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People spend a substantial fraction of their lives indoors (often 80-90%) and so these locations can represent a significant fraction of exposure to air pollution. Indoor air quality is a complex phenomenon but has been studied far less than air quality outdoors. In the absence of indoor sources of pollution, indoor air quality is determined by ingress of outdoor air, balanced with pollutant loss processes such as deposition to surfaces and through ventilation. In reality most enclosed spaces have a wide range of indoor emissions including from buildings materials, furnishings, the use of combustion appliances such as gas and solid fuel cookers, boilers and stoves, the consumption of solvent-containing products, and the use of consumer products (e.g. cleaning and personal care products). Individuals themselves are a source of emissions that include CO2, human bio-effluents and biological aerosols such as viruses. Some factors are outside an occupant’s control, such as building fabric or ventilation in public spaces or the workplace, however individual behaviour and activities are a significant determinant of indoor air pollutant concentrations. Consequently, a person’s actions can directly influence the concentrations they experience. This contrasts with outdoors where concentrations are to a large degree controlled through the aggregation of collective societal emissions. Since dispersion is much more limited indoors, compared to outdoors, even modest emissions indoors can result in high indoor concentrations.There is extensive qualitative information on how individual processes, materials and activities can lead to emissions indoors, including the detailed chemical speciation of the pollutants released. Many of the key outdoor pollutants are found to be important indoors, such as particulate matter (PM2.5), nitrogen oxides (NOx), and carbon monoxide (CO), although there is limited evidence of whether the toxicity of PM indoors differs from outdoors. There are aspects of pollution found indoors that are notably different to outdoors. Mould and damp can lead to elevated concentrations of biological aerosols when compared to those found typically outdoors. The indoor environment can accumulate much higher concentrations of volatile organic compounds (VOCs) than are found outdoors in the UK, due to their release from construction and furnishing materials and use of cleaning and personal care products.Whilst comprehensive inventories exist that list the myriad different chemicals that are emitted indoors, AQEG found only limited information that places those emissions on a quantitative footing, e.g., expressing emissions of a pollutant as mass per unit of activity, person, or consumption. In this report, the National Atmospheric Emissions Inventory (NAEI) has been a key source of information on indoor emissions in the UK. Although the NAEI is not designed specifically for the purposes of evaluating indoor air quality, sources of pollution arising from buildings are significant to outdoor air quality and their emissions at a national scale are captured and reported as part of transboundary emissions obligations. Particularly notable are emissions of VOCs, of which >14% occur indoors according to the NAEI (for contrast only 0.1% of NOx and 0.7% of PM2.5 emissions occur indoors). Major sources include aerosol propellants and decorating products such as paints and varnishes.A complex mix of ventilation and product emission regulations and guidelines have an impact on indoor air quality, but these are not always well-integrated with one another or used to their best effect. Standards for acceptable ventilation rates are included in Buildings Regulations in the UK; high VOC content products such as paints have been regulated through EU Directives, and numerous labelling schemes exist for construction products across Europe, but not in the UK. Less well defined are standards for acceptable concentrations of air pollutant indoors. Advisory health-based guideline values on selected indoor air pollutants issued by WHO and UKHSA (formerly PHE) do not have any statutory underpinning. In the workplace there are limits on occupational exposure to a range of airborne chemicals. These assume that the time spent in these settings is limited and those exposed are healthy adults, so they are set at high time-weighted concentrations. Occupational indoor air quality standards are likely not appropriate for a wider population that includes children, elderly and vulnerable individuals.A major area of uncertainty identified relates to current concentrations of indoor air pollution in UK homes and their trends over time. Most AQEG reports on outdoor air quality can draw on extensive observational data collected through national, local authority and research networks, on many different pollutants, and often over multi-decadal periods. No such datasets exists for indoor air quality in the UK. Instead, the only quantitative evidence on indoor air quality comes from individual research studies in specific indoor micro-environments (e.g., homes, schools, transport, rail stations, shops etc.) with fragmented and inconsistent pollutant speciation. Most research studies report information for only a small number of pollutants over a short period of sampling, providing only a snapshot of concentrations and with limited data on occupant activities.Since it is impossible to measure everywhere at once, outdoor air quality management assumes that given suitable criteria, representative assessments of concentrations can be made from a limited number of representative monitoring locations such as roadside, urban background and rural. It is however challenging to characterise a ‘representative’ indoor space that can be used as a reference point or a baseline against which other locations can be compared. A consequence is that it is impossible to generate a holistic and quantitative picture of current concentrations in UK buildings, or how this may have changed over time. It is also challenging to use measurements to evaluate those processes that determine indoor air quality, or to draw general or widely applicable conclusions on the effectiveness of interventions. Compared to outdoors, conducting detailed observations in homes is practically difficult and resource intensive, and each experiment runs the risk of being unrepresentative of other indoor locations. What emerges from research measurements of indoor air is the exceptional heterogeneity of chemicals found, and with a far greater range of concentrations than are encountered in typical ambient outdoor air in the UK.There are currently rather limited capabilities to model and predict indoor concentrations (or personal exposure). For outdoor air, there is comprehensive model infrastructure to estimate concentrations of pollution at any given point in space or time, through combining emissions data, chemical mechanisms and meteorological fields. Outdoor models are routinely tested against observations to evaluate their performance, and in some cases, observations are used to improve model forecasts. The indoor environment lacks this same degree of predictive capability, in large part because of the uncertainty in potential contributing emission sources. This compromises attempts to estimate exposure and health effects, or the use of models to evaluate potential interventions. There are detailed chemical mechanisms that describe indoor gas and particle-phase reactions, developed as extensions of schemes used for outdoor models. However, these models frequently lack building and occupant-specific emission rates into indoor spaces, or parameterisation of ventilation, temperature, relative humidity, lighting, and air exchange of individual buildings. The key role played by occupant behaviours in controlling factors such as ventilation and frequency of use of consumer products that emit, means that identical homes can often experience widely differing levels of indoor air quality.Determining whether indoor or outdoor air quality is the greater contributor to overall exposure is not straightforward. For any individual, it will depend uniquely on time spent in each environment, their home, its location, ventilation, choices of activity indoors and crucially which pollutant is being considered. The home is also not the only indoor environment that people experience. Elevated concentrations of pollution have been reported in studies of air quality in UK schools and hospitals, of particular significance given they are occupied by more vulnerable groups. Transport micro-environments are also a significant route of exposure, inside cars, buses and trains and transport hub buildings. Looking across the literature, peak indoor reported concentrations of PM2.5 can often be higher than those that are experienced outdoors. For NO2 the picture is mixed; outdoors at the roadside concentrations are often higher than are typically reported indoors, except when there is unextracted gas cooking. For biological aerosols, carbon monoxide and many VOCs, literature reported indoor concentrations in the UK are often significantly higher than outdoors.There are numerous interventions that would likely improve indoor air quality including eliminating emissions from highly polluting sources such as solid fuel burners, improving building quality, and the development of lower emission product standards with accompanying labelling. Reducing emissions from these sources would also benefit outdoor air quality as well.Anticipated improvements in outdoor air quality, as set out in the Clean Air Strategy, should also feed through into better indoor air quality, since air exchange will remain a key factor in determining indoor concentrations. However, it should be noted that in some urban and road-side locations ozone concentrations are likely to increase and that if brought in to buildings nearby could increase rates of indoor air chemistry. There are direct opportunities to further improve indoor quality through increased ventilation in buildings (including homes, commercial and public spaces), an issue which has increased significantly in public prominence during the COVID-19 pandemic. The enclosed nature of indoor spaces makes them amenable to air quality improvement through active air filtration systems for particulate matter, although these may incur tradeoffs that include capital / operational costs and long-term changes in exposure to bioaerosols that may have uncertain impacts on health. Caution is noted regarding other air cleaning technologies such as those using UV light, ozone, peroxyl radicals or ionizing reactions, which have the potential to be detrimental to indoor air quality through the creation of harmful secondary pollutants.