Research and analysis

Response to the Air Quality Expert Group (AQEG) report on indoor air quality

Published 18 December 2025

Summary

This statement provides COMEAP’s response to the Air Quality Expert Group (AQEG) report on Indoor Air Quality (AQEG, 2022). In it, we make observations in response to AQEG’s report and suggestions for further research. We anticipate that the statement will be useful to healthcare and public health professionals who wish to inform patients about sources of indoor air pollution and possible risks to health. We hope that other organisations with an interest in the health effect of air pollutants, such as patient groups, health charities and other third sector and medical organisations, will also find it helpful. Some of the guidance and other information which is signposted within the statement will likely also be of interest to local authorities, regulators and those with responsibility for designing, constructing and maintaining buildings, as well as to the public. We also hope that the research recommendations will be useful to government departments, research councils and other organisations intending to fund future research on indoor air quality and health.

Indoor air quality can be affected by pollutants infiltrating from outdoors and also by emissions from indoor sources. Because most people spend most of their time indoors, exposure to indoor pollution makes an important contribution to overall personal exposure to air pollutants. There are differences between the types and concentrations of pollutants found indoors and outdoors. The health effects of some indoor pollutants are well established. Nonetheless, because indoor air quality is not routinely measured, less is known about the scale of the potential health risks arising from exposure to other indoor air pollutants.

Therefore, there is a need for more information about pollutants in indoor environments in the UK, and the risks that they pose to health. We have identified evidence gaps and research needs. A coordinated programme of measurements, which could be undertaken by an Indoor Air Quality Observatory, would provide valuable information on exposures, but funding of other relevant research is also needed. We recommend a focus on pollutants which are known to be hazardous to heath, in order to assess the current level of risk and enable the appropriate prioritisation of interventions.

Some policies implemented to address other concerns have the potential to increase concentrations of pollutants indoors. In particular, we recommend that an integrated approach should be used to assess policies and interventions which are intended to contribute towards net-zero carbon emissions, or to improve indoor or outdoor air quality, in order to maximise co-benefits to health and to avoid unintended consequences (for example, adverse effects on indoor air quality or increased carbon emissions).

Introduction

In November 2022, the independent Air Quality Expert Group (AQEG), which advises Defra and the devolved administrations, published a report on Indoor Air Quality (AQEG, 2022). The report reviews the available evidence covering aspects such as the differences between the types and concentrations of pollutants found indoors and outdoors, the regulatory frameworks relevant to indoor sources of pollution, and possible exposures in different types of indoor microenvironments. AQEG also identified a number of data gaps which need to be addressed in order to allow a better understanding of the health risks posed by exposure to air pollutants indoors, and to inform thinking about how these risks could be mitigated.

This statement is intended as a response to AQEG’s report on indoor air quality. In it, we make some initial observations in response to AQEG’s report and also suggest additional research that would make a valuable addition to policy-relevant knowledge in this field. The statement is not intended as a comprehensive review of the topic. Instead, it mentions some key pollutants and aspects, and signposts other sources of information. We note that there is relevant on-going research, which will inform our future considerations.

The focus of the statement is potential exposures to air pollutants in private and public indoor environments such as at home or at school. It does not consider occupational exposure. Nonetheless, we recognise that the epidemiological literature on occupational exposures is a valuable source of information on the health hazards posed by some of the pollutants experienced indoors. The statement does not consider the interaction of the effects of exposure to air pollution with those arising from heat or cold.

Future indoor air quality is also a concern, because the increasing air tightness of buildings, implemented to improve energy efficiency and contribute to achieving net-zero carbon emissions, can adversely affect indoor air quality and the health of occupants if suitable ventilation is not installed (see, for example, Milner and co-authors, 2023).

We note that, as well as influencing exposures while people are indoors, the high concentrations of pollutants produced by some indoor sources, such as cooking, solid fuel combustion and use of consumer products, can also affect outdoor air quality. However, this topic is not addressed further in this statement.

As well as AQEG’s report, other authoritative bodies have also highlighted the impact of indoor air quality on health and made recommendations. Reports jointly published by the Royal College of Physicians (RCP) and the Royal College of Paediatrics and Child Health (RCPCH) on outdoor and indoor air pollution (Every breath we take: the lifelong impact of air pollution (2018), The inside story: Health effects of indoor air quality on children and young people (2020) and A breath of fresh air: responding to the health challenges of modern air pollution (2025)) provide information on sources, types and effects of indoor air pollution, as well as possible mitigation actions. The Chief Medical Officer (CMO) for England’s annual report for 2022 on air pollution highlighted a need to focus on the indoor environment (CMO, 2022) and the Royal Society of Chemistry’s report of a workshop on indoor air quality (RSC, 2023) lists a number of challenges facing researchers in this field, proposes solutions and makes recommendations for further research. In the US, the National Academies convened a committee to examine PM_2.5 indoors, and to evaluate mitigation solutions (NASEM, 2024).

In addition, we expect that guidance from the National Institute for Health and Care Excellence (NICE) Indoor air quality at home will be useful, including to local authorities and health care professionals, and to professionals involved in design and construction of buildings. For people living with a lung condition, information provided by Asthma and Lung UK will also be of interest: Indoor air pollution and allergies. Throughout the statement, we also mention a number of sources of more detailed reports and guidance on specific topics, which readers might also find useful.

Observations in response to AQEG’s report

Pollutants indoors

As noted by AQEG, pollutants arising from both outdoor and indoor sources contribute to exposure while indoors. Some sources found indoors, and some pollutants, do not generally occur outdoors. There are well established risks to health from exposure indoors to some sources and pollutants. Examples include smoking, damp and mould, carbon monoxide, radon and asbestos. Legislation, guidelines, action plans or information resources have been developed to mitigate these risks.

Environmental tobacco smoke

Smoking in enclosed public and workplaces has been prohibited in the UK since 2006 to 2007. Tattan-Birch and Jarvis (2022) found an increase (from 63% in 1998 to 93% in 2018) in the percentage of children living in smoke-free homes, coinciding with the introduction of the smoking ban in public places. Nonetheless, exposure to secondhand smoke can still occur in indoor settings, such as at home or in private vehicles. Exposure to secondhand smoke increases the risk of the same adverse health effects as active smoking, which include lung cancer, heart disease and adverse birth outcomes such as premature birth and low birthweight, and children who live in a house in which smoking occurs are more likely to suffer from breathing problems including asthma (NHS, 2022).

In recent years, there has been an increase in the use of electronic cigarettes (vaping) (Jackson and co-authors, 2024) and a study in England found that adults are much more likely to vape indoors than smoke indoors (Tattan-Birch and co-authors, 2024a). However, a study found that exposure to nicotine was lower in children exposed by secondhand vaping indoors than in those exposed by secondhand smoking indoors (Tattan-Birch and co-authors, 2024b).

Radon

Radon is a radioactive gas formed from uranium, which occurs in rocks and soils throughout the UK [1]. Outdoor air concentrations are normally very low – a few Bq m⁻³ – whereas radon concentrations indoors can exceed 10,000 Bq m⁻³. The likelihood of high concentrations within a building depends upon its location (the underlying geology), construction, and occupant behaviour related to factors such as heating and ventilation (Public Health England (PHE), 2018). Radon exposure increases the risk of lung cancer and has a strong synergy with cigarette smoking. The UK has a National Radon Action Plan (PHE, 2018) intended to reduce high individual radon exposures and the overall level of radon exposure to the population. A draft revised plan was published for consultation in November 2023 (UKHSA, 2023). The post-consultation version is awaiting publication. Information on radon, including for householders, employers, local authorities, housing associations, and professionals involved in building or selling houses, is available from UKradon – Home.

Asbestos and other fibres

Exposure to asbestos, which was widely used in construction in previous decades, can also pose a risk to health in indoor environments. We do not discuss asbestos further in this statement; it has been well-studied and health risks associated with exposure are well recognised. They include thickening of the lung lining, lung cancer and mesothelioma (UKHSA, 2024a) [2]. Man-made vitreous fibres (MMVF) can also be found in indoor air (Allen and co-authors, 2023).

Carbon monoxide

Carbon monoxide (CO), for example arising from incomplete combustion in faulty fuel burning appliances, is well recognised as a hazard indoors. Symptoms following short-term exposure include headache, nausea, vomiting and altered consciousness, with more serious effects in severe cases. Long-term exposure to low concentrations may cause lethargy, headaches, nausea and flu-like symptoms and some studies have also linked it to adverse neuropsychological and cardiovascular effects (UKHSA, 2022). Accidental poisoning due to exposure to high concentrations of CO is responsible for around 20 deaths a year in the UK (HSE, 2023a) [3]. Examining the narrative verdicts from coroners, Close and co-authors (2022) found that, of the records with relevant information, 59% of fatal unintentional non-fire related CO poisonings in England and Wales occurred within a dwelling, and that appliances using the domestic piped gas supply were the most common source of the CO (36%). Similarly, data on CO exposures reported to the UK National Poisons Information Service (NPIS) indicated that approximately 60% occurred in the home, and that gas boilers and other gas appliances accounted for approximately 35% of exposures (Gentile and co-authors, 2022). Close and co-authors (2024) found that, in England, there was a clear trend in unintentional non-fire related CO fatalities with increasing deprivation, with half of the deaths occurring in the 2 most deprived quintiles of the population. Further information on the health effects from exposure to CO can be found at Carbon monoxide: health effects, incident management and toxicology.

The World Health Organization (WHO) (2010, 2021) have proposed health-based guidelines for concentrations of CO indoors. Guidance has been developed to help environmental health professionals identify CO exposure and take appropriate action (PHE, 2015a) and to help healthcare professionals diagnose poisoning and identify CO exposures (PHE, 2015b,c; UKHSA, 2025). Within the UK, there are country-specific regulations regarding the provision and installation of CO alarms in new or existing homes with carbon-fuelled appliances. In Scotland, the regulations apply to all dwellings: new or existing, rented or owner-occupied [4, 5]. In England [6], Wales [7] and Northern Ireland [8, 9, 10], CO alarms are required where new or replacement combustion appliances are installed, including in new homes. There are also specific requirements related to rented accommodation, either private (Northern Ireland [11] or both private and social (England [12] and Wales [13]).

Bioaerosols, damp and mould

Bioaerosols experienced indoors may be from different sources, and at higher concentrations, than those outdoors. Indoor sources include mould, dust mites, pets and people. Indoor environments contain a complex mixture of microorganisms (fungi, bacteria and viruses) – live or dead, or fragments of these. House dust mites, animal dander (small flakes of skin or hair) and some fungi are common allergens and can cause exacerbation of allergic rhinitis and asthma symptoms. Damp conditions favour the growth of fungi and dust mites, and it has been suggested that they may also promote bacterial growth resulting in higher concentrations of bioaerosols in indoor air (WHO, 2009). Higher humidity may also enable aerosolized bacteria to remain viable and infectious for longer (Lin and Marr, 2020). Humidity also influences the time viruses can remain infectious in aerosols, but the relationship is not consistent (Oswin and co-authors, 2022, Lin and Marr 2019). In general, enveloped viruses remain infectious for longer at lower relative humidity, and nonenveloped viruses (which are usually more environmentally robust) the reverse, however there are exceptions to this (Yang and Marr, 2012; Aganovic and co-authors, 2022).

WHO has produced guidelines on dampness and mould indoors (WHO, 2009) and, more recently, the Department of Health and Social Care (DHSC), Department for Levelling Up, Housing and Communities (DLUHC) and UK Heath Security Agency (UKHSA) (DHSC DLUHC UKHSA, 2023) published guidance to landlords on understanding and addressing the health risks of damp and mould in the home. Mould is formed by some filamentous fungi, and can grow in damp conditions in a wide range of temperatures typically found indoors. Some moulds are opportunistic pathogens (such as Aspergillus species, which causes aspergillosis), and mould spores can be allergenic (WHO, 2009). Health effects following exposure include asthma, rhinitis, respiratory infections and respiratory symptoms such as bronchitis, cough and wheeze. These can sometimes be severe, or even fatal: an inquest into the death of 2-year old Awaab Ishak found that he had died from a respiratory condition caused by mould exposure (North Manchester Coroners Service, 2022). This event led to ‘Awaab’s Law’, which means that social housing landlords have to comply with new requirements regarding the timescales for investigating and addressing health hazards such as damp and mould [14].

Nitrogen oxides

Nitrogen oxides (NOx) can infiltrate indoors and are also emitted from indoor sources (notably during cooking using gas appliances). There is a sizeable literature investigating the health effects associated with concentrations of nitrogen dioxide (NO₂) that arise from burning gas indoors for heating or cooking. These studies provided the evidence base for the derivation of the WHO Air Quality Guidelines for Indoor Air Quality (2010) [15] for NO₂. The majority of the available studies have investigated respiratory effects. For example, systematic reviews on studies investigating associations of asthma and wheeze with indoor NO₂ and gas cooking have been published (including Lin and co-authors, 2013; Li and co-authors, 2023).

Volatile and semi-volatile organic compounds

More than 800 chemicals have been identified in indoor air (INDAIRPOLLNET [16] Working Group 3, undated), the majority of which are organic compounds. These include volatile and semi-volatile organic compounds (VOCs, SVOCs) emitted from consumer products used for cleaning and personal care, as well as by occupants and other activities such as cooking. VOC concentrations are generally much higher indoors than outdoors, owing to the numerous sources of these in materials and products used in the home; indoor concentrations can be reduced by efficient ventilation. Because building materials are a major source of VOCs in indoor air, the highest concentrations are often experienced in new buildings, or following construction work.

WHO (2010) have developed Guidelines for Indoor Air Quality for selected VOCs. More recently, the UK Health Security Agency (UKHSA, formerly Public Health England, (PHE, 2019a)) published Indoor Air Quality Guidelines for a wider range of VOCs. These were based on work by Shrubsole and co-authors (2019) who reviewed health-based guideline values for 11 VOCs in indoor air, and compared these with available data on concentrations indoors.

Building on, and expanding, the work by Shrubsole and co-authors (2019), Halios and co-authors (2022) carried out a comprehensive literature review and identified 65 health-relevant VOCs which had been measured in European residences. Concentrations, emission rates and health effects that had been found (in epidemiological or experimental studies) to be associated with exposure to individual VOCs were reported. Health effects reported to be linked to the VOCs found indoors included irritation of the upper airways and eyes as well as adverse respiratory, cardiovascular, neurological and carcinogenic effects. Of the 65 individual VOCs, 52 were from sources associated with building and construction materials (such as bricks, wood products, adhesives and materials for flooring installation) and 41 were linked with consumer products (passive, electric and combustible air fresheners, hair sprays, deodorants). Some VOCs were noted as being produced during combustion, for example from wood stoves, kerosene space heaters and burning candles or incense. This wide range of sources means that the VOC mix indoors is different from that outdoors, and less well understood. Although some of the VOCs emitted may not be harmful themselves at the concentrations found, they have the potential to produce health-relevant pollutants as a result of chemical reactions indoors, or to affect health because of combined exposures.

Formaldehyde can be released from a range of products including construction and insulation materials, consumer products and furniture and can also be generated from combustion sources. It is an irritant and a carcinogen (UKHSA, 2024b) and is also regarded as likely to be causally linked to a number of other health endpoints including the prevalence of current asthma or the degree of asthma control (US Environmental Protection Agency (USEPA), 2024). Clark and co-authors (2023) estimated that 2.5% of asthma cases in England were attributable to formaldehyde concentrations in the home. In 2023, HSE launched a Regulatory Management Options Analysis (RMOA) to consider whether to implement restrictions on the use of formaldehyde in order to reduce concentrations in indoor air (HSE, 2023b).

Landeg-Cox and co-authors (2025) carried out a comprehensive review of studies in which concentrations of SVOCs had been measured in air or dust in European residences. A total of 298 individual SVOCs were identified in the 84 studies examined. The review focused on the 67 SVOCs detected in European residential micro-environments for which health effects (irritant, carcinogenic, cardiovascular, endocrine, respiratory or neurological) have been reported. These chemicals included flame retardants, phthalates (used as plasticisers solvents and denaturants in a range of consumer products), pesticides and biocides, polychlorinated biphenyls (PCBs; previously used as dielectric fluids and in building materials), per- and poly-fluorinated alkyl substances (PFAS; used in products for their water-resistant, stain-resistant, flame-resistant, and anti-stick properties) and polycyclic aromatic hydrocarbons (PAHs; produced by combustion, including for heating and in some consumer products). The review authors explain that SVOCs from all of these classes can be emitted from building and construction materials, furnishings and consumer products. They also note that both ‘legacy’ compounds (for which manufacture and use has been banned or restricted) and newer alternatives were found in the studies reviewed.

Airborne particulate matter

Fine particles (PM_2.5) in outdoor air can infiltrate indoors. Occupant activities such as dusting and vacuuming can also cause resuspension of particles into the air. Indoors, particulate matter can be generated during combustion (burning fossil or solid fuels, use of candles or incense) and during cooking. Concentrations are potentially higher indoors, although usually for shorter periods of time. For example, a review by Audignon-Durand and co-authors (2023) found that some domestic activities, such as frying or grilling food or using a hairdryer, can produce high levels of ultrafine particles (UFPs). Particles can also arise indoors from reactions driven by ozone.

There is a body of evidence from lower income countries regarding the health effects associated with exposure indoors to high concentrations of particles generated from burning fuels (such as wood, coal or kerosene) in inefficient stoves or open fires for cooking (WHO, 2024). However, there is very little information relevant to higher income countries as to whether the particles generated indoors, or outdoor particles aged by chemical reactions indoors, are more or less harmful to health than those experienced outdoors. NASEM (2024) noted that there is evidence that exposure to PM_2.5 indoors (of combined outdoor and indoor origin) causes acute respiratory symptoms, although the evidence for effects on the cardiovascular and other systems is more mixed or limited. Nonetheless, they conclude that there is ample evidence that indoor PM_2.5 causes adverse health effects. This view is based partly on the understanding that a large fraction of PM_2.5 exposure indoors is to PM_2.5 of outdoor origin, for which a large body of evidence indicating adverse health effects exists. UKHSA published a systematic review on solid fuel combustion exposure and respiratory health in adults in Europe, USA, Canada, Australia and New Zealand (Guercio and co-authors, 2022). Evidence from this review suggests that burning solid fuels such as coal and wood indoors could contribute to the risk of chronic obstructive pulmonary disease (COPD) and lung cancer.

There is growing interest in the emerging topic of possible risks from exposure to nano- and microplastics in air. There may be higher concentrations, and different compositions, of airborne microplastics indoors compared with outdoors. However, it is difficult to draw conclusions from the available literature. The majority of available studies do not focus specifically on particle sizes relevant to inhalation risk, and analytical methods commonly used are not able to detect and identify microplastic particles within the PM₁₀ and PM_2.5 fractions. A recent synthesis of evidence reporting microplastic number concentrations in air, or deposition from air, was inconclusive regarding the relative concentrations indoors and outdoors (O’Brien and co-authors, 2023) [17]. However, the authors suggest that, assuming that microplastic sources are indoors, concentrations indoors would be expected to be higher than outdoors. They also note that the compositions and concentrations of airborne microplastics indoors would be expected to vary between buildings, as they likely reflect differences in the characteristics and contents of the internal space, and occupant behaviour. The available evidence on nano- and microplastic air pollution is outlined in our statement on this topic (COMEAP, 2025).

Indoor chemistry and dispersion

The chemistry of the indoor environment is different from that outdoors (Weschler and Carslaw, 2018), meaning that there are different relationships indoors between secondary pollutants and their primary precursors. Reactions at surfaces are much more important in indoor chemistry than outdoors, and photochemistry is less important compared with outdoors. For example, ozone deposition to surfaces, including to skin, is one of the key drivers of indoor emissions of long-chain carbonyls (Wisthaler and Weschler, 2010). Ozone also drives gas-phase chemistry indoors, resulting in products such as formaldehyde and particulate matter – for example, following the use of cleaning products. Outdoor ozone concentrations are expected to increase in the future, partly driven by climate change. This suggests that these processes could potentially become more important, as a result of ingress of more ozone from outdoors.

The concentrations of air pollutants will vary in different areas of a building. As well as being due to different sources being located in different rooms (or areas) within a building, this also reflects internal pollutant dispersion, dilution, and deposition mechanisms. For example, although cooking generally takes place in the kitchen, it affects concentrations of organic aerosols throughout a home, particularly if the kitchen door is open and if the extractor fan is not working properly. Concentrations elsewhere in the building, of pollutants originating from the cooking activity, will be lower than in the kitchen, due to dispersion. In addition, there will be a time delay for this dispersion.

Exposure in private and public indoor spaces

The age of a building, and of the fittings and furniture, will influence indoor sources of pollutants. Indoor air quality at home is influenced by the size and design of homes and the number of occupants in the household. This has consequences for social inequalities, as summarised by Dimitroulopoulou and co-authors (2022). The literature suggests that unequal exposures may arise via poor quality housing, a lack of education regarding the harm of indoor secondhand smoke, location near congested roads and higher occupant density resulting in greater resuspension of particles indoors (Ferguson and co-authors, 2021). A review of the literature investigating socioeconomic inequalities in exposure to indoor air pollution in high-income countries (Ferguson and co-authors, 2021), found that households of lower socioeconomic status were at risk of exposure to higher levels of PM, NO₂, VOCs and secondhand smoke indoors. In addition, increased indoor pollutant concentrations have been observed in low-income housing following retrofits that increased airtightness (Shrubsole and co-authors, 2016; Fabian and co-authors, 2016). Residents in homes which are not owner-occupied may have less agency (real or perceived) to improve indoor air quality, for example through interventions to reduce damp and mould. Ferguson and co-authors (2021) suggest that more research is needed to determine the specific mechanisms that underpin the socio-economic differences in exposure. These might be complex and specific to different pollutants. For example, the review by Ferguson and co-authors (2021) reported that indoor radon concentrations were higher in more affluent households, and a study in Brown and co-authors (2015) found that the relationship between socioeconomic and lifestyle factors and indoor air quality in French residences was different for different VOCs.

Although much time indoors is spent in residential buildings, exposure also occurs within public indoor environments. Some indoor microenvironments – such as the home, school or office – contribute to overall personal exposure because of the amount of time which individuals spend in them. Others, such as transport microenvironments, might be of interest because of high concentrations of pollutants, even though most people would be expected to spend only relatively short amounts of time there. Smith and co-authors (2016) estimated that, in London, travel contributes approximately 15% of daily NO₂ exposure and 9% of daily PM_2.5 exposure, and that exposures were higher during inactive travel (for example, in a car or using public transport) than active travel (such as walking or cycling).

Exposure within different transport microenvironments, such as private vehicles, buses, trains and stations, will differ (Mitsakou and co-authors, 2021; Thornes and co-authors, 2017) depending upon ingress of outdoor pollution, and sources specific to the mode of transport. The air quality in some transport microenvironments may reflect specific indoor sources of pollutants. For example, COMEAP (2019) previously considered the elevated concentrations of PM_2.5 in the London Underground. These originate from sources within the network, principally the mechanical wear of train components including wheels, current collector shoes and brake blocks. Non-rolling stock sources include rail wear, biological particles and textile fibres associated with passengers. These are emitted as primary sources and then re-suspended by movements of the train and people (COMEAP, 2019). Mechanical ventilation may play an important role in removing pollutants from some transport environments, such as subway stations, but can also act as a source of exposure to other environmental public health hazards, including noise (Wen and co-authors, 2020).

Inside schools, kindergartens [18] and other public settings for children, pollutants such as aldehydes, VOCs and SVOCs are emitted from building construction materials, furnishings and fittings. In addition, cleaning, and some children’s activities (such as art and crafts), can increase indoor concentrations (WHO, 2022).

Mitigation, guidelines, policy development and regulation

Good ventilation is important to maintain good air quality indoors. It has also been recognised as important in reducing viral transmission indoors. However, the efficiency of active ventilation and filtration systems changes over the lifetime of a building, and is dependent upon good maintenance and appropriate operation.

Ventilation may be compromised due to the implementation of adaptation and mitigation policies related to climate change, such as increasing air tightness to improve energy efficiency in new and existing buildings, particularly if the energy efficiency measures are poorly installed, operated, maintained and serviced. In contrast, other approaches to achieving net-zero carbon emissions avoid such trade-offs between energy efficiency and indoor air quality and, therefore, may have health co-benefits. For example, as the AQEG report notes, the decarbonisation of homes is likely to improve indoor air quality by removing substantial NOx, PM_2.5 and CO sources (such as gas cookers, gas boilers, and gas and solid fuel fires) if renewable electricity is used instead. The benefits for indoor air quality will be less if gas combustion is replaced by combusting hydrogen as a fuel, although CO emissions from this source would be eliminated.

It is therefore important that an integrated approach is used to assess proposed policies and interventions intended to mitigate climate change or to improve indoor or outdoor air quality, in order to avoid unintended consequences and to maximise co-benefits to health.

Interventions which reduce sources of pollution are the most effective for improving indoor air quality (CMO, 2022) and are highest in the air pollution intervention hierarchy (PHE, 2019b). However, the type of interventions to improve indoor air which are most appropriate will depend upon the building and the specific circumstances (NASEM, 2024). NICE (2020) concluded that “Evidence showed that giving people advice on specific pollutants and their sources can help them reduce the pollution levels in their homes and improve their health. Evidence also showed that giving people advice on how to reduce or prevent indoor air pollution is cost effective for people who are already ill, because it can prevent their condition worsening.” The NICE guidelines also comment that “the committee looked at evidence for specific interventions such as air filtering systems or air purifiers. But they agreed that buildings vary so much that it wouldn’t be practical to make any specific recommendations in this area.” A systematic review by Cheek and co-authors (2021) reported that using portable air cleaners based on filtration resulted in short-term reductions in PM_2.5 concentrations indoors and therefore had the potential to offer health benefits, although the direct evidence for health benefits was weak. The use of some other types of air cleaning technologies (for example ozone generators and plasma-based technologies) produces pollutants which are harmful to health (Cheek and co-authors, 2021, SAGE-EMG, 2020).

The policy and regulatory options to address indoor pollution are different from those which apply to outdoor air pollutants. As discussed by AQEG (2022), possible approaches include building regulations (for example, ventilation requirements) and the regulation of products which emit health-relevant chemicals such as VOCs. The RSC (2023) report mentions regulation, labelling and measurement of emissions from construction and decoration products, and also of ventilation, as examples of good practice in other countries. The CMO (2022) also suggested a role for regulation to reduce emissions from products and appliances used indoors.

AQEG (2022) drew attention to the fact that groups who may be particularly susceptible to adverse effects from exposure to pollutants, such as children, older people or people with health conditions, spend significant amounts of time in indoor environments which, from a regulatory perspective, are primarily regarded as occupational settings. As well as schools and pre-school settings, these include residential and nursing care homes for older people, and hospitals.

The CMO’s 2022 annual report suggests a role for regulation to achieve good air quality in buildings which are public spaces (CMO, 2022). On this topic, Morawska and co-authors (2024) proposed that indoor air quality standards should be mandatory for indoor public spaces, and suggested a focus on PM_2.5, carbon monoxide (CO) and carbon dioxide (CO₂, as a proxy for other contaminants) in addition to a ventilation standard. However, these proposals are made in the global context, and might not be appropriate or practicable for implementation in all countries.

In the European context, INDAIRPOLLNET (2023) also acknowledged that different policy and compliance approaches may be needed for private residential buildings and public and private buildings open to the public. INDAIRPOLLNET (2023). proposed an approach to identifying 4 groups of pollutants for which regulation indoors might be needed. The proposed approach includes identifying “ubiquitous” pollutants (such as radon, CO₂, CO, NO₂, O₃, PM_2.5), “high hazard substances” (such as benzo[a]pyrene, ethylene oxide, benzene, lindane, styrene and formaldehyde), “low to moderate risk substances” where regulation would be expected to reduce the burden of disease, and classes of chemicals regarded as “low harm but important precursors” (such as total VOCs). However, it was recognised that this approach would be likely to produce a list of pollutants too large to monitor on a regular basis. Therefore, a tiered regulatory approach was suggested, in which CO₂ (a proxy for occupancy effects), NO₂ (a proxy for outdoor pollution, namely traffic) and PM_2.5 (a combination of indoor and outdoor sources) were frequently assessed. It was suggested that other pollutants could be assessed less frequently, for example when a new or newly renovated building was commissioned, and/or when there were health concerns or complaints.

A database of national Indoor Environmental Quality (IEQ) guidelines has been compiled by the International Society of Indoor Air Quality and Climate’s (ISIAQ) Scientific and Technical Committee (STC) 34 (Dimitroulopoulou and co-authors, 2023). The aim of the database is to help countries develop appropriate standards. The ISIAQ committee’s view (Sani Dimitroulopoulou, pers comm) is that countries should modify internationally agreed guidelines (such as those recommended by WHO) or standards (for example, ISO) to suit regional characteristics and needs. Relevant considerations might include factors such as climate, building types, occupant characteristics, and cultural differences, which can all impact on the applicability and feasibility of adopting international guidelines and standards.

We do not intend that this statement recommends or endorses any specific approaches to the development of guidelines or regulation. Nonetheless we note that, together with information on exposures, health-based guideline concentrations could be used to assess the need for mitigation action and inform decisions about appropriate regulatory responses, even if there is not an intention to introduce them as statutory indoor air quality standards.

Both AQEG (2022) and the CMO (2022) have noted that there is no obvious single government department with ownership of indoor air quality policy. Like AQEG and the CMO, we welcome the establishment (in 2021) of a cross-Whitehall working group on indoor air quality. We consider that the cross-government co-ordination provided by such a group will be important in ensuring holistic and effective approaches to research, policy and regulation. We would therefore encourage the reactivation of this group, which would provide a suitable forum for the discussion of the developing evidence, and for advice from AQEG and COMEAP to be implemented.

Recommendations for future work

Measurement and exposure data needs

We agree with AQEG on the need to measure concentrations of pollutants indoors. A number of relevant research projects are currently underway in the UK (such as those funded by Wave 2 of the Clean Air Programme Projects – Clean Air Programme and those listed by POST, 2023). Our understanding of the work undertaken to date is that it has demonstrated that it is possible to conduct measurements within homes, and has provided valuable information on pollutant concentrations, including that they are very heterogeneous between residences (see, for example, Carslaw and co-authors, 2025). However, if such measurements are to be useful as exposure information for epidemiological studies, they will need to be undertaken at scale, in order to allow the inclusion of enough people to provide studies with sufficient statistical power to detect associations with health outcomes. Monitoring targeted towards assessing exposures of vulnerable populations is also needed.

Proposals for an Indoor Air Quality Observatory in the UK have been made [19]. A coordinated programme of measurements such as that undertaken by the French Indoor Air Quality Observatory (Mandin, 2020) would make a valuable contribution to the understanding of indoor environments in the UK. Nonetheless, a complementary national programme of research funding is also likely to be needed.

We think that a focus on assessing exposure to pollutants which are known to be hazardous to health should be a priority. This would allow an assessment of the risk posed to individuals and the population, based on current evidence of known health risks, and enable interventions to be prioritised appropriately. A workshop could identify actionable evidence and prioritise research that would inform early action to mitigate risks.

We suggest the following data needs:

i) Long-term measurements of concentrations of pollutants indoors would be valuable, ideally through an approach that would allow temporal trends in emissions and concentrations to be understood. To be representative, this would need to be undertaken in a large number of different buildings and for a number of years.

ii) Measurements that allow source attribution, and evaluation of the contribution of exposure from different sources (indoor and outdoor) would be valuable to inform policy.

iii) Monitoring is needed to provide insight into exposures in indoor environments that are currently under-represented in the literature. These include rented homes (properties in both social and private rental sectors, and of different sizes and designs) and public indoor spaces (for example, in residential care homes, hospitals, prisons, gyms, transport settings, retail, concert and sports venues and in workplaces such as offices).

iv) Measuring pollutants concentrations in different areas of buildings, for example different rooms within the home.

v) Concurrent indoor and outdoor measurements are important to estimate infiltration efficiency and the extent to which indoor exposures are determined by outdoor concentrations.

vi) More standardised data on time-activity patterns would be useful. This could be based on the time activities of individuals though their day and on the utilisation of public indoor spaces such as passenger numbers or retail footfall.

Research to inform and develop modelling

Further work to inform and develop concentration and exposure modelling would be valuable, including:

vii) Continued improvement of exposure modelling with the use of time-activity patterns and measurement data, and also development or improvement of modelling assessments that differentiate personal exposures from indoor or outdoor sources. Examples of recent and ongoing work include the SPF-funded APEX, DIMEX, Wellhome and INGENIOUS projects.

viii) Improved models to estimate the dispersion of pollutants and physical processes within buildings would allow better exposure estimation. This could build upon tools developed in the SPF-funded Indoor Air Quality Emissions Modelling System (IAQ-EMS).

ix) As it will not be possible to monitor in every building, further work is needed to understand the various processes impacting on indoor air quality, so that this can be used to inform estimation of concentrations of pollutants in a wide range of indoor environments. Factors to be considered include, for example:

• understanding the sources of indoor air pollutants and the composition of products that lead to emissions indoors
• chemical processes in indoor environments and the formation of secondary pollutants
• understanding the impact of ventilation on the movement of air pollutants around buildings

The SPF-funded IAQ-EMS, Wellhome and INGENIOUS projects will contribute to understanding.

Health effects

More research relevant to assessing and evaluating the risks to health arising from indoor air pollution is needed. For example:

x) Monitoring of pollutants expected to be hazardous to health might help identify exposures posing a potential health risk. This would allow the implementation of interventions to reduce exposure and mitigate risks. In order to make this monitoring representative, it might need to be undertaken in a large number of different buildings and for significant time periods. Alternatively, personal measurements might also be useful to address this.

xi) Epidemiological studies investigating associations of health effects with indoor exposures to pollutants would increase the evidence base and allow more direct estimates to be made of the public health importance of exposures indoors. Research that considers the transferability of CRFs derived from outdoor air pollution epidemiology to indoor air pollution health impact assessment would also be valuable.

xii) Research to understand the toxicity of pollutants emitted and formed indoors. This should include investigation of the toxicity of particles produced by various indoor sources, and be designed to help understand the differential toxicity of particles indoors compared with those outdoors. Experimental research, such as that ongoing in the UKRI-funded HiPTox project [20], which is performing studies of human and animal exposures relevant to outdoor and indoor sources, can also contribute to the understanding of the causal mechanisms linking pollutants to adverse responses.

xiii) Developing a library of reference materials representative of particulate matter exposure indoors would be valuable, to allow consistent and comparable toxicological studies to be undertaken. Collecting sufficient particulate matter from a single building is unlikely to be practical so generating synthetic samples based on detailed analysis may be necessary. In addition, a sample from a single building would be unlikely to be representative of the range of particles present in different buildings.

xiv) Studies comparing health or physiological endpoints resulting from exposures indoors and outdoors, including effects beyond the lung.

xv) Research to investigate whether there are interactions, such as increased sensitisation, between chemical and biological pollutant exposures indoors. Some evidence and research gaps in this area is available from the SPF BioAirNet network.

xvi) Research to understand impacts of indoor air pollutants on vulnerable groups including children, pregnant women, elderly people and those with chronic health conditions. The UKRI-funded CleanAir4V, Wellhome and RESPIRE studies will add to the knowledge on this.

The experience of the projects undertaken as part of the SPF Clean Air Programme will provide useful information regarding the challenges and successes in performing community- and participatory-focussed air quality research. This experience should be leveraged to enable future projects to avoid encountering similar barriers. In addition, researchers may find information (Diez et al, 2024) on the use of small sensors available from the SPF QUANT (Quantification of Utility of Atmospheric Network Technologies) project, useful.

In relation to indoor PM_2.5, NASEM (2024) similarly recommended research on indoor aerosol characteristics and the influence of particle origin on health effects. Other research recommendations included the development of new technologies for monitoring PM indoors, studies of the health benefits of interventions intended to reduce exposure indoors, and the development of more appropriate and effective air cleaning technologies. We note particularly NASEM’s recommendation that future research should aim to understand how social, cultural and behavioural factors influence exposure, health effects and the implementation of known control measures.

Summary recommendations

Our main recommendations can be summarised as:

a) Risks to health from exposure indoors to some sources and pollutants are well established and it will be important to continue to reduce these exposures and mitigate risks. Nonetheless, there is a need for more comprehensive information on the types and concentrations of pollutants in indoor environments in the UK, and the risks that they pose to health. A coordinated programme of measurements could make an important contribution to this knowledge; this could be achieved by establishing an Indoor Air Quality Observatory to undertake this role. Nonetheless, a complementary national programme of research funding is also likely to be needed. We recommend a focus on pollutants which are known to be hazardous to heath, in order to facilitate an assessment of the level of risk and to allow interventions to be prioritised appropriately.

b) An integrated approach should be used to assess policies and interventions intended to mitigate climate change or to improve indoor or outdoor air quality, in order to avoid unintended consequences and to maximise co-benefits to health. The reactivation of a cross-Whitehall working group on indoor air quality could play an important role in this, and in co-ordinating efforts to address health effects attributable to indoor air pollutants. We think that applying the interventions hierarchy – prioritising prevention of emissions – would be most effective in improving indoor air quality.

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COMEAP

Chair                     

Professor Anna Hansell (University of Leicester)

Members     

Dr Suzanne Bartington (University of Birmingham) (from April 2024)

David Birchby (Logika Group) (from April 2024)

Professor Alan R Boobis (Imperial College London) (until April 2024)

Professor Nicola Carslaw (University of York) (until April 2024)

Ruth Chambers (lay member) (until April 2024)

Professor Martin Clift (Swansea University)

Professor Francesco Forastiere (Imperial College London) (from April 2024)

Professor Roy Harrison (University of Birmingham)

Professor Mathew Heal (University of Edinburgh)

Dr Mike Holland (EMRC and Imperial College London) (COMEAP Member until April 2024, co-opted Member from May 2024)

Professor Klea Katsouyanni (University of Athens, Greece and Imperial College London)

Dr Haneen Khreis (Texas A&M Transportation Institute and University of Cambridge) (from April 2024)

Professor Duncan Lee (University of Glasgow)

Professor Matthew Loxham (University of Southampton) (from April 2024)

Professor Mark Miller (University of Edinburgh)

Dr Ian Mudway (Imperial College London) (from April 2024)

Rosie O’Carroll (Lay member) (from April 2024)

Tom Parkes (Lay member) (from April 2024)

Professor Gavin Shaddick (Cardiff University)

John Stedman (Ricardo Energy and Environment) (Member until April 2024, co-opted Member from May 2024)

Dr Heather Walton (Imperial College London) (until November 2023) 

Dr Abigail Whitehouse (Queen Mary University of London) (from April 2024)

Associate Members

Dr Dimitris Evangelopoulos (Imperial College London) (from June 2024)

Dr Rob Ferguson (University of Essex) (from June 2024)

Dr James Milner (London School of Hygiene and Tropical Medicine) (from June 2024)

Secretariat responsible for supporting the development of this statement

Alison Gowers (UK Health Security Agency)

Dr Christina Mitsakou (UK Health Security Agency)

Acknowledgements

We thank Professor Sani Dimitroulopoulou (UKHSA) for her input and valuable comments while developing the statement, as well as Professor Nicola Carslaw (University of York) for her continued input after the end of her term on the Committee.

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[1] Indicative atlas of radon potential for Great Britain version 3

[2]  Advice on health risks of asbestos would fall within the remit of the Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment (COC) rather than COMEAP. For example, the COC has previously provided independent advice on public health risks from asbestos (in the context of relative vulnerability of children compared to adults): ‘Relative vulnerability of children to asbestos compared to adults’

[3] The figure of ‘around 20 deaths a year’ is based on a rolling average for 5 years (2018 to 2022), for the average number of accidental poisonings by other gases and vapours (ICD-10 code X47) and where the secondary cause of death was the toxic effect of carbon monoxide (ICD-10 code T58).

[4] Chapter 17, Satisfactory fire and carbon monoxide detection: tolerable standard guidance

[5] Fire and smoke alarms: the law

[6] Combustion appliances and fuel storage systems: Approved Document J

[7] Approved document J: combustion appliances and fuel storage systems

[8] Building Regulations (Northern Ireland) 2012

[9] Carbon monoxide alarms, controlling risk together Health and Safety Executive for Northern Ireland

[10] Technical booklet L Department of Finance

[11] Smoke, Heat and Carbon Monoxide Alarms for Private Tenancies Regulations (Northern Ireland) 2024

[12] Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022: guidance for landlords and tenants

[13] Renting Homes (Fitness for Human Habitation) (Wales) Regulations 2022

[14] Awaab’s Law: Guidance for social landlords – Timeframes for repairs in the social rented sector

[15] WHO (2010) did not develop specific guidelines for PM<sub.2.5</sub> and PM₁₀ in indoor air, because the Air Quality Guidelines for outdoor air were regarded as also being applicable to indoor spaces

[16] INDAIRPOLLNET (INDoor AIR POLLution NETwork) is a European COST Action network which has focused on defining the knowledge gaps and challenges for indoor air and provided suggestions for future approaches https://indairpollnet.york.ac.uk/home

[17] The definition of microplastics in this review includes tyre wear from road transport

[18] ‘Kindergartens’ is the term used by WHO for nurseries

[19] See, for example, Indoor Air Quality Observatory Stakeholder Engagement Workshop – Breathing City

[20] GtR (ukri.org) (viewed on 26 June 2024)