the wider microbiome of buildings, few countries have legally enforceable thresholds for indoor biological agents. Where present, these specify levels at which bioaerosols or defined areas of surface bio-contamination become legally intolerable. In England, design requirements specified in the Building Regulations Approved Documents serve to guide developers toward providing building designs that will prevent dampness or related IAQ problems. A microbiological threshold as an indicator of things going wrong in a building is not specified. Indoor dampness is certainly a key risk factor, and its avoidance should be central to the design and longevity of new buildings.

The reasons why microorganisms colonize a building, the health risks they pose, and whether they can be measured reliably make setting an IAQ threshold for biohazards challenging. This is despite adverse health outcomes from exposures being supported by published studies. The presented evidence demonstrates that problem buildings can expose their occupants to poor IAQ and surface bio-contaminants, with people affected in different ways. This includes respiratory irritancy, inflammation, and allergy from exposure to biohazards. Infections can also occur, particularly in densely populated indoor environments, or during infectious disease outbreaks.

When considering possible improvements to building stock in England, the wider UK, and elsewhere, it is helpful to identify knowledge gaps from the evidential findings, since these often identify pathways for future investigation. Addressing these might help to improve health outcomes and in this review, as knowledge gaps were captured during data extraction, with some key topics presented below. A larger list of all knowledge gaps from reviewed papers is available in Supplementary Table S7. In addition to a number of studies already described in this review, others provided additional focus on colonization/contamination mechanisms, including vulnerable materials and fomites [108,109], airborne transfer [110], mycotoxin emissions [111] and microbes integrated in to building materials and furnishings [112,113]. [Fundamental to this is the measurement of such contaminants and the inconsistencies reported in analytical approaches [114,115].

In summary, areas of research that might help to mitigate indoor microbiological hazards are likely to require multi-disciplinary approaches to address the following:

1.

Larger scale studies looking at building type/ventilation/resident behaviors and their effect on indoor moisture and mold contamination [6,65] **. Such studies could be combined with home awareness initiatives to inform residents about the best strategies to improve home ventilation and how this can, for example, quickly reduce indoor moisture levels.

2.

Characterization of microbiological exposures (mold and bacteria) and their adverse respiratory health effects [1,8,18,19,20,22,28,29]. This would have to be undertaken on a statistically robust scale, with real residential or workplace cohorts and appropriate control settings, ideally to include expert IAQ measurements and other building observations, rather than relying on less objective questionnaire studies.

a.

These studies would ideally use a combination of culture and DNA-based methods to maximize bioaerosol data gathering and to minimize the detection bias associated with any single analytical approach. It was recognized in this review, by virtue of the authors’ experience in bioaerosol sampling techniques, that some analytical methodologies had limitations, as also described in a research paper [116] and a review [117]. This was factored into this review and its paper selection in terms of the quality of the papers selected for inclusion, i.e., by rejecting papers where there were recognized inadequacies in sampling methods or their description. Those selected for inclusion were considered to provide valid data within the methodological variability, as acknowledged above.

3.

Assessing the impact of seasonal changes on the levels of indoor bio-pollutants and whether this situation is shifting with climate change [23,25,78]

a.

Can the design of our homes and workplaces control for such effects?

4.

Do standard design features in modern residential buildings support the effective control of airborne infectious microorganisms, e.g., under isolation conditions such as those recommended during COVID-19? [13,35,36,44,46,66,71].

a.

For example, is sufficient spacing possible in smaller properties to limit the spread of infection agents by staying in one naturally ventilated room?

b.

And is this approach effective at diluting indoor air without spreading contaminants?

** Such studies would be made more complex by including assessments of cause and effect on residents’ health. The evidence is already strong for these associations, so a greater focus on simple interventions could be the focus.

5. Conclusions

Understanding the extent and level of harm caused by exposure to indoor microorganisms is complex. It is evident that indoor microbiological pollutants are linked to conditions like asthma, but the causes and health effects may be multifaceted and involve triggers [64]. Interrogating the impact of indoor microorganisms therefore requires an understanding of the types and levels of exposure in specific situations. This may involve performing studies over extended time periods, using large cohorts and effective control populations to obtain statistically robust data. Interventional studies, such as those concerned with building improvements, must consider ventilation controls or other mechanisms intended to limit indoor moisture and associated microbiological contamination. Overall, these study requirements are challenging to satisfy when they often necessarily involve people’s homes or workplaces. One approach to address this is to use self-reporting questionnaire studies to collect multiple data on health and environment, from study populations.

When disease causation is linked to indoor microbiological exposure, any conclusions may be based on exposures to a range of biological hazards. Data are often qualitative, rather than specifying threshold levels for the microorganism detected and study methodology, including microbiological sampling and analysis approaches, and they can influence these measurements. In addition, the types and levels of microorganisms may only be suggestive of human exposure, being based on background measurements (in air samples), because few studies outside of the occupational context include air sampling to assess an individual’s personal exposures.

Published research shows substantial variation in reported exposure levels, influenced by factors such as the context of exposures. Additionally, health outcomes are shaped by an individual’s susceptibility to the particular agent, whether that be infectious [93] or allergenic [105]. Allergic disease conditions may take time to develop, often after multiple exposures to certain types and levels of microorganisms. Conversely, infections can occur via the airborne or surface transmission route after a single low-level exposure to a particular pathogen. Such infections depend on the infectious dose of the pathogen and susceptibility of the human host [5,44,106]. The concentrations at which microorganisms become harmful may therefore occur across a range of exposure levels, rather than at well-defined thresholds.

In view of the above, establishing threshold IAQ concentrations for hazardous microorganisms remains challenging, particularly given the variable likelihood of human infection or sensitization. This situation is further exacerbated by the mixtures of biological and chemical hazards and exposure scenarios in different indoor settings [107]. However, an awareness of indoor conditions that can lead to such exposures, and a more widespread understanding of appropriate controls that can mitigate them, would almost certainly improve the health of building occupants.