Written submission from Professor Nicola Carslaw (AIR0043)
Introduction
This evidence is provided on behalf of the INGENIOUS project team led by Professor Nicola Carslaw (project PI), at the University of York in the UK. The INGENIOUS project aimed to better understand air quality in 309 occupied homes in Bradford, and to understand the sources, transformations and fates of air pollutants within them. More details on the aims and objectives of the project, sampling protocols, the study area and homes, can be found on our website, and in our overview paper.
1. What are the main causes and sources of air pollution?
Although most air pollution research, funding, and regulation focuses on outdoor air quality, in developed countries like the UK, we spend 80-90% of our time indoors, with around two thirds of our time spent in our homes. Sources of air pollution in homes broadly derive from:
● building materials (mainly volatile organic compounds, VOCs)
● furnishing and decorative materials (mainly VOCs)
● occupant activities
○ cooking (particulate matter (PM), nitrogen oxides (NOX), VOCs, carbon monoxide (CO))
○ cleaning (VOCs)
○ air freshener use (VOCs)
○ candle, incense burning (PM, NOX, VOCs, CO)
○ personal care products (VOCs, PM)
● outdoors (PM, NOX, ozone).
VOC concentrations tend to be much higher indoors than outdoors and have a more diverse chemical fingerprint. Outdoor VOCs often reflect vehicle emissions, such as benzene, toluene and xylenes. Indoor VOCs tend to be dominated by fragrance compounds such as the terpenes (e.g. lemon, pine, lavender aromas), as well as aldehydes, and alcohols.
Ozone concentrations are much lower indoors than outdoors. Ozone has few indoor sources, apart from high energy devices such as photocopiers and laser printers, which are absent from most homes. Some homes may use air cleaners, some of which produce ozone (see response to question 3). Ozone sticks to surfaces (walls, carpets), and surface area to volume ratios are much higher indoors than outdoors. Ozone surface chemistry then produces VOC emissions.
The NOX concentration indoors is often lower than outdoors, unless the home uses gas for cooking. In that case, NO2 (nitrogen dioxide), PM, CO and other pollutant concentrations from gas combustion (e.g. benzene) can be elevated.
In the absence of indoor sources, indoor PM concentrations depend on outdoor concentrations and ventilation rates. Outdoor sources of air pollutants can be important for properties on busy roads, owing to vehicle emissions. However, unlike NOX, PM2.5 (fine particulate matter) concentrations tend to be homogenous over large spatial areas. Moreover, as soon as there are occupant activities within homes, they will dominate indoor PM concentrations.
Figure 1 shows PM2.5 concentrations in a home in Bradford during the INGENIOUS study. Indoor concentrations track outdoors when there are no occupant activities. The high concentrations of indoor PM2.5 correlate with cooking activities (from the timestamps and activity diaries we asked the occupants to provide), which were responsible for exceedingly high concentrations in many homes. We currently do not fully understand the differential toxicity of particles formed through different sources. Whether vehicle PM have a worse impact on health than cooking PM is still largely under-researched in large-scale air pollution epidemiology.
Figure 1: PM2.5 concentrations measured in a Bradford kitchen and outdoors at a nearby urban background station in March 2023.
Finally, there are biological pollutants, such as mould and pollen. Mould can have a profound impact on human health, as for the tragic case of Ishaq Awaab who died from exposure to mould. During the INGENIOUS study, 40% of the homes we sampled in Bradford reported the presence of mould in at least one room. The most commonly affected rooms were the adults’ bedroom (19%) followed by the child’s bedroom (18%) and bathrooms (16%).
The drivers of mould growth in residential buildings are not purely physical (i.e. poor insulation, thermal bridging) but also related to underheating and the price of fuel, overcrowding, poor ventilation and other behavioural patterns (i.e. drying clothes indoors). Underheating of homes was common in our sample (30% of rooms <18 °C during occupation), stressing the need for Net Zero strategies to deliver climate, health and social gains. Cold and damp homes cost the NHS ~1.4 billion annually, and full societal costs are £18.5 billion a year. We also found significant evidence for poor ventilation particularly in childrens’ bedrooms, where nighttime CO2 levels exceeded 2,000 ppm 20% of the time.
2. What evidence exists of the extent of air pollution directly or indirectly impacting the health of individuals or communities in England?
As part of the INGENIOUS study, we investigated whether PM2.5 concentrations were correlated with deprivation levels. Figure 2 based on Cheung et al. (2025) shows the mean daily average PM2.5 concentration/mean number of days exceeding the WHO daily threshold of 15 μg/m3 by deprivation level. In both cases, it can be seen that the most deprived families in our study were exposed to the highest PM2.5 concentrations.
Figure 2: Levels of deprivation versus PM2.5 concentration (A) and number of days exceeding the WHO threshold of 15 μg/m3 in the sampled Bradford homes.
Those in the most deprived quintile experienced PM2.5 concentrations that were around two times higher than the least deprived (24.0 vs 12.7 μg/m3 respectively). There was a link between occupancy and PM2.5 concentrations: PM2.5 concentrations were higher in households with more occupants. In addition, South Asian families experienced higher concentrations than White British families (23.5 μg/m3 vs 17.1 μg/m3), indicating that ethnic minority groups might be exposed to higher indoor air pollution exacerbating existing health inequalities. We also found higher PM2.5 concentrations in rented homes, smoking households, terraced/semi-detached houses and houses with gas cooking appliances.
3. What are the wider environmental impacts of air pollution, and what are their cascading effects?
Current policy ignores indoor air quality (IAQ) with the exception of occupational exposure, and some guidance around ventilation rates. It would be impractical to regulate IAQ in peoples’ homes, but would be possible to regulate some of the products we bring into our homes. For instance, VOC emissions from paints were reduced following the EU Paints Directive (2004/42/EC). A similar approach could be applied to cleaning products, scented candles, insulation materials etc. At the very least, a traffic light system (akin to that used to highlight sugar and salt content in food) could indicate whether VOC content was low or high, to give consumers the necessary information to make an informed choice about products they bring into their homes.
An overlooked aspect of air pollution exposure is the impact of buildings and the activities of their occupants on outdoor air quality. As part of the INGENIOUS project, we investigated the contribution of commercial establishments (restaurants, beauty salons, and mechanical repair shops) to outdoor VOCs. In densely populated urban centres, commercial clusters can significantly elevate local VOC levels. Figure 3 shows the distribution of restaurants in Bradford, and the restaurant source factor, showing the impact of these emissions on the surrounding area. High VOC concentrations extend beyond the discrete restaurants, showing that emissions from within buildings can impact outdoor air quality. As vehicle emissions continue to be reduced in urban areas with improving vehicle technology, sources from buildings will become increasingly important for urban air quality management.
Figure 3. Left panel: Discrete point-source locations for restaurants (n = 576). Right panel: Modeled source factor intensity at 10 m resolution grid averaged across all measurements, using a normalized scale to illustrate the regional extent of plume dispersion.
Tighter regulations should be considered for IAQ in public spaces such as schools, hospitals, care homes, and civic buildings that often accommodate the most vulnerable groups of the population etc. There have already been calls for such a move from leading scientists in the field.
Since the COVID-19 pandemic, there has been increased use of air cleaning technologies to remove air pollutants indoors, as well low-cost sensors to monitor air pollutant concentrations. There is currently no regulation around either of these technologies. Given that some air cleaning technologies have the potential to generate air pollution through their operation as highlighted by the SAGE Committee during COVID, we favour regulation around their use.
Furthermore, consumers need to have confidence that the readings on readily available low-cost sensors for the home are providing accurate information about the concentrations of air pollutants they purport to measure. This step is necessary both to avoid providing a false sense of security, but also, unnecessary concern if pollutant concentrations are over-estimated. Knowing the air pollutant concentrations in your home does not necessarily mean you are equipped with the knowledge and/or ability to act on the information and may just lead to additional stress.
We would like to see a change in the testing procedures and regulations of these two technologies and other items we bring into our homes - building materials, furnishings, cleaning products etc. At the moment, such items are tested in isolation in carefully controlled chambers with clean air. Such controlled testing is no substitute for the conditions in our homes where temperature and relative humidity can change in response to e.g. cooking and showering, and there is a mixture of activities and pollutants. The testing procedure should reproduce the more realistic conditions that occur in occupied homes, and test for the full range of pollutants that can be formed. Regulation should then ensure that only those items that meet carefully defined parameters are approved for use.
4. Are the current national targets and performance for air pollution, such as those in the Air Quality Environment Act target delivery plan and the 10-year Health Plan, adequate, ambitious and wide-ranging enough to provide adequate protection for public health and the environment, and how do they compare with WHO recommendations?
The Environment Act 2021 targets outdoor air, especially PM that is monitored on a national scale. While such initiatives may reduce population exposure to outdoor-generated particles, they are currently inadequate because they ignore 90% of our exposure to air pollution, which happens indoors. Indoor exposure is not only often higher than outdoors, but more diverse due to variable sources and complex chemical interactions between pollutants. The lack of routine monitoring of indoor environments means there is no evidence to develop efficient environmental policy.
5. Do local authorities in England have the resources and powers to enforce existing legislation and regulation to improve local air quality?
To achieve decarbonisation of the UK’s domestic heating, which currently accounts for 14-18% of national emissions (CCC, 2020), rapid national-scale housing retrofits are essential and are currently being implemented by the “Warm Homes Plan”. Retrofits primarily involve building fabric improvements, which should deliver substantially better insulation of walls, windows, floors and roofs, and reduce air leakage. These changes may result in changes in the hygrothermal properties of buildings resulting in unintended consequences, such as mould growth.
We note anecdotally that Local Authorities are struggling to convince households to take up retrofit opportunities, not helped by recent headlines, but also a reluctance to suffer the disruption, concerns about rent increases afterwards etc. In addition, there is a need to have such a low income to qualify for financial assistance, that for many it remains unattractive even with financial assistance. As houses become more airtight, there is a risk that indoor-generated pollution gets trapped indoors for longer, resulting in higher exposures with poorly quantified health effects. We suggest that retrofitting homes to reduce energy losses needs greater consideration of IAQ and for there to be information readily available about retrofitters that meet a relevant standard of competence (plus see related comments below).
6. Does the Government provide sufficient funding and devolved powers to comprehensively monitor air quality? Is data capture and analysis sufficient to provide a detailed and accurate assessment of air quality within England?
There is no routine monitoring of IAQ, so it is impossible to accurately assess the air quality exposure of the UK population. Outdoor pollutant concentrations are not a proxy for indoor exposures for the reasons detailed in this summary. Because IAQ depends critically on behaviour, finding representative locations for routine monitoring will be challenging, and need to be carefully planned. There have been calls for a UK indoor air observatory that would move towards routine monitoring, but to date, no funding to support one.
Our INGENIOUS project has demonstrated that even monitoring for 2 weeks at a time in 309 homes in Bradford allows some conclusions to be drawn, such as those around air pollution exposure and deprivation. We also measured high concentrations of CO2 in children’s bedrooms due to inadequate ventilation, and high PM concentrations in kitchens from cooking activities. More comprehensive monitoring such as that carried out during INGENIOUS (more pollutants and for longer periods) would provide crucial information around IAQ and its potential impacts on health.
7. How joined up is government in planning, policies and action towards national targets and fostering communication and data sharing between departments?
With respect to IAQ, there is no one government department that is responsible. It is relevant for DHSC, DESNZ, MHCLG, DfE, Defra and DfT. A coordinated approach is needed to bring together the relevant actors to develop rigorous and relevant policy.
For instance, UK Government policy around climate change means that we are attempting to make buildings more energy efficient through a massive programme of retrofitting, namely additional insulation. However, insulation makes buildings more airtight. There are numerous reasons why this would be concerning, not least the number and diversity of indoor sources of pollution, and the sometimes high air pollutant concentrations in homes. Making buildings more airtight means that exposures in the home will increase, potentially compounding a risk to health that is not yet fully understood. We therefore suggest that efforts to retrofit need strong alignment with regulation around ventilation, for example through PAS2035.
Some sources of poor IAQ are challenging to regulate, such as cooking and drying of clothes indoors. However, ventilation is regulated, and this is an effective means to improve IAQ (Du et al. 2019, Stamp et al. 2021, Hernandez et al. 2026). Moreover, new regulations could ensure that automatic cooker hoods (that vent outdoors) are a necessary requirement for all cooking appliances, and that tumble dryers (or communal laundry facilities) are provided in all properties without outdoor drying space.
There are also instances where joined up government policy could provide co-benefits. For instance, a policy to remove gas cookers from homes will help with our climate change commitments (assuming homes switch to green electricity), but will also improve IAQ, as it will lower exposures in homes to NOX and PM. A cross-governmental approach is required to ensure that we make the most of these win-win opportunities and avoid unanticipated adverse impacts.
8. How well is the Government spreading awareness of the impacts of poor air quality and promoting action being taken to tackle the issue?
There is now a lot of attention around outdoor air quality, but very little on IAQ. It was good to see the CMO mention the importance of IAQ in his 2022 annual report. However, most members of the public we talk to are still unaware that IAQ is an issue, and it would benefit from more government involvement. To start with, this could mainly be around education. The increasing use of low-cost sensors in residential environments by the general public could have a clear positive impact by increasing awareness of household exposure and empowering them to reduce environmental risks through behavioural change. However, we need to avoid putting stress on households who have no agency to improve their situation and there needs to be concomitant pressure (regulation) on landlords to pay for improvements in sub-standard housing rather than blaming tenants (e.g., for drying clothes indoors when they have no choice). Such regulation could be added as an extension to Awaab’s Law.
9. What are the economic or freedom of choice arguments for or against further action on air pollution?
The economic arguments for improving indoor air pollution support action because household exposure reduces productivity and places a substantial burden on health services. Freedom of choice arguments support action when framed around correcting structural constraints.
Improving health inequalities could involve: Interventions for deprived subgroups suffering the highest exposure burden; prioritising exposure reductions for children or the elderly due to the potentially large health benefits that could be realised; designing flexible, performance-based regulations; shifting costs towards actors with capacity (for example, large landlords, manufacturers of high-polluting products/ appliances).
10. How does UK air quality regulation compare with international counterparts? What comparisons or best practice can be learned from other countries? Has the UK kept pace with its international counterparts?
The balance between energy use, IAQ and thermal comfort calls for scientifically solid evidence and well-established guidelines for indoor exposures. The ISIAQ (International Society for Indoor Air Quality and Climate) has developed a database to share worldwide guidelines on IAQ, thermal comfort and ventilation. Germany and France have more developed systems for monitoring and managing IAQ with mandatory VOC emission labelling (France) and CO2 monitoring in Germany as a proxy for ventilation, while the USA (i.e. ASHRAE standards), Japan and South Korea incorporate WHO-aligned IAQ guidelines.
In the UK, there are governmental regulations for asbestos CO, NO2, formaldehyde and CO2 as well as temperature, and additional governmental guidelines for multiple VOCs, radon and relative humidity; however, these are not legally enforceable for homes, or indoor environments in general. As a result, the UK is lagging behind some high-income countries, which incorporate more structured IAQ regulations in their building codes or public health strategies.
In addition, CO2 is not systematically used as a proxy for ventilation in residential environments as is the case elsewhere in Europe, and regulations primarily focus on workplace/school ventilation. The UK suffers from the oldest housing stock in Europe with poor insulation and ventilation which makes our situation more challenging. Together with the increased rates of fuel poverty and climatic conditions (temperate and often humid), mould growth further deteriorates IAQ in homes.