Indoor Air Quality in Buildings: A Comprehensive Review on the Factors Influencing Air Pollution in Residential and Commercial Structure

Worldwide people tend to spend approximately 90% of their time in different indoor environments. Along with the penetration of outside air pollutants, contaminants are produced in indoor environments due to different activities such as heating, cooling, cooking, and emissions from building products and the materials used. As people spend most of their lives in indoor environments, this has a significant influence on human health and productivity. Despite the two decades of indoor air quality (IAQ) research from different perspectives, there is still a lack of comprehensive evaluation of peer-reviewed IAQ studies that specifically covers the relationship between the internal characteristics of different types of building environments with IAQ to help understand the progress and limitations of IAQ research worldwide. Therefore, this review of scientific studies presents a broad spectrum of pollutants identified in both residential and commercial indoor environments, highlighting the trends and gaps in IAQ research. Moreover, analysis of literature data enabled us to assess the different IAQs in buildings located in different countries/regions, thus reflecting the current global scientific understanding of IAQ. This review has the potential to benefit building professionals by establishing indoor air regulations that account for all indoor contaminant sources to create healthy and sustainable building environments.


Introduction
Research on the urban population has confirmed that people spend more than 90% of their daily lifespan in indoor environments. Apart from residential indoor environments, people spend a large proportion of their time in offices, educational institutes, and other different commercial and industrial buildings. Specific research in North America has shown that adults tend to spend 87% of their time in buildings, and the remainder of their time is spent in vehicles (6%) and outdoors (7%) [1]. As people spend a majority of their time in indoor environments, exposure to indoor air pollutants has a significant impact on both human health and effectiveness in the workplace. However, research on air quality has mostly focused on the outdoors, whereas indoor air quality (IAQ) and its impacts have received considerably less attention until the last decade [2]. Recently, both scientists and the public have focused on risks associated with IAQ because research has established that indoor air is more contaminated than outdoor air [3]. Due to continuous changes in living style and the materials used in indoor environments, there have been significant changes in terms of the nature and complex compositions of indoor air pollutants, which opens up avenues that need to be investigated in detail.

Patterns of Time Spent Indoors
Different life activities cause people to spend a majority of their time in different types of buildings, including residences, offices, schools, and restaurants ( Figure 1). The air quality in indoor environments is a significant determinant of human health and wellbeing. Several studies have established links between positive human health impacts and improved indoor environments [4][5][6]. Low IAQ results in unwanted health conditions, including death in the worst-case scenarios. This highlights the importance of the IAQ of any indoor space where humans spend a majority of their time. . The total number of participants was 9196, and approximately 87% of the time spent in indoor environments was in residential buildings, office buildings, restaurants, and other indoor places, such as malls, stores, schools, churches, public building, salons, health clubs, parking garages, auto-repair shops, and laundromats [1].
To better understand the factors affecting overall IAQ, the assessments of IAQ should be based on different types of buildings. Therefore, it is essential to understand the relation between IAQ and different residential and commercial buildings because building codes and regulations vary based on the type and end purpose of the building.

Indoor Pollution Sources and Health Impacts
The energy crisis of the 1970s introduced the importance of energy savings in buildings, which ultimately led to more airtight and insulated buildings worldwide [7]. For energy savings, reduced amounts of fresh air are circulated in air conditioning systems. Moreover, with the advent of improved living standards, more synthetic materials and chemicals are being used in buildings for indoor construction and decorating purposes. Additionally, pesticides, cleaning agents, air fresheners, and gases from cooking are other sources of indoor air pollution.
Because inadequate ventilation, lack of air conditioning systems, human activities, and numerous materials, chemicals, and gases mainly influence indoor pollution, different organizations, such as the United States Environmental Protection Agency (US EPA) and World Health Organization (WHO) have recognized IAQ as a multi-disciplinary phenomenon and classified pollutants into several categories. According to the WHO, in the year 2000, over 1.5 million deaths were caused by indoor air pollution [8]. Moreover, indoor air pollution has been recognized as the third main reason for disability-adjusted life years worldwide [9]. Table 1 provides some pollutants in indoor environments and their health impacts.

Contaminants
Sources Possible Consequences Ref.

Biological Contaminants
Allergens Furry pets, dust mites Asthma [10,11] Endotoxins Presence of cats and dogs, contaminated humidifiers, storage of food waste, lower ventilation rate, increased amount of settled dust Asthma, reduced lung function [12,13] Dampness and mold Unattended plumbing leaks, leaks in building fabric, hidden food spills, standing water Upper respiratory symptoms, cough, wheeze, and asthma [14,15] Chemical Contaminants

Smoke Tobacco smoke
Premature mortality, lung cancer, coronary artery disease, childhood cough and wheeze, respiratory illness, infant death syndrome [16,17] Coal & biomass fuels combustion product Cooking and heating

Purpose of Study
With the continuous growth of the population and economy, demand for high quality of life has introduced different new elements in indoor building environments. Additionally, different types of buildings are changing with time, which has an impact on IAQ and human health. Therefore, it is of utmost importance to analyze the IAQ in diverse end-use buildings to determine each possible indoor pollutant in specific kinds of buildings that are responsible for adverse health impacts. Despite the two decades of IAQ research from different perspectives, there is still a lack of organized evaluation of peer-reviewed IAQ studies that specifically cover both residential and commercial building environments. These would help to understand the factors influencing IAQ in different types of building  environments along with the capacity to highlight the progress and limitations of IAQ  research worldwide. A wider understanding of the relation between different building  characteristics and air pollutant concentrations is required to enable possible sustainable  solutions for better IAQ. To address this gap, we reviewed scientific studies that focused on both residential and commercial building IAQ in different parts of the world. Therefore, the trends and gaps in scientific research for both the residential and commercial sectors that focus on quantitative changes in air parameters due to IAQ have been identified in this review. Moreover, we reviewed the internationally recognized IAQ standards and the sampling techniques applied in peer-reviewed studies. The breadth of this review was undertaken to support and accelerate future research on the design of optimal building environments to provide the best possible IAQ benefits for future healthy indoor spaces.
To achieve these objectives, this review includes scientific studies from different relevant scientific databases. The remainder of this review paper is structured under four major headings that discuss (1) international IAQ standards and assessment methods, (2) residential buildings and IAQ assessment, (3) commercial buildings and IAQ assessment, and (4) the conclusions and future scope.

Methodology
This review is formulated based on peer-reviewed journal articles from several renowned databases, such as ScienceDirect, Wiley Online Library, and Taylor & Francis. We mostly focus on papers published in the last twenty years to realize the periodical progress in scientific research. A few journal papers from other databases are also reviewed here (such as ACS), as mentioned in Figure 2. As this review considers the IAQ of both residential and commercial buildings, the home spaces where people spend majority of their time, as well as IAQ in two different commercial buildings (offices and educational institutes), have been reviewed. Other types of commercial buildings, such as hospitals, malls, and restaurants, are beyond the scope of this review because the percentage of time spent in these spaces is insignificant as compared to that spent in offices and educational institutes. In this review, we attempt to describe the progressive trend of IAQ research around the world; therefore, peer-reviewed journals across the world were considered. Along with the journal papers, a few conference papers and government reports were also analyzed to enhance the quality of the review.
A database search was completed using several combinations of keywords, e.g., IAQ, residential building, commercial building, office, school, indoor air pollution, educational institute, home, and IAQ standards. These keywords were searched in the journal title, abstract, and keywords for primary selection of peer-reviewed papers (phase 1 in Figure 2). A total of 1095 peer-reviewed journal articles and other papers, and reports were returned after the primary search from where 414 articles were selected after the careful screening of the article titles, keywords, and quick screening of the article's outline. Screening in phase 1 has been done to eliminate those articles that are qualitative in nature and focus on IAQ for other types of commercial buildings such as hospitals, malls, restaurants which are beyond the scope of this review study. To search conference papers and reports, these keywords were searched only in the title. After the preliminary search, phase 2 included a three-stage literature filtration process. In the first step of the literature filter (filtration step 1), elimination was performed after careful reading of the abstracts, based on the inclusion criteria. This phase resulted in 84 studies. Next, we screened the full texts of the remaining 84 studies thoroughly, which resulted in 69 studies for further review. Finally, duplicate articles were eliminated, resulting in 61 papers for final review. The selected studies were classified into specific categories according to the aim of the review. Figure 2 shows the literature search overview with the selection criteria.

IAQ Standards & Assessment Methods
The quality of indoor air is crucial because people spend a significant portion of their time in different indoor spaces and also because of the presence of numerous pollution sources in indoor spaces, such as traditional and newly developed building materials, finishing products, furniture, cooking systems, and cleaning agents. Therefore, several international organizations worldwide, such as the WHO, have set guidelines and threshold values to maintain an optimal IAQ (    Health problems due to IAQ, which are more commonly respiratory-related diseases and allergies, have increased the importance of IAQ measuring techniques and associated tools. Therefore, device types and monitoring systems of different indoor air pollutants were extensively reviewed. Table 3 shows a summarized list of identified indoor pollutants and devices used for pollutant detection.  Air collected on adsorbent tubes and analyzed by gas chromatographymass spectrometry Sampling rate: 100 mL/min for 100 min [58] Organic vapor sampler, adsorbed on activated charcoal column, analyzed by gas chromatographymass spectrometry

Residential Buildings and IAQ Assessment
Realizing the potential risk of indoor air pollution, the Hong Kong government began establishing IAQ objectives in the last two decades. To fulfill this target, research has been performed to investigate the IAQ of local residential flats in Hong Kong [49]. Six housing were selected based on some pre-criteria, including three public rental houses and rest are private housing. Pre-criteria was developed to select those six homes, which includes housing type, highly populated location and finally housing not containing any newly purchased furniture during the sampling of indoor air. All the selected homes were occupied during the experiment, and both the kitchen and living room air was collected for investigation. The results of the investigation indicated that compared to the living room, the concentration of CO 2 and PM 10 were 14% and 67% higher in the kitchen, respectively. Similarly, the count of mean total bacteria was also 23% higher in the kitchen. Insufficient ventilation was indicated as the major reason for elevated CO 2 level in the kitchen, whereas the impact of outdoor air infiltration, infrequent housekeeping, and mode of cleaning were found to have significant relation with higher PM 10 level. The influence of cooking using liquefied natural gas was considerably higher in the case of VOC release when compared to natural gas-based cooking. Both new and established dwellings were assessed in Melbourne, Australia, to determine the VOC and HCHO levels [60]. Although the results indicated lower concentrations of VOCs in established homes; however, compared to the outdoors, these concentrations were four times higher. Moreover, attached garages, faulty wool carpeting, and site contamination were highlighted as sites of higher VOC concentration inside homes. New or renovated buildings yielded one-or two-orders higher VOC emission as compared to established buildings.
A study was conducted in Singapore to determine the relationship between the air quality inside bedrooms and sick building syndrome (SBS) for both naturally ventilated (NV) and air-conditioned (AC) systems [61]. The measured CO 2 levels in NV bedrooms were lower than that in AC bedrooms. However, an opposite trend was observed in the case of particulate levels because NV bedrooms were found to have higher particulate levels. Another part of the assessment revealed higher SBS symptoms for residential occupants sleeping in AC bedrooms. As a part of the periodic assessment of building rules and regulations in England and Wales, several air quality parameters were examined in 37 homes [51]. This detailed study indicated gas cooking systems, occupancy, and location of the house as the three major contributors to a high level of inorganic gas emission. In accordance with the Canadian Environmental Protection Act (CEPA), a study was performed that investigated residential homes in Ottawa, Canada [58]. Compared to outdoor air, all targeted VOCs were present at a significant level in indoor air; however, their values were lower than that in the results of a study conducted in the previous year.
To determine the relationship between indoor PM 10 and oxidative damage to plasmid DNA, a study was performed in China [62]. Houses with smokers and those with non-smokers were selected for air sampling. An investigation employing different technologies revealed that the PM 10 generated in the living rooms and kitchens of smokers was more toxic and could cause 50% plasmid DNA damage, whereas the homes of nonsmokers contained lesser bio-reactive PM 10 . The investigation determined that soot and unknown fine particles were responsible for plasmid DNA damage. To collect data on indoor pollutants, the Observatory on Indoor Air Quality in France examined a total of 567 dwellings and focused on over 30 different pollutants; they published two separate research studies [46,63]. The major VOCs found in these dwellings were formaldehyde, toluene, acetaldehyde, m/p-xylenes, and hexaldehyde. Simultaneous measurement of 20 VOCs in both indoor and outdoor spaces revealed that for a majority of the compounds, the median levels were considerably higher than those in outdoor spaces. Later in 2017, assessment of the same sampling size (567 dwellings) in mainland France was conducted to observe the relationship between measured air pollutants concentration and perceived IAQ [64]. In 2018, another study was conducted by OQAI (mandated by the French government) to assess the IAQ in energy-efficient buildings, which included both newly built and retrofitted buildings, based on standard quantification methods and questionnaires [47]. These energy-efficient buildings have been defined as consuming 40-75 kWh/m 2 of energy per year in the case of newly built housing and 64-120 kWh/m 2 per year in the case of retrofitted buildings. An analysis of the experimental data revealed higher concentrations of hexaldehyde, alpha-pinene, and limonene as compared to those from the abovementioned French studies. IAQ assessment in selected urban slams in Delhi, India for all three seasons (summer, rainy season, and winter) revealed 10 times higher air pollutants concentration during the winter period compared to the permissible limit [65]. Household characteristics such as occupants age, family size, types of kitchen and fuel, window opening facilities have been described in this study. However, the information related to building structures and arrangements (e.g., flooring type, furniture used) were not mentioned.
Despite the progress of the Korean government in the IAQ sector, a consumer agency survey reported that 14.5% of participants experienced SBS. Thus, a study was conducted to investigate the effects of several environmental factors on the IAQ of newly built apartments before and after occupancy, including construction characteristics, temperature, humidity, and occupation duration [54]. This investigation concluded that the average pollutant levels were in accordance with the guidelines set by the Ministry of Environment, except for the levels of formaldehyde and toluene. The study also attempted to correlate the pollutant behavior with temperature and humidity and strongly suggested evaluating the IAQ based on the load ratio of major pollutant sources. Finally, the study observed a reduction in the pollutant levels to approximately half of the initial values after one year of occupancy. A comparative study on aromatic VOCs was performed for residential houses in China and Japan during the period from 2006 to 2007 [66]. Smoking-related investigations were not the focus of this study, which was a limitation because smoking is a major source of indoor VOCs. The concentrations of investigated indoor VOCs were considerably higher in China as compared to those in Japan and the outside VOCs, indicating higher indoor VOC concentrations than those outdoors in the case of China; however, the Japan case study yielded consistent results for both indoor and outdoor VOCs. The carcinogenic analysis conducted in this study revealed an alarming 10 times higher exposure risk in China as compared to that in Japan. IAQ assessment in highly populated and polluted Lodi Province, Italy was performed for both gaseous pollutants and particulate matters [67]. Investigation in both summer and winter seasons concluded crossing the threshold value set by WHO for PM and NO 2 for some cases, where CO and O 3 level were found satisfactory. Although, reduction of pollutants produced through indoor sources was highlighted. However, no clear information about household characteristics was provided. Indoor Air Pollution and Health (IAPAH) study in Ireland and Scotland focused on IAQ assessment in homes with open combustion source [68]. Households with four different heating fuel were assessed including peat, coal, wood, and cooking gas, along with households with no open combustion source but having smoker occupants. Analysis of the air parameters concluded satisfactory level of pollutants in the households using gas stoves or solid fuels according to the WHO guideline values. However, poor IAQ occurred in the households with cigarette smokers. To characterize the patterns of airborne VOCs, an extensive study was performed in Leipzig, Germany; this study measured 60 different VOCs using a survey [56]. They used two different methods to allocate VOC compounds to their source of origin. An analysis of the enormous amount of sampling data concluded that occupant behavior, furnishing materials, ventilation, natural activities, and/or a combination of these factors significantly influence IAQ. To study the IAQ in harsh desert climates, an evaluation of gaseous and particulate matters was performed in Emirati houses in the UAE [69]. An attached garage (<5 m from the house), the kitchen, and central AC systems were found to be mainly responsible for indoor PM 2.5 and PM 10 , whereas an attached kitchen, smoking, and split AC were found to be significantly correlated with indoor CO levels.
The IAQ in a Leadership in Energy & Environmental Design (LEED)-certified green building in the US was assessed on an annual basis to ensure safe limits of indoor air pollutants [70]. Although green building regulations include certain steps, such as more supply of fresh air to the indoors and a selection of safe materials, to address the IAQ during the design phase, limited experimental data confirming the improvement of IAQ during the operation phase is available. The assessment revealed several benefits of the air quality in green buildings as compared to conventional buildings. According to the ASHRAE regulation, CO 2 levels and relative humidity measurement data were at satisfactory levels. However, this study suggested that additional sensitive assessment techniques were required to more accurately assess the IAQ in green buildings. Impact of green renovation on IAQ was assessed in low-income housing apartment in Arizona, US before and after the retrofitting. Simultaneous air sampling (3 times: before, immediately after and 1 year later the renovation) and questionnaire survey concluded higher initial formaldehyde level which however met international standards after 1 year of retrofitting, except for 4% units [71]. 32% sampled houses in Macedonia were found to exceed the recommended limit of TVOC where the mean TVOC values were found to have a range from 50-2610 µg/m 3 [6]. Analysis of the measured data in Ireland suggested that buildings exceeding the annual average concentration of 100 Bq/m 3 should be further investigated in terms of their ventilation systems and current operations [59]. The small amount of sample data has been listed as a limitation of this study. The overall assessment concluded that passive houses mostly correspond with the threshold radon limit and perform better compared to conventional houses.
Both quantitative and qualitative investigations of IAQ in rural communities (lowincome families) were conducted to assess the impact of cooking fuels on PM 2.5 and CO concentration in Paraguay [72]. A much higher concentration of PM 2.5 and CO were found in houses using charcoal and wood compared to the houses using electricity and LPG for cooking, exceeding the recommended value by WHO. Impact of building retrofitting activity on IAQ was assessed in multifamily housing in Lithuania and Finland [73]. Significant reduction in both fungal and bacterial concentration was found after retrofitting activity in Finnish housing, where the opposite trend was found for selected VOCs. Investigation in Lithuanian housing concluded significant increase in radon concentration as a result of retrofit. To assess the IAQ in both new and renovated housing with gas cooking burners, a study was performed in California where the resident density was higher [74]. This comparative study indicated 165% increase in NO 2 , 18% increase in CO 2 , 25% decrease in formaldehyde, and 4% decrease in PM 2.5 when compared to a recent study performed in code-compliant ventilated California buildings. The later study was performed in 70 detached buildings in California and resulted in a significant decrease in formaldehyde and PM 2.5 compared to the study conducted for the new homes in California for the period 2006-2007 [75]. Although having limitations, these results are invaluable for the improved IAQ in future retrofit housing. Figure 3 represents the range for five different indoor air pollutants concentration in residential buildings for selected countries discussed above, while Table 4 has summarized different aspects of residential IAQ studies.

Commercial Buildings and IAQ Assessment
The IAQ in schools has been one of the major concerns among researchers, because children are more susceptible to air pollutants than adults, and children spend a significant amount of time in schools. Similarly, the IAQ in office buildings has also been of particular concern because it significantly affects the productivity of workers. Realizing the research gap in this field, in 2003, a large number of commercial buildings (20 offices, 4 schools, 1 hospital, and 1 nursing home), as well as dwellings and building products, were assessed to investigate the presence of indoor VOCs in Australia [76]. A total of 163 VOCs were identified in indoor environments as well as during the product analysis process, and a majority of the observed VOCs were released from different indoor building materials and a few surface finishes and appliances. The Korean government has also expended significant effort for improving educational conditions; as a part of these efforts, a few studies have been performed to characterize IAQ. To assess the severity of indoor air pollutants based on the age of school buildings, a study focusing on 55 different schools was conducted in Korea [51]. The factors that contributed to indoor air pollution in schools were listed as emissions of chemicals from building materials or furnishings and unsatisfactory ventilation. The HCHO concentration was found to be significantly higher than the standard value established by the Korean government. Considering the higher susceptibility of younger kids as compared to higher grade children, another study focusing on preschools was conducted in Korea [55]. The study concluded that preschools in urban areas have considerably higher indoor pollution levels as compared to the outside and rural preschool environments in Korea. In naturally ventilated office space inspection, influencing factors for indoor particulate matters were indicated as the nearby construction activity, indoor human movement, tobacco smoke, and computer operation [77]. Investigation of school IAQ in West Macedonia, Greece highlighted the presence of many times higher PM 10 concentration compared to the outdoor in normal operation period [78]. To determine the correlation between indoor pollutant levels in several primary schools, a study was conducted in Belgium [79]. This study investigated 7-8-year-old school kids for exposure to NO 2 , SO 2 , O 3 , BTEX (benzene, toluene, ethylbenzene, xylene isomers), and PM 2.5 . Higher benzene concentrations were observed in lower level classrooms, and the carpet in classrooms was determined to be responsible for the higher PM 2.5 concentrations.
The impact of airborne fungi on the IAQ inside AC offices was assessed in compliance with the new IAQ policy of 2003 in Hong Kong [80]. The assessment yielded satisfactory results that indicated a decrease in the count of airborne fungi after applying the new IAQ policy, except for the CO and NO 2 emission rates. To investigate ammonia (NH 3 ) contamination in an office located in Beijing, a study combining standard experiments and a questionnaire was conducted [42]. The results of this study were compared to the results gathered from the office of the same company located in Stockholm. This comparative study revealed a significantly higher concentration of NH 3 (3-6 ppm) in the Beijing office, whereas its concentration was <0.1 ppm for the office in Stockholm. Similarly, a considerably higher benzene concentration was observed in Beijing; the concentrations were 26.8 and 0.4 µg/m 3 for Beijing and Stockholm, respectively. This study indicated that a concrete additive was the probable source of NH 3 emission.
A study performed in Michigan State, USA for nine elementary and middle schools resulted in satisfactory bioaerosol and VOC levels [44]. The possible sources of VOCs were listed as a combination of indoor and outdoor sources along with occupant behavior, while carpets were identified as a possible source of bioaerosols. In total, 37 semi-randomized small and medium commercial buildings (e.g., offices, restaurants) were investigated in California, USA to monitor the particle concentrations [81]. Continuous measurement of the sample buildings resulted in an indoor-outdoor particulate matter ratio of less than one for the majority of the buildings. This study also indicated the disadvantage of low-efficiency filters in most of the observed buildings, which allowed outdoor particles to enter inside. Apart from the building materials and other processes inside buildings, the role of humans in IAQ was assessed based on a university classroom in Boulder, Colorado, USA by a continuous VOC measurement process [57]. The VOC measurement analysis confirmed that respiratory emissions from human beings and reaction of O 3 with their skin lipids affects the indoor VOC concentration. These human activities were found to be responsible for 40% of the daytime VOC. To investigate the IAQ for a range of climates in the US, another research group performed a simulation study using the building energy model "EnergyPlus," and it was found that PM 2.5 was mostly affected by changing weather patterns and ventilation systems, where CO 2 , HCHO, NO 3 , and O 3 concentrations were independent of ventilation patterns [82].
Particulate matters (PM) are considered as critical pollutants in most parts of India; this was determined while investigating a school in Chennai, India [45]. The significant concentrations of PMs were found to exceed the National Ambient Air Quality Standards in India. Rather than indoor pollution sources, outdoor sources, including ambient particles emitted by traffic, were found to be responsible for these concentrations. Another study focused on non-commercial buildings (offices and educational institutes) and IAQ was conducted in Delhi, India [52]. This study highlighted the significant impact of occupant density on indoor air pollution in the case of non-residential buildings. The CO 2 concentrations in two office buildings were significantly higher than the ASHRAE baseline; moreover, the measured concentrations of pollutants in educational buildings were lower than those in office buildings. Ductless air conditioning systems and ineffective air circulation systems were the primary contributors to the higher PM 2.5 in the office buildings. A study conducted in two states of the UAE revealed the poor IAQ conditions in 16 elementary grade classrooms [83]. The measured concentrations of TVOC, CO 2 , and particles were 815 µg/m 3 , 1605 ppm, and 1730 µg/m 3 , respectively, whereas the recommended values for these concentrations established by the Dubai Municipality were 300 µg/m 3 , 800 ppm, and 150-300 µg/m 3 , respectively.
To understand the influence of children's activity on IAQ, an evaluation was performed in nursery schools located in Poland [84]. Investigated indoor pollutant concentrations were found to be higher compared to the outside ones, such as PM 2.5 and PM 10 that were in the ranges of 41.17-106.06 µg/m 3 and 68.26-149.81 µg/m 3 , respectively, exceeding WHO indoor guidelines. The effect of increased ventilation on classroom IAQ was investigated in 18 classrooms in the Netherlands [85]. Results indicated a much lower concentration of endotoxin, b(1,3)-glucan, and PM 10 because of increased ventilation, whereas there was no such effect on PM 2.5 and NO 2 levels. However, analysis of Italian classrooms concluded that greater manual airing resulted in higher sub-micrometric particulate penetrations in indoor classroom environments [43]. An investigation of indoor air in office environments focused on PM 2.5 and PM 10 on both normal and dusty days over a 2-month period in Qatar [86]. The study concluded that significant concentrations of PMs in indoor spaces are mainly caused by ventilation, faulty building envelopes, and windows. In comparison with the ASHRAE and US EPA IAQ standards, concentration of CO 2 and particulate matters were much higher during a study of Qatari schools, and outdoor PM levels were identified as the main reason for high indoor particulate matter levels [87]. A study of Turkish university classrooms concluded higher CO 2 and PM levels and observed a radon concentration that was lower than the standard value set by the International Commission on Radiological Protection, but it was higher than other worldwide values [88]. To increase the knowledge about IAQ in modern offices, a project was performed in Europe that focused on newly built and refurbished office buildings in Europe [53]. All the indoor pollutants, except xylene, showed significant variations for different seasons (summer and winter). The association between human health and IAQ assessment indicated a higher PM 2.5 concentration, whereas formaldehyde, ozone, acrolein, α-pinene, and D-limonene concentrations were in acceptable ranges. This study recommended to perform a pollution assessment in at least two different time periods during a year along with an assessment of both ground and highest floors. Under the same project, another study showed a correlation between aldehyde, VOC levels, and factors associated with building structures and resident behavior [89].
Recently, new low-energy regulations for new buildings in the European Union stipulated that all new schools built in this region should follow airtight and energy-efficient envelope. To investigate the IAQ of these newly built low-energy preschools in Sweden, a comparative study was performed [48]. A strong relationship was found between IAQ and the functioning level of the ventilation system. The comparative study concluded that the preschool manufactured with Swan eco-labeled materials emitted fewer initial TVOCs compared to preschools made with conventional materials. In compliance with this trend, seven low-energy schools in Sweden were also investigated, and analysis of the data confirmed a satisfactory temperature and CO 2 level as set by the Swedish guidelines [90]. Investigation of IAQ in schools located in a highly polluted area of southern Italy was performed, which resulted in higher CO 2 , NO 2 , PM 2.5 , and endotoxin levels, whereas the Der p 1 allergen level was found to be below the threshold limit [91]. Table 5 has summarized the IAQ studies in different locations.

Conclusions and Future Scope
This paper, which discusses the last 20 years of indoor air research, aimed to review the IAQ sector from different aspects to understand the interaction between IAQ and building environments, which were mainly residential and a few commercial buildings. Since people spend over 85-90% of their time in different buildings, the IAQ of different types of building can have a major impact on human health. Therefore, this paper has done a review on the current state of the art and knowledge related to the IAQ of different residential and commercial buildings. Moreover, to understand the progress in IAQ research around the world, studies from different countries were reviewed. Region/location-specific review can also be beneficial to identify the major indoor air pollutant in each location, which need to be addressed for long term solutions. Hence, this review aimed to benefit building professionals when creating new indoor air regulations, considering major air pollutants, all indoor contaminant sources, and related health impacts, to create healthy and sustainable building environments.
Most of the developed countries consider and follow IAQ regulations during the design and maintenance phase of building environments through appropriate measures. However, this scenario is not similar in developing or underdeveloped countries, where poor IAQ disproportionately affects children, women, and elderly persons [92]. Despite the severe impact of exposure to indoor air pollutants, there is still a lack of proper scientific research on IAQ in most developing and underdeveloped countries/regions. Analysis of peer-reviewed journals during this review indicated that primarily developed and a few developing countries are more interested in exploring IAQ in terms of the human health impact, whereas underdeveloped countries still lack IAQ-focused research. The pattern of indoor air pollutants in developing and underdeveloped countries and the consequences to health should be studied more, which can provide a baseline to determine more beneficial IAQ policies in these regions. Therefore, more research is needed in these regions to ensure healthy and sustainable building environments worldwide. Along with indoor pollution sources, the situation of IAQ is worse in some regions because of outdoor climatic conditions, such as high humidity, temperature, and dust intensity, such as in GCC countries. However, studies that have focused on the IAQ situation in GCC countries have mostly excluded detailed VOC evaluations.
The reviewed studies commonly examined some parameters, such as PM, volatile matters, carbon dioxide, and carbon monoxide; however, most have focused on selected VOCs. Although a few studies have analyzed VOCs in detail, most limited their studies to estimating TVOC, benzene, toluene, xylene, and ethylbenzene. Most studies have preferred to use gas chromatography-mass spectrometry to analyze VOCs, showing that it is the most popular detection method for VOCs. Among the reviewed studies, analysis of carcinogenic air pollutants, such as radon, was rare. Additionally, few studies have clearly reported the building materials in walls or floors, whereas others did not mention the finishing type, furniture material, cleaning agent, household activities, which are highly critical elements for analyzing IAQ. Similarly, most studies focusing on commercial building's IAQ have not specified the specific detail of the indoor materials that has the most impact on the air pollution. However, building structure and/or materials, surface finishes, and resident's activity in general have been indicated as the major reasons for the elevated VOC concentration in the reviewed commercial buildings. Similarly, outside PM level and/or nearby construction process, tobacco smoke, presence of carpet, human movement have been identified for rise in indoor PM level where concrete additives has been indicated as the responsible element for higher indoor NH 3 concentration. Moreover, inter-relation model or equation between pollutants concentration and pollution source inside building environment was not clearly presented in the reviewed studies. Therefore, this study recommends more studies focusing on detailed assessment of exposure concentration along with the identification of responsible sources in each type of building environment.
Of note, direct comparison of indoor air pollutant levels is difficult and not straightforward because evaluations have been conducted over different time periods, using different instruments and sampling techniques, and in different indoor environments. Thus, it is highly recommended that more detailed scientific studies be conducted by following standardized regulations, which will allow for an inter-comparison of IAQ from studies in the future to close the existing knowledge gaps regarding IAQ.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.