Improving Indoor Air Quality through Standardization

: Human beings experience a large fraction of their exposure to air pollutants in indoor environments. Air pollution is a large environmental health risk, and exposure to ambient air pollution and indoor air pollution contribute equally to the total number of fatalities worldwide. Although legislative authorities have established limit values for ambient outdoor air and stack emissions, there are inconsistent and variable national and regional limit values for gaseous substances and airborne particulate matter in the built environment (schools, homes, healthcare facilities, ofﬁces, and other public spaces). This lack of regulation is unsurprising, because indoor spaces are characterized by complex air chemistry, and their construction materials and types of activities vary signiﬁcantly. The current understanding of indoor pollutants, including short-lived oxidants, degradation of VOCs, particle formation, and particle composition, is incomplete. It is necessary to identify and assess emerging pollutants and their toxicity, and to consider new consumer products and green construction materials and their impact on indoor air quality (IAQ). Learning from IAQ surveys and audit protocols, research methodologies should be regularized for cross-research comparisons. Some indoor air quality guidance and standards have been written, and several more are in development, with the international ISO 16000 series of indoor standards leading the way for improving indoor air data quality. The WHO has established some ambient air limit values which can mostly be translated into indoor limit values. The built environment needs to harmonize energy efﬁciency, thermal comfort and air quality standards and guidance. In this review, we discuss the next steps for improving international, regional and national standards and guidance, leading to better and more complete indoor air quality regulations.


Introduction
In developed countries, people spend a large fraction of the day in indoor environments [1][2][3][4], with two important consequences: Indoor air quality (IAQ) directly affects human health and wellbeing, with airborne particulate matter (PM), oxidizing gases (ozone, nitrogen dioxide, chlorine) and volatile organic compounds (VOCs) all causing respiratory, pulmonary, and neural damage [5].The resulting burden on national healthcare systems is very significant [6].The construction, usage, renovation and demolition of buildings account for 40% of the total energy consumption [7,8]; therefore, reducing building energy requirements is a climate change priority.
A large body of scientific studies has shown that exposure to indoor air pollutants has adverse effects on the human health.The known health impacts are reduced cognitive function, heart and respiratory symptoms, stroke, chronic obstructive pulmonary disease and lung cancer [9][10][11].Requirements for future air quality management approaches must tackle the individual's exposure to airborne pollutants [12].An individual's exposure strongly depends on temporal and spatial pollutant concentrations, fluctuations in pollutant emission patterns, the characteristics of the built environment, and human behavior [13,14].Protecting human health and also conserving energy are two conflicting priorities; good ventilation is pursued for better air quality, but energy savings are also pursued through sealed buildings and the reduced performance of ventilation systems.
Society attempts to resolve such conflicts through legislation and standardization procedures.Governments set the maximum allowed pollutant concentrations and establish building regulations, mainly for ventilation; however, ensuring that these regulations are followed requires guidance documents (i.e., technical specifications and standards).Despite the fact that a comprehensive amount of scientific insight into the physico-chemical features of pollution in the built environment (and ventilation) is available, practical application of this scientific information via guidance documents to help building designers, building operators, and facility managers is incomplete or missing.
In this review, we assess the body of normative documents on IAQ, and how these documents are elaborated, either through legislation or standardization processes (Section 2).The selection criteria for normative documents of global recognition presented in this review are detailed in Section 3. The review results of documents from legislation processes and from standardization processes are presented in Sections 4 and 5, respectively.Finally, the review results are discussed (Section 6), and conclusions are drawn (Section 7).

The Body of Normative Documents on IAQ
Globally, many nations have enforced ambient air quality standards with legally binding force as a means to protect public health.In several countries, these ambient air quality standards are oriented towards the values proposed in the 'WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide, and sulfur dioxide'.The latest issue of the 'WHO air quality guidelines' is, for the first time, applicable to both ambient and indoor air [5].Even though IAQ is of paramount importance for the health of the occupants of the built environment, there are no legally binding IAQ guidance documents.This situation represents a stark contrast to the legally binding character of ambient air quality guidance documents.The constitutional basis and legislation must cover all fields of law, and thus cannot focus on the technical details necessary to measure, assess, and improve air quality.Therefore, legislation and regulations are supported by administrative provisions, assuring the equal treatment of relevant persons by the executing administration.Following administrative provisions, technical specifications and standards provide practical guidance for people needing to fulfill the legal requirements laid down by regulations.This construction of the 'legal pyramid' allows for increasing attention to detail via technical specifications and standards.It is also easier to revise technical specifications and standards than regulations (see Figure 1).
Standards 2023, 3, FOR PEER REVIEW 2 strongly depends on temporal and spatial pollutant concentrations, fluctuations in pollutant emission patterns, the characteristics of the built environment, and human behavior [13,14].Protecting human health and also conserving energy are two conflicting priorities; good ventilation is pursued for better air quality, but energy savings are also pursued through sealed buildings and the reduced performance of ventilation systems.Society attempts to resolve such conflicts through legislation and standardization procedures.Governments set the maximum allowed pollutant concentrations and establish building regulations, mainly for ventilation; however, ensuring that these regulations are followed requires guidance documents (i.e., technical specifications and standards).Despite the fact that a comprehensive amount of scientific insight into the physico-chemical features of pollution in the built environment (and ventilation) is available, practical application of this scientific information via guidance documents to help building designers, building operators, and facility managers is incomplete or missing.
In this review, we assess the body of normative documents on IAQ, and how these documents are elaborated, either through legislation or standardization processes (Section 2).The selection criteria for normative documents of global recognition presented in this review are detailed in Section 3. The review results of documents from legislation processes and from standardization processes are presented in Sections 4 and 5, respectively.Finally, the review results are discussed (Section 6), and conclusions are drawn (Section 7).

The Body of Normative Documents on IAQ
Globally, many nations have enforced ambient air quality standards with legally binding force as a means to protect public health.In several countries, these ambient air quality standards are oriented towards the values proposed in the 'WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide, and sulfur dioxide'.The latest issue of the 'WHO air quality guidelines' is, for the first time, applicable to both ambient and indoor air [5].Even though IAQ is of paramount importance for the health of the occupants of the built environment, there are no legally binding IAQ guidance documents.This situation represents a stark contrast to the legally binding character of ambient air quality guidance documents.The constitutional basis and legislation must cover all fields of law, and thus cannot focus on the technical details necessary to measure, assess, and improve air quality.Therefore, legislation and regulations are supported by administrative provisions, assuring the equal treatment of relevant persons by the executing administration.Following administrative provisions, technical specifications and standards provide practical guidance for people needing to fulfill the legal requirements laid down by regulations.This construction of the 'legal pyramid' allows for increasing attention to detail via technical specifications and standards.It is also easier to revise technical specifications and standards than regulations (see Figure 1).The normative documents reviewed in this work result either from legislative processes (regulations) or from standardization processes (guidance documents comprising technical specifications and (pre-)standards).
Legislation comprises the exercise of power and the function of making rules (including laws) that have the force of authority by virtue of their promulgation by an official government body.
Standardization comprises all systematic activities executed by stakeholders with the overall aim to harmonize (im)material objects.These systematic activities can be at the national, regional or international level.A recognized Standards Developing Organization (SDO) uses dedicated processes to identify and involve stakeholders in the consensusbuilding process amongst all interested parties.Clear documentation of these processes by the recognized SDO and communication of them to the general public are mandatory.Furthermore, the FRAND criteria (fair, reasonable and non-discriminatory treatment of all stakeholder interests) have to be fulfilled in all standardization sub-processes.The consensus-building process assures general consent among all stakeholders involved in standardization.Finally, consent is characterized by the absence of implicit and explicit objection to the essential content.For this reason, a consensus-building process considers all aspects of the stakeholders, and tries to clear out all counter-arguments.Consensus does not necessarily mean unanimity [15].Unlike peer-reviewed research articles which express the view of one or a few authors, guidance documents state the agreed view of a plurality of experts.

Methodology
This work focuses on the standardization process and analyzes how standardization is influenced by legislation, and how standardization can drive legislation on a national, regional and international level.
In the following, three essential types of normative documents are considered for determining the requirements to monitor and improve IAQ, namely in regulations, standards and pre-standards, and technical specifications for the management of IAQ.
A regulation is a normative government-approved act with general application, which is addressed to abstract categories of persons.Regulations are legally binding in their entirety; they may not be applied incompletely, selectively, or partially, and they are directly applicable.For indoor air quality, regulations address fundamental key questions oriented towards general environmental protection approaches (the polluter pays principle, principle of sustainable development, principle of preventive action, principle of cooperation, principle of subsidiarity).A standard is a normative document published by an SDO.It results from a standardization activity hosted by an SDO on a national, regional or international basis.In this standardization activity, a full consensus amongst all stakeholders must be achieved via the consensus-building process.The resulting normative document provides specifications for general and/or recurrent activities, or for the results (immaterial and/or material) of these activities.Trusted scientific and technical results are the knowledge basis of standards (i.e., the state of the art).The overarching objective of a standard is to create social benefits.Standards make things work by providing specifications (guidance or requirements) for products, services and systems.Importantly, adherence to a standard is voluntary (unless laid down by a regulation or required by a business contract), but meeting the requirements of a standard does not confirm that legal requirements have been met [15,16].Standards are elaborated upon by SDOs with full membership in the International Organization for Standardization (ISO), and with corresponding mandates.A pre-standard is a normative document published by an SDO.It results from a standardization activity hosted by an SDO on a national, regional or international basis.In this standardization activity, a partial consensus is reached amongst a limited number of stakeholders.A pre-standard specifies recommendations for possible products, processes or services.Rapidly evolving and newly emerging scientific and technical results form the knowledge basis of pre-standards.Certain objections about the content might lead to the situation in which a pre-standard will not be published as a standard.The intention of applying a pre-standard is to gain experience, which can then help to create a standard.Some pre-standards documents have a limited lifetime.Following a period of review by all members of the governing body of the SDO, a pre-standard can be converted into a standard, if approved, and after any agreed modifications by the SDO [15,16].Technical specifications expand the law.Technical specifications are normative documents that have not been fully ratified by the SDO, but are still useful, and can be used as guidance for regulators.They may be statutory (and must be followed) or not statutory; however, if they are not followed, a person or organization may still be liable to prosecution.Technical specifications and, frequently, annexes in standards provide guidance.In contrast to standards, technical specifications are not fully ratified by an SDO who has full ISO membership and corresponding mandates.
Most IAQ regulations, (pre-)standards, and technical specifications result in either emission reduction requirements ( exposure-driven approach) or ventilation requirements ( ventilation-driven approach), which are two different approaches for tackling the management of IAQ (see Table 1).Legally binding requirements for permitted concentrations of airborne pollutants, pollutant emission rates and ventilation rates.
(Pre-)Standards Technical Specifications Ventilation-related aspects (mechanical/non-mechanical ventilation, outdoor air quality) Activity-related aspects (e.g., basal metabolic rate (BMR), physical activity ratio (PAR), activity-specific emissions) Occupancy-related aspects (e.g., number of occupants, profiles of occupants, time of occupancy) Material-related aspects (e.g., pollution sources, construction materials, furnishing emissions) Guidance on how to fulfill legally binding requirements with standardized sampling and analysis, harmonized data quality objectives, standardized IAQ surveys, and consideration of specific and general building characteristics.
The data presented in this review on IAQ-relevant regulations were retrieved from the databases of the International Society of Indoor Air Quality and Climate (ISIAQ), the World Health Organization (WHO), the United States Environmental Protection Agency (EPA), and the Occupational Safety and Health Administration (OSHA) of the United States Department of Labor (Section 4).Additionally, IAQ-relevant guidance documents retrieved from the databases of the ISO, European Committee for Standardization (CEN), American National Standards Institute (ANSI), American Society for Testing and Materials (ASTM), American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), American Conference of Governmental Industrial Hygienists (ACGIH), and VDI/DIN (Verein Deutscher Ingenieure/Deutsches Institut für Normung) Commission on Air Pollution Prevention (Section 5) were analyzed to identify priority areas for future legislation and standardization and for future research directions.The scope of the legislative and resultant SDO documents covers both exposure-driven and ventilation-driven approaches.Generally, the reviewed documents pursue the overarching aim of monitoring and reducing indoor airborne pollutants to improve wellbeing, while also optimizing building ventilation.This paper concludes with a discussion of how legislation drives standardization, with recommendations on how to drive legislation through standardization, and identifying priority areas for future standardization (Section 6).
The scope of the reviewed normative documents comprises the built environment, specifically residential spaces, offices and workspaces, hospitals, schools, kindergartens, sports halls, restaurants and bars, theatres, cinemas and other public spaces.Workrooms and workplaces, which are already subject to national and regional regulations for health and safety inspections, are not considered in this review.This study focuses on normative documents published by international, European, and North American SDOs, and does not claim to be complete.This study has selected normative documents from these SDOs that have achieved global recognition: IAQ-relevant regulations reviewed by ISIAQ, subsequently prioritized and finally compared with guidance documents from WHO, EPA, USA Federal government rules, published in the Federal Code Register (eFCR) [17].and OSHA (Section 4).
• Parameters for the assessment of IAQ were identified and prioritized based on the Indoor Environmental Quality Guidelines Database of ISIAQ.This database on global indoor environmental quality guidelines is updated continuously.The IAQ parameters from the ISIAQ database are further compared with recommendations for policy-makers provided by the WHO.The WHO, through the guidelines review committee, ensures that WHO guidelines are based on the best available evidence and are consistent with internationally accepted best practices.Finally, the IAQ parameters are also complemented with recommendations from the US EPA and OSHA.
Globally recognized IAQ-relevant guidance documents, standards and technical specifications retrieved from the databases of ISO, CEN, ANSI, ASTM, ASHRAE, ACGIH, and VDI/DIN (Section 5).
• Citizen demands on consumer product manufacturers are driven by health effects related to IAQ.This is reflected by numerous product labels for 'low-emission' products used for marketing purposes.In a review performed by ISIAQ, the government guidelines and regulations on different IAQ parameters of 35 nations were compiled.The results are reported in Figure 2 for each individual IAQ parameter.
It is clear that temperature plus formaldehyde, carbon dioxide, carbon monoxide, radon and ozone concentrations are the dominantly legislated IAQ parameters.Each of these parameters are subject to government guidelines or government regulations in more than 50% of the countries considered in the review [18].Based on the key questions (the scope of normative documents) defined in Table 1, IAQ parameters are grouped into different categories with respect to their scope, and in order of decreasing occurrence in Table 2.This list includes specific VOCs and bioaerosols which are not regularly monitored.It is clear that temperature plus formaldehyde, carbon dioxide, carbon monoxide, radon and ozone concentrations are the dominantly legislated IAQ parameters.Each of these parameters are subject to government guidelines or government regulations in more than 50% of the countries considered in the review [18].
Based on the key questions (the scope of normative documents) defined in Table 1, IAQ parameters are grouped into different categories with respect to their scope, and in order of decreasing occurrence in Table 2.This list includes specific VOCs and bioaerosols which are not regularly monitored.
The list of priority parameters based on the ISIAQ review shown in Table 2 comprises all parameters for the assessment of ambient and indoor air quality proposed in the 'WHO global air quality guidelines' (PM 2.5 , PM 10 , O 3 , NO 2 , CO), except for SO 2 .Dealing primarily with ambient outdoor air, the guidance limit values for particles and NO 2 were reduced in the latest 'WHO global air quality guidelines' [5].Black carbon (BC), ultrafine particles (UFP), and sand and dust storms (SDS) were highlighted as important pollutants, but adequate data to set limit values were lacking.The 2010 'WHO guidelines for indoor air quality: selected pollutants', which focuses on IAQ, has highlighted specific VOCs including naphthalene, trichloroethylene, tetrachloroethylene and formaldehyde [19].
Asbestos, biological pollutants, CO, formaldehyde, NO 2 , Rn, PM, and VOCs are listed by EPA as indoor pollutants, and are also covered by the ISIAQ ranking.In addition to the ISIAQ parameters, EPA also includes Pb, pesticides, environmental tobacco smoke (ETS), and wood smoke in the priority list of IAQ parameters [20].
The OSHA parameters for the assessment of IAQ (CO, CO 2 , Rn, biological contaminants, and VOCs) are covered by the ISIAQ ranking, with the exceptions of pesticides, Legionella, and dampness [21].

Health Classifications
Different approaches are used when evaluating the impact of IAQ on health.Regulations and resulting guide values are usually based on either epidemiological or toxicological data.The derived guide values can lead to emission reductions and/or ventilation requirements.Guide values should be and are frequently accepted as the limit values in regulations.
It is necessary to distinguish between the four types of guide values [22]: Hygienically justified guide values are based on the observation that the probability of health problems increases with increasing concentration.Some examples are guide values for TVOC and CO 2 .
Toxicologically justified guide values are based on dose descriptors.Dose descriptors facilitate the identification of the interrelation between a specific effect of a substance and the dose at which the effect is observed.Such dose descriptors are the basis for deriving the so-called no-effect threshold levels for human health (i.e., the derived no-effect level (DNEL) or reference dose (RfD)) and the environment (the predicted no-effect concentration (PNEC)).Toxicological studies are the knowledge base of dose descriptors.Typically, they are expressed as the lethal concentration (LC), lethal dose (LD), no-observed-adverse-effect Level (NOAEL), and lowest-observed-adverse-effect Level (LOAEL).They are used for hazard classification and risk assessment.Risk-related guide values are consulted for the health assessment of carcinogenic pollutants in indoor air, and for the specification for genotoxic carcinogens with the most sensitive endpoint.
Odor guide values are used to report the concentration of a substance perceived by at least half of the test persons.One level of classification differentiates between a noticeable odor and a significantly annoying odor.

IAQ-Relevant Guidance Documents
Limit values for air quality in some workplaces are legally enforced.Unfortunately, although there are guidance documents for some pollutants and minimum fresh air rates, there are relatively few national or regional limit values for gaseous substances and airborne particulate matter that have broad legally binding force.Currently, the air quality in the built environment is mostly monitored and enforced by building and ventilation regulations.Owing to the significant health impacts of IAQ, related research has accelerated during the last two decades.This can be seen in the increasing number of scientific publications and international standards [23].
The Appendix A lists guidance documents from several SDOs.These normative documents are separated into 14 categories, defined by their target pollutant or purpose.Each category is discussed below.

Inorganic Gases
The guidance documents for specific pollutant gases, which either define the test method or the method of evaluating and validating the analyzers, are as follows.
Nitrogen dioxide can be measured using wet chemistry.ASTM D1607 and ISO 6768 detail the Griess-Saltzman reaction, while OSHA Method ID-214 and EN 16339 explain extraction and analysis using ion chromatography.Chemiluminescence is the reference method, as detailed in EN 14211.ISO 16000-15 gives guidance for a NO 2 sampling plan.Cavity-attenuated phase shift (CAPS) is rapidly becoming an accepted alternative technology for NO 2 measurements according to the federal equivalent method (FEM) or the federal reference method (FRM).
Ozone analyzers are calibrated using gas titration (ISO 15337) or using a transfer standard (ASTM D5011).Ozone is measured using the ultraviolet photometric method, as described in ISO 13964, EN 14625 and ASTM D5156.An alternative continuous method, ASTM D5149, uses ethylene chemiluminescence to measure ozone.
The carbon monoxide reference method is non-dispersive infrared spectroscopy (NDIR), as specified in EN 14626 and ASTM D3162.ASTM D6332 explains how to measure the response time of carbon monoxide alarms.EN 50291 and EN 50292 provide guidance for the validation, installation, use and maintenance of CO alarms.
Carbon dioxide concentration is measured using either NDIR, the reference method, or cavity ringdown spectroscopy (CRDS).ASTM is drafting WK74360, a standard for validating CO 2 monitors.The sampling strategy for CO 2 measurements is specified by ISO 16000-26, and the use of CO 2 as a tracer gas is explained in ISO 12569.EN 50543 describes the use of portable CO and CO 2 instrumentation.
Sulfur dioxide is measured either using ultraviolet fluorescence, EN 14212, or using wet chemistry, as described in ASTM D2914-15.
Ammonia is measured using a diffusive sampler; EN 17346 is the reference method for measuring NH 3 .
Hydrogen sulfide can be continuously determined with a lower detection limit of 1 ppbv, using the change in reflectance, as explained in ASTM D4323.
References to these documents are listed in the Appendix A, Table A1.

Formaldehyde
Planning for formaldehyde concentration measurements in indoor air is explained in ISO 16000-2.Specifications for the analysis of formaldehyde emissions and the sorption characteristics of materials (and the products for their indoor application) are given in ASTM D6007-14, ISO 16000-23, and ISO 16000-24, respectively.The analytical procedures are defined in dedicated normative documents for the determination of formaldehyde (and other carbonyl compounds) using active (ISO 16000-3 and ASTM D5197-16) and diffusive (ISO 16000-4) samplers with solvent desorption and high-performance liquid chromatography (HPLC).These analytical procedures have been validated for the quantification of a range of aldehydes and ketones.
References to these documents are listed in the Appendix A, Table A2.

VOCs
Organic compounds eluting from n-hexane up to and including n-hexadecane on a gas chromatographic column (specified as a 5% phenyl 95% methyl polysiloxane phase capillary gas chromatographic column) are considered to be volatile organic compounds (VOCs: ISO 16000-6).Guidance documents specify sampling strategies as well as analytical methods for the determination of VOCs in indoor air, and in air sampled from test chambers (ISO 16000-5, ISO 16000-6, ASTM D7339-18, ASTM D6330).These methods are based on sorbent sampling tubes with subsequent thermal desorption and gas chromatographic analysis.Several guidance documents are also available for diffusive sampling techniques for VOCs (ISO 16017-1, -2, ASTM D3609, ASTM D4298, EN 14412), detector tubes (ASTM D4490), and canister sampling (EPA Method TO-15, ASTM D5466, ASTM D8283-19).Additional test procedures are described for ambient air measurements of gases and vapors (EN 13528-1, -2, -3) and benzene (EN 14662-1, -2, -4, -5).Complementary support for the determination of analytical parameters, for test methods for VOC detectors, for the determination of VOC emission profiles, and for the subsequent exposure risk characterization are given in ASTM D6196, ISO 16000-29, ASTM D6177-19, and ASTM D6485-18, respectively.Emissions from bedding sets can be measured by following ASTM D6485-18 and D6177-19.
References to these documents are listed in the Appendix A, Table A3.

Mould
Inhaling spores and metabolic products of mold can trigger allergic and/or irritating symptoms in humans.In addition, the growth of mold can lead to strong odor impairments.In rare cases, some types of mold can also trigger infections in certain at-risk groups.
Epidemiological studies have shown that damp and moldy buildings increase the risk of respiratory symptoms, respiratory infections and asthma symptoms in residents.Therefore, mold growth in buildings should be avoided.ASTM D7338 specifies procedures to assess fungal growth in buildings, whereas ASTM D6329 specifies the evaluation procedure of indoor materials to support microbial growth.The standard practice for collection of total airborne fungal structures via inertial impaction is described in ASTM D7788.The standards of the ISO 16000 series provide support for the detection and enumeration of mold sources in indoor areas using sampling by filtration (ISO 16000-16), a culturebased method (ISO 16000-17), sampling by impaction (ISO 16000-18), sampling strategy (ISO 16000-19), the determination of total spore count (ISO 16000-20), and sampling from materials (ISO 16000-21).The enzyme activity method (ISO 16000-22) and the total cell count method for building materials (ISO 16000-35) are under development, and ISO 16000-19 specifies the sampling strategy.Additional test procedures are described for ambient air measurements of airborne pollen grains and fungal spores (EN 16868) and molds (EN 16115-1).The sampling and analysis of bioaerosols are defined in CEN/TS 16115.
References to these documents are listed in the Appendix A, Table A4.

Particulate Matter
Oxidation of VOCs in the presence of nitrogen oxides (NO x ; corresponds to mostly the sum of NO and NO 2 ) leads to the formation of OVOCs (oxygenated volatile organic compounds) and oxidants such as O 3 .Compared to their organic precursors, OVOCs have a lower vapor pressure and increased water solubility.Hence, they can deposit on surfaces, especially in indoor environments with high humidity (e.g., bathrooms, kitchens), wherein the surfaces can be contaminated with OVOCs.They can also condense by forming secondary organic aerosols (SOA).Many VOCs and OVOCs are toxic or carcinogenic, and can cause diseases of the respiratory tract and the cardiovascular system.Furthermore, airborne particles have adverse health effects at exposure levels determined in urban regions, wherein limit values for PM 2.5 and PM 10 have been set.Test procedures for airborne particles in indoor environments address general sampling strategies (ISO 16000-34), the PM 2.5 -fraction (ISO 16000-37), and the particle number concentration (PN or PNC: ISO 16000-42).Complementary guidance documents have been developed for ambient air measurements (EN 12341, CEN/TS 16976, CEN/TS 17434).ASTM D8405-21 specifies how to validate PM 2.5 particle monitors.Standards and guidance for ultrafine particles are being drafted by ISO, ASTM and CEN working groups.
References to these documents are listed in the Appendix A, Table A5.

Other Pollutants
Various materials continuously emit trace substances into the indoor air.These can be contaminated surfaces such as wood preservatives and asbestos.Alternatively, newly introduced building materials and synthetic products are some of the strongest emitters of organic chemicals, and some of these are also nuisance odor sources.Since today's consumer behavior is determined by a pronounced awareness of health, manufacturers of consumer products brand their products as so-called 'low-emission' products, as a marketing tool.Despite the fact that most organic chemicals occur in very low concentrations, public interest in low-emission products has increased, as some of the identified organic compounds are irritating or even toxic, and in some cases have high odor intensities.These issues are tackled by dedicated guidance documents for the measurement of asbestos (ISO 16000-7, ISO 16000-27), flame retardants and plasticizers (ISO 16000-31, ISO 16000-12, -13, -14), phthalates (ISO 16000-33), amines (ISO 16000-38, ISO 16000-39), wood preservatives (ISO 16000-12, -13, -14), pesticides (ASTM D6333-17), radon (ASTM D6327-10), incomplete combustion products (ISO 16000-12), refrigerant gases (EN 50676), and odors (ISO 16000-28, ISO 16000-30).Additional test procedures are described for ambient air measurements of polycyclic aromatic hydrocarbons (EN 12884, EN 16362, ISO 12884, ASTM D6209), benzo[a]pyrene (EN 15549), and odors (EN 13725, EN 16841-1, -2).A new guidance standard for determination of airborne per-and polyfluorinated alkyl substances (PFAS) is under development by ASTM WK81752.
References to these documents are listed in the Appendix A, Table A6.

Ventilation
Ventilation guidance documents consider a ventilation system's design, control and testing, and performance.The guidance documents listed in the Appendix do not include detailed ventilation equipment requirements.
Most design guidance documents consider specific built environments.ANSI/ASHRAE/ASHE 170 provides guidance for healthcare facilities.ANSI/ASHRAE/IES 90.1 sets standards for energy usage in commercial and high rise buildings in the USA.VDI 2262-3 is the German guidance document for good air quality through ventilation in the workplace; BS 5925 explains the design of natural ventilated buildings in the UK, while BSI-BS PD CR 1752 gives general guidance for ventilation systems.ASHRAE 154 defines the requirements for commercial cooking operations.The ASHRAE handbooks are references for ventilation system design and the corresponding technical requirements.
US federal regulations for ventilation systems can be found in the 'ACGIH industrial ventilation manual'.
The most important sets of ventilation guidance documents are ASHRAE 62.1 and 62.2.ASHRAE 62 has gone through several revisions since 1989, and is the basis of using CO 2 for demand control ventilation, which is the compromise between energy efficiency and good air quality.ASHRAE 62 defines an acceptable level of outside air for good air quality by dilution, while allowing ventilation system turndown when the built environment is unoccupied or has low occupancy.
The surge in using CO 2 monitors to determine adequate ventilation for safer environments started with ASHRAE 62.There are now dozens of commercially available low-cost CO 2 monitors; almost all are based on NDIR optical technology.An undesirable addition to CO 2 monitoring is the use of metal oxide chemiresistors as very low-cost CO 2 surrogates; they are not validated, and there are no standards or guidance documents that support the use of this alternative technology.Unfortunately, a simple CO 2 concentration measurement is not adequate for understanding the functioning of the entire ventilation system without considering other parameters.ASTM D6245, recently rewritten, explains the capabilities and pitfalls of CO 2 measurements when evaluating room ventilation.
Ventilation systems in the USA are tested and balanced following ANSI/ASHRAE 111; in Europe, EN 16798 specifies the energy performance of buildings and its interrelation with indoor air quality, as well as how to measure ventilation flow rates for both mechanically ventilated and passive duct systems.ASTM E741 and ISO 12569 explain the use of tracer gas dilution to determine the air change rate in a single zone.
Ventilation equipment is tested and validated using several standards.Two relevant standards are ISO 16890 for air filters and EN 13141-2 for testing the components of residential ventilation systems.
Properly maintained air filters are critical to good air quality resulting from mechanical ventilation systems.EN 779 and EN 15780 explain the maintenance and performance inspection of air filters.
References to these documents are listed in the Appendix A, Table A7.

IAQ Surveys
IAQ is assured by considering the built environment from several aspects.Guidance documents are written for different purposes, including building design rules for optimum air quality, how to perform an IAQ audit, inspecting and measuring ventilation and air quality, and considering sustainability.
Good air quality begins with good design.ISO 17772-1, ASHRAE 55, and EN 15251 specify the requirements for thermal comfort, indoor air quality, and lighting and acoustics.For residential buildings, ASTM E267 suggests performance statements for both air quality and thermal comfort.The UK building services association, BSRIA, published TG12/2021, a general guide to IAQ.
Air quality surveys are undertaken either periodically as part of the building maintenance schedule, or when air quality problems or sick building syndrome are a concern.The 'EPA walkthrough inspection checklist' provides guidance for an IAQ audit.ASTM D7297 is a suitable standard when air quality problems arise in residential buildings, and ASTM D1357 explains more generally how to organize an IAQ audit.The requirements of a combined energy and air quality audit are specified in ANSI/BPI-1200-S-2017.
The method for performing the specific measurements required for building inspection is detailed for ventilation systems in ANSI/ASHRAE/ACCA 180.When measuring pollutants, ASTM D6306 explains where to locate diffusive samplers.ASTM D6245-18 is an important standard; closely allied with ASHRAE 62, it explains the use of CO 2 monitoring not only to evaluate the ventilation system but also to determine the air change rate (ACH), which is the simplest measurement for ensuring good air quality through dilution with fresh air.
IAQ surveys include examination of the ventilation system.EN 15780 and the UK institution CIBSE's publication KS 17 should be referenced when examining ventilation systems, and ISO 16000-8 when determining the local mean age of air in buildings for characterizing ventilation conditions.General aspects for sampling strategies, investigation of buildings for the occurrence of pollutants, and a management system for IAQ are specified in ISO 16000-1, ISO 16000-32, and ISO 16000-40, respectively.Specifications for the assessment and classification of indoor air quality are currently elaborated upon by ISO 16000-41.Ventilation for schools is specified in the UK in the 'ESFA building bulletin 101', and HSE HSG 258 is specific to workplace ventilation.REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, has published 'guidebook no.11', which defines how to monitor local exhaust ventilation (LEV).
A more inclusive approach to building audits is the 'NSF/ANSI sustainability assessment', which reviews all materials in the building, including flooring, walls, and soft furnishings.Each section of this exhaustive document covers specific pollution sources.
Due to the COVID-19 pandemic, many recent documents have emphasized the need for good ventilation with recommended ACH, based on recent results.These recommendations will change and improve as more data become available.A typical example of these documents is the 'REHVA COVID-19 guidance'.
References to these documents are listed in the Appendix A, Table A8.

Air Cleaners
Air purifiers and air cleaners remove contaminants from indoor air to improve IAQ.These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and claim to reduce or eliminate second-hand tobacco smoke and microbiological contaminants (bacteria, fungi, viruses).Commercially graded air purifiers are either small stand-alone units or larger units that can be integrated into air-handling units or to heating, ventilation and air-conditioning (HVAC) system units found in the medical, industrial, and commercial sectors.Industrial air purifiers can also be used to remove impurities from air before processing; pressure swing adsorbers or other adsorption techniques are typically used for this purpose.Standardized test chamber procedures are available to test the suitability of air purifiers for reducing airborne bacteria (ISO 16000-36) and airborne fungi (ISO 16000-43), as well as increasing the perceived indoor air quality (ISO 16000-44, under development).
The 'EPA ENERGY STAR Program 2.0' sets performance requirements for room air cleaners.
References to these documents are listed in the Appendix A, Table A9.

Test Chambers
Low-emission materials and objects help to ensure that indoor air pollutants and odorous substances are minimized.Interior material testing (e.g., building products) plays a central role; the material selection is often not at the discretion of the room user, but is part of the building construction.A declared aim of European legislation is to protect the health of building users.A specification of this requirement can be found in a basic document drawn up by the scientific committee on health and environmental risks, published by the European Commission, in which the avoidance and limitation of pollutants in indoor areas are explicitly mentioned [24].Adequate requirements of the health compatibility of construction products are determined, thereby enabling reliable product selection.
In addition to indoor air measurements, the characterization of indoor materials and building products can contribute to the development of sustainable materials and products for indoor applications.Emission test procedures are carried out in test chambers/test cells to determine emissions of VOCs (ISO 16000-9, -10, ASTM D5116-17, ASTM D7143-17, ASTM D6803, ANSI/BIFMA M7.1, CDPH/EHLB V1.2 2017, ASTM D7706-17, ASTM D6670-18, ASTM D7706-17), semi-volatile organic compounds (ISO 16000-25), and odors (ISO 16000-28).Specific procedures for the investigation and assessment of the interior air of road vehicles are given in the ISO 12219 series of standards.The test piece is placed in a test chamber and then exposed to defined conditions.Air samples are taken from the test chamber, and are treated in the same way as air samples from indoor environments.In addition to the requirements for test chambers and test cycles (ISO 16000-3, ISO 16000-4, ISO 16000-6, ISO 16000-9, ISO 16000-10), separate standards also specify sample management (ISO 16000-11) and performance testing for sorbent building materials for the reduction of formaldehyde (ISO 16000-23) and VOCs (ISO 16000-24).
European Standard EN 16516 specifies a horizontal reference method for determining the emission of regulated hazardous substances from construction products into indoor air.This method can be used for volatile organic compounds, semi-volatile organic compounds, volatile aldehydes, and ammonia.It is based on the use of a test chamber and subsequent analysis of the trace gases and VOCs.
References to these documents are listed in the Appendix A, Table A10.

Diffusive Samplers
Since diffusive samplers are used almost exclusively for monitoring VOCs, the relevant guidance documents are reviewed in VOC Section 5.3.
References to these documents are listed in the Appendix A, Table A11.

Analytical Methods
EPA 40 details test methods and procedures for testing gases, VOCs and particles.ASTM D3249 is a general guide for using air quality analyzers.
More specifically, ASTM D7911 explains use of reference materials when determining bias error for VOC measurement.ASTM D8141 is the standard to use when determining the area-specific emission rates of VOCs and SVOCs when modelling indoor VOC concentrations.
References to these documents are listed in the Appendix A, Table A12.

Limit Values
Threshold limit values (TLVs, frequently called limit values, or LVs) are the maximum permissible concentrations set for gases, PM and VOCs.These limit values are set by regional, national or local governments, and should be part of legislation, making them legally binding; however, limit values and their enforcement are different for each country.Limit values are set for different averaging periods: a 15 min short term exposure level (STEL), a 1 h STEL, an 8 h time weighted average (TWA), a 24 h TWA or an annual TWA.
The 'WHO global air quality guidelines' historically set the accepted reference limit values for ambient air in most countries.The guidelines were updated in September 2021, with tighter levels for NO 2 , PM 2.5 and VOCs [5].These guidelines are for ambient, not indoor air, but are frequently used as both indoor and outdoor LVs.'WHO guidelines for indoor air quality: selected pollutants' (2010) reviews NO 2 , radon, polycyclic aromatic hydrocarbons (PAHs), naphthalene, trichloroethylene and tetrachloroethylene, with recommended TLVs where possible.
EU Directive 2008/50/EU set the limit values for ambient air in all European countries.This directive is currently under review, and will consider the 2021 'WHO guidelines' when reviewing this directive.
EPA 40 CFR PART 50 sets the legal requirement for ambient air in the USA, and OSHA CFR Part 1910 sets the limit values for workers.These LVs include gases, particles and many VOCs.ASHRAE 55 and ISO 17772 set the limit values for thermal comfort in the built environment, but these do not include gases, particles or VOCs.The USA relies on the OSHA LVs for the built environment, but these exclude residential buildings, which are included in the US EPA Building Air Quality Guide.
There are few limit values for indoor air.Most countries default indoor air LVs to the ambient outdoor air LVs, but these include only a few VOCs, the most common being benzene and formaldehyde.
References to these documents are listed in the Appendix A, Table A13.

Data Quality
Data quality is paramount to indoor air studies.Many algorithms and models are used for IAQ data analysis, and comparison of these different methodologies relies on guidance documents for data analysis.There are many guidance documents for general data analysis, and there are two guidance documents that are specific to air quality.The common use of machine learning (ML) to analyze air quality data does not remove the need for statistical analysis of the ML-corrected data.
Estimation theory, originally proposed by Fisher in 1925, is the basis of the statistics used to analyze air quality data.The accepted single measurement of data quality is expanded uncertainty, and the central document is 'JCGM guide to the expression of uncertainty in measurement (GUM)'.JCGM members include ISO, International Union of Pure and Applied Chemistry (IUPAC), and International Electrotechnical Commission (IEC), as well as other groups.ISO 20988 and ASTM D7440 both detail the use of GUM when applied to air quality data.ASTM D22 work item WK21341 is developing a guidance document for determining uncertainty in VOC measurements.
Detailed statistical analysis is specified in ASTM D5289 and CEN TC264 CR 14377.These two guidance documents cover the level of confidence, confidence intervals, bias, instability and upper and lower limits of detection.ISO 5725-1 and -3 specify the accuracy of measurement methods and results.ASTM D5280 specifies the evaluation of performance characteristics of air quality measurement methods using linear calibration functions.
To ensure clarity, ASTM D1356 details the correct terminology when reporting air quality, and ASTM D1914 defines the correct units and factors.ASTM D5791 explains how to use statistics when planning an IAQ investigation, and ASTM D5157 shows use of statistics to evaluate air quality models.
References to these documents are listed in the Appendix A, Table A14.

How to Select National, Regional or International Standardization
National standards differ between nations due to different views about what needs to be standardized.Standards with national scope are most frequently prepared in conjunction with unique national legislation.Globally, most nations have agreed to a centralized approach on standardization, and implemented a national SDO that is also a full member of ISO.In contrast, several countries follow a decentralized and thus sector-specific approach.The standardization system of the United States is very diverse, with at least 600 nongovernmental SDOs.Germany, with around 150 non-governmental SDOs, is in the middle of the spectrum that ranges from diverse to centralized [15].
Several regional SDOs have global impact, such as the Association of Southeast Asian Nations (ASEAN), the Arab Industrial Development and Mining Organization (AIDMO), and CEN.Most deal with issues such as promoting trade by harmonizing standards in the given region.CEN is, i.a., strongly committed to standardization in the field of air quality.Generally, there are two specialties in the European standardization system: (i) forced harmonization, meaning the European standard must be adopted, unchanged, by all members, and they must also withdraw all conflicting national standards; and (ii) standards as policy tools, meaning the European Commission can ask CEN to draft a European standard by means of a standardization mandate, leading to a harmonized standard that applies to specific EU legislation [15].
The main goal of international standardization is to facilitate the global exchange of goods and services, and to promote international cooperation in technical, economic, scientific, and intellectual fields.ISO standardizes topics of global relevance so that the international standard can be used as broadly as possible in global markets.ISO sets voluntary standards which do not legally demand harmonization.Difficulties in harmonization may arise if the current situation is stable and not everyone is willing to accept the voluntary ISO standard.Hence, voluntary standards are best suited to markets in which standards are widely variable or rapidly changing [15].

How to Use Standardization to Ideally Drive Legislation
Considering Section 6.1, three avenues of standardization can be identified that provide unique opportunities for successful standardization in the field of IAQ.These avenues follow the principle of subsidiarity; thus, each of the three avenues comprises national, regional and international standardization and is oriented towards a dedicated scientific question: How can we protect occupants from adverse health effects due to indoor air pollution?
• Specific national needs for the protection of building occupants from the adverse health effects caused by indoor air pollution are ideally addressed through national standardization efforts, since the indoor exposure strongly depends on, e.g., the typical prevailing exposure pattern, building infrastructure, and demography.This is reflected by various national limit value concepts.• The specific regional characteristics of building products and materials for indoor applications demand regional harmonization for material-related emissions.EN 16516 is a widely accepted example of regional standardization in the field of assessment of the release of dangerous substances from construction products.• Putting sampling and measurement techniques on an equal basis is best addressed on an international level to elaborate minimum requirements acting as starting point for further regional and/or national standardizations, meeting specific regional and/or national needs.This is the underlying approach of the development of many standards of the ISO 16000 series.
How can we accomplish energy-efficient building operations for improved IAQ?
• Specific national needs for the accomplishment of energy-efficient building oper- ations for improved IAQ are ideally addressed through national standardization efforts, since the energy-efficiency of a building operation strongly depends on, e.g., the availability of energy, building infrastructure, and climate.This is reflected by various national minimum air exchange rates.• The specific regional characteristics of occupancy, room use, and room setup (furnishing) demand regional harmonization of ventilation requirements.EN 16798 is a widely accepted example of regional standardization in the field of harmonized ventilation approaches.• Putting design, sampling and measurement techniques on an equal international basis is also best addressed on an international level, as outlined above.
How can we create awareness and acceptance of IAQ management requirements?
• Specific national needs depend on, e.g., socio-economic status, availability and access to information, as well as the societal distribution of the environmental burden of disease and wellbeing.Therefore, these needs should be addressed on a national basis by providing national guidance documents on IAQ management for a specific target group.• Regional harmonization for a roadmap towards, e.g., environmental justice, sustainable development, and energy efficiency should be tailored to world regions that exhibit comparable pre-conditions in the mentioned policy fields.• IAQ assessment approaches should be put on an equal basis by defining minimum international limit values that can in due course be adapted to specific regional and/or national needs.

Priority Areas for Future Standardization
Support for guidance documents on short lived oxidants and particles is needed.Measurement methods for monitoring long-lived airborne pollutants (e.g., O 3 , NO x , and particulate matter) both indoors and outdoors are standardized and internationally recog-nized (EN 14211; ISO 13964; EN 12341).However, there is a lack of standardized methods and strategies for the measurement of airborne short-lived trace gases [25,26].
Short-lived oxidants yield highly oxidized organic compounds in reactions with VOCs.Highly oxidized species are subsequently involved in new particle formation through condensation.Additionally, epidemiological and toxicological studies show that many organic compounds and ultrafine particles as trace constituents cause adverse health effects.In this context, oxidative stress is the common underlying biological mechanism [27][28][29].This fact has initiated initial future regulatory activities by ISO and CEN, but guidelines on sampling and analysis need further development.
Therefore, standardized methods for real-time, continuous indoor measurements of VOCs, as well the predominant radical precursor in indoor environments, HONO, are required.Complementary standardized methods for the in situ measurement of short-lived oxidants such as OH, HO 2 , RO 2 , NO 3 , and other reactive oxygen species are needed to facilitate new scientific insights into indoor air chemistry and resultant health effects.These measurement techniques are commonly used for the investigation of photochemistry in the lower troposphere [25][26][27][28][29][30][31][32].
IAQ guidance documents need to adapt to the dynamic changes of the portfolio of products for indoor application.Steadily shortened market cycles of new consumer products with indoor applications are leading to swiftly changing emission patterns in indoor environments.Consequently, future analytical methods for assessing IAQ need to be very flexible.Current ISO standards exclusively focus on discontinuous target-screening for characterizing IAQ.Conventional target-screening resorts to a pre-defined analytical parameter set to enable interference-free measurements of the target species.Following this approach of conventional target-screening does not allow for adaptation to swiftly changing exposure scenarios.Complementary guidance documents for online measurement methods is just one future requirement.
The development of online measurement strategies for emerging indoor pollutants is urgently needed.Emerging indoor pollutants are substances that are not yet considered in routine monitoring programs, and which may be candidates for future regulation, depending on research into their toxicity, potential health effects, and occurrence in the environment, as well as our ability to monitor them with the required uncertainty and within the limit of detection (LOD or LDL).Non-target-screening enables simultaneous measurement of a vast amount of substances by identifying environmental pollutants without a preceding selection of the compounds of interest (an open set of unknown or untargeted substances).This approach provides a plethora of analytical information in a short time period.Detection sensitivities that can be very different for each group of substances, resulting in considerably lower sensitivity and a higher LDL compared to targetscreening, are the subject of ongoing research in the field of non-targeted analysis [33].Future guidance documents for IAQ assessment will need to confront the potential and the limitations of non-target screening.
Air quality guide values need global harmonization.Guide values normally rely on dose-response assessment, described by the terms NOAEL and LOAEL.However, to date, there are no harmonized European or international guide values; rather, each country uses different approaches.National variations can be seen, for example, with formaldehyde and α-pinene, wherein various guide values differ by a factor of 10 [34].
Existing guide values need to be revised and re-evaluated for several reasons:

Chemicals are being reclassified constantly;
There is a lack of harmonization of global, regional and national guide and limit values; Guide values are missing for emerging indoor air pollutants (e.g., VOCs and PFAS); Ultrafine particles in the built environment are not regulated; There is a lack of distinction between the different indoor environments: residences, public spaces and commercial premises; Existing guide values have been derived for single substances or substance classes without considering real environments, which have complex mixtures of different pollutants.
There is a continuing need to standardize field measurement campaigns for indoor air research.Buildings are unfortunately very heterogeneous with respect to building materials, construction, occupancy and activity.To be efficient, intensive field measurement campaigns are feasible only at a limited number of measurement sites.Prior definition of scientific objectives, the indoor environment type, and the minimum number of investigation sites and buildings is mandatory.International standards can assist the systematic categorization of indoor environments, ensure compliance with consistent measurement strategies, and thus support dataset comparability.This approach is similar to outdoor field studies, wherein the chemical regime-defined by the prevailing NO x concentration and the reactivity of air masses towards OH-is consulted for the sake of the comparability of measurement datasets.The categorization outlined above does not exist for the oxidative capacity of indoor environments.
Future ISO standards need to be developed for IAQ modelling, and therefore have to comprise data quality objectives, corresponding quality indicators, emission factors, and emission inventories.Implementing a management system for controlling different aspects of IAQ can have enormous benefits in terms of comfort and health.Managing IAQ must anticipate and resolve the target conflict between energy management, thermal comfort, and hygienically acceptable air quality.A management system for controlling different aspects of IAQ was recently addressed in ISO 16000-40.This "Indoor Air Quality Management System" (IAQ-MS) was designed to be integrated with other management systems, including ISO 9001, ISO 14001 (environmental management) and ISO 50001 (energy management).The goal of IAQ-MS is to define an integrated building system that ensures the building management system is environmentally friendly, energy-efficient, and provides a healthy and comfortable air quality for the building occupants [45].Specific technical guidance for the implementation of an IAQ-MS is still required.

Summary and Conclusions
Regulations and guidance documents are intertwined, operating on international, regional and national levels.National and regional regulatory differences should be confronted and normalized, possibly following the ISO's lead on standards through the ISO 16000 series, and the WHO's lead on guidance and limit values.
Guidance documents for ambient outdoor air are already established.Work continues on UFPs and VOCs, but the bulk of the required test methods and sampling methods are in place.
Test and sampling methods for the gases and particles found in both outdoor and indoor air are in place, but established indoor limit values to drive forward regulations are unfortunately missing.Indoor air chemistry is subject to specific boundary conditions; the required guidance documents for indoor-specific pollutants need to be developed.This is a difficult task, considering the abundance of VOCs in indoor air.
The indoor environment is dynamic and complex, both in the short term and long term, and considering the problems of solid-gas partitioning onto surfaces and particles.Research on indoor air is not standardized, meaning the opportunity for cross-study comparisons is missed.IAQ audit control documents exist and should be used as templates for IAQ research.These audit templates need to be extended for research to cover both short-term and long-term exposure in homes, offices, schools and hospitals.
Standardization can drive legislation.New IAQ guidance documents are being drafted and current documents updated, based on current and emerging scientific evidence.Scientists, local activists, trade associations, and politicians must organize to define the limit values for pollutants that are neglected health risks.
A concerted effort may lead to national or regional legislation for indoor air pollutants.However, it is necessary to consider the specific requirements for air quality management approaches in different indoor environments.In private homes, it is challenging to implement IAQ compliance strategies on a voluntary basis.In this context, it is important to understand that CO 2 is a major factor when it comes to IAQ assessment, but is only an indicator of ventilation rate; information on the indoor air pollutants discussed in Sections 4 and 5 must be made available and communicated effectively to the general public.In indoor public spaces, different air quality management approaches are required to address both the building operational costs as well as the costs of absenteeism and presenteeism.These approaches balance the dual requirements of air pollutant abatement in the building and in the immediate vicinity of the building, as well as minimizing energy usage while implementing adequate mechanical ventilation.
Finally, it is important to prioritize future air quality management approaches from a health risk management perspective.It is impossible to regulate exposure limits and/or to standardize the measurement of the hundreds of airborne trace constituents that exist in indoor air.A threefold approach differentiating between (i) ubiquitous airborne pollutants, for which exposure limits are already in place (CO 2 , CO, NO 2 , O 3 , PM 2.5 ); (ii) highly hazardous substances (carcinogenic, mutagenic, toxic substances), for which exposure limits partially exist and potentially need to be adapted to permanent occupancy situations, also considering vulnerable groups; and (iii) low-to-moderate health risk substances, the regulation of which is expected to reduce the burden on healthcare systems.

Figure 1 .
Figure 1.Construction of the 'legal pyramid' describing the body of normative documents comprising documents resulting from legislation processes (constitutional law, legal acts, regulations and

Figure 1 .
Figure1.Construction of the 'legal pyramid' describing the body of normative documents comprising documents resulting from legislation processes (constitutional law, legal acts, regulations and administrative provisions), as well as guidance documents resulting from standardization processes (technical specifications and standards).The number of normative documents increases with increasing attention to technical details.Revising normative documents is also easier for documents paying greater attention to technical details.

Figure 2 .
Figure 2. Percentage of countries that adopted government guidelines or government regulations for different IAQ parameters.Government guidelines/government regulations from 35 countries were reviewed.This figure is based on data extracted from the ISIAQ Indoor Environmental Quality Guidelines Database.

Table 1 .
Scope of normative documents for managing IAQ, grouped by intervention approach.

Key Question for IAQ Management Priority Parameter
Percentage of countries that adopted government guidelines or government regulations for different IAQ parameters.Government guidelines/government regulations from 35 countries were reviewed.This figure is based on data extracted from the ISIAQ Indoor Environmental Quality Guidelines Database.

Table 2 .
Priority parameters retrieved from government guidelines and government regulations grouped into different categories with respect to their scope, and in order of increasing occurrence (each in more than 5% of countries in the ISIAQ Indoor Environmental Quality Guidelines Database).

Table A2 .
Guidance documents for formaldehyde.

Table A3 .
Guidance documents for VOCs.

Table A4 .
Guidance documents for mold.

Table A5 .
Guidance documents for particulate matter.

Table A6 .
Guidance documents for other pollutants.

Table A7 .
Guidance documents for building ventilation.

Table A8 .
Guidance documents for IAQ surveys.

Table A9 .
Guidance documents for indoor air cleaners.Standard method for assessing the reduction rate of culturable airborne bacteria by air purifiers using a test chamber ISO 16000-43 Standard method for assessing the reduction rate of culturable airborne fungi by air purifiers using a test chamber ISO 16000-44 Test method for measuring perceived indoor air quality for use in testing the performance of gas phase air cleaners EPA ENERGY STAR 2.0 ENERGY STAR Program requirements for room air cleaners

Table A10 .
Guidance documents for test chambers.

Table A11 .
Guidance documents for diffusive samplers.Indoor, ambient and workplace air-sampling and analysis of volatile organic compounds using sorbent tube/thermal desorption/capillary gas chromatography-diffusive sampling EN 13528-1 Ambient air quality-diffusive samplers for the determination of concentrations of gases and vapors-requirements and test methods-general requirements

Table A12 .
Guidance documents for analytical methods for IAQ-assessment.

Table A13 .
Guidance documents for air quality limit values.

Table A14 .
Guidance documents for air quality data quality.