Search

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.

Air pollutants in the exhaust emissions from internal combustion engine (ICE) vehicles result from unburnt fuel, high temperature combustion of fuel in the presence of air, and from exhaust after-treatment processes. As many of these pollutants are harmful to human health, impact climate as greenhouse gases, or both, emissions from vehicles have been subject to regulation for many years. Europe-wide standards on vehicle exhaust were introduced in the early 1990s and have been progressively made more stringent, extending the range of regulated pollutants included and reducing the permissible levels of emissions in new vehicles introduced to the fleet. A review of the development and evolution of exhaust emissions standards of relevance to UK vehicles is provided in Chapter 2 of this report.To comply with these standards, technologies for reducing emissions of regulated pollutants have been developed and fitted to vehicle exhaust systems. In combination with advances in engines, these have substantially reduced emissions of many of the regulated pollutants, and a review of the key technologies has been undertaken. Whilst there has been considerable success in substantially reducing exhaust emissions of pollutants such as carbon monoxide (CO), hydrocarbons (HCs) and particulate matter (PM), exhaust emissions from ICE vehicles remain a major source of nitrogen oxides (NOx) and an important contributor to poor urban air quality.Ambient atmospheric measurements have previously revealed important disparities between urban and roadside concentrations of nitrogen dioxide (NO2) and those that would be projected based on the prevailing vehicle emissions standards of NOx. It is now understood that exhaust gas after-treatment technologies introduced on diesel vehicles have not always been effectively implemented and not delivered the anticipated reductions in emissions when in real-world use. The on-road effectiveness of exhaust emissions controls has been found to be highly variable even between vehicles that are technically compliant with the same regulatory standards. There is now greater appreciation of the need to evaluate and demonstrate vehicle emissions performance under real-world driving conditions. On-road emissions have been higher than those measured during laboratory based tests, although in the past five years that gap has closed for some vehicles, and improvements continue. Future European standards will move close to demanding equivalence between on-road and laboratory performance. Real-world driving emissions are reviewed for various vehicle types in Chapter 3.The dynamic nature of the vehicle fleet means that vehicles with a range of different exhaust control technologies and historic standards are in use on UK roads. Between 1990 and 2015, the UK vehicle fleet transitioned to a greater proportion of diesel cars, compared with gasoline, a change that was motivated by predicted fuel economy savings and lower CO2 emissions per mile driven. The relative contributions of exhaust emissions from diesel, gasoline, and more recently hybrid electric vehicles continue to evolve as new technologies develop, and the regulatory framework and consumer preferences (in part driven by economic policy such as fuel tax) change. The vehicle fleet will continue to change over the next decade with older more polluting vehicles leaving the fleet and newer cleaner vehicles joining. This report provides, in Chapter 4, modelled estimates of the vehicle fleet composition over time and the resulting exhaust emissions using best available estimates from inventories used for international reporting. Understanding how the future ensemble of vehicles on the roads will contribute to air pollution emissions is significant for policy development since this information can support the evaluation of interventions such as low emissions zones, and the likely compliance with air quality directives. This report was drafted prior to the COVID-19 crisis and projections of future vehicle use are based on knowledge and understanding at that time. Since March 2020 traffic volumes have varied significantly depending on national restrictions and this has influenced roadside concentrations of many vehicle-derived pollutants. AQEG provided an initial analysis of these effects to Defra in June 2020. The post-COVID fall in new vehicle sales and economic conditions which may affect transport in the future are not assessed here.Ambient observations of air pollution, particularly at the roadside, provide an independent assessment of vehicle exhaust emissions, trends in these emissions over time, and by extension whether emissions controls are working as intended. Methods to assess vehicle exhaust emissions using UK ambient air quality data for a range of different pollutants are presented in Chapter 5, which also includes examples of where ambient observations have detected unanticipated trends in emissions not predicted based on laboratory tests.Chapter 6 identifies possible future interventions and technologies to reduce exhaust emissions and their impacts, including after-treatment approaches. The longer-term impacts on vehicle exhaust emissions from increased use of ICE-electric hybridisation propulsion and driver automation technologies are considered, as are potential approaches to retrofitting as a means to improve emissions performance for existing vehicles. Potential regulatory approaches to managing exhaust emissions are also reviewed, including the possible impacts of changed vehicle inspection regimes, new fuels standards and external traffic management controls on vehicle movements and on idling.Chapter 7 provides the AQEG recommendations.

Ozone near the planetary surface is considered an air pollutant because of its adverse effects on human health, crop yields, and ecosystems more broadly. In addition, ozone in the free troposphere is a greenhouse gas with climate impacts. Ozone is a relatively long-lived air pollutant so can be transported for periods of days to several weeks over large distances. The concentrations that arrive over the UK in the prevailing westerly winds from the Atlantic are influenced by the cumulative effects of emissions that occur throughout the mid-latitudes of the Northern Hemisphere.UK population and ecosystem exposure to ozone is frequently lower than the north Atlantic baseline due to dominant ozone sink processes of removal by reaction with local emissions of NO and deposition to the terrestrial surface. In spring and summer, net ozone production from UK emissions, in addition to advection from mainland Europe, leads to episodes of elevated concentrations.Control of surface ozone is complex since it is not emitted directly from any source but is formed in the atmosphere in a non-linear manner from reactions involving nitrogen oxides (NOx), methane, volatile organic compounds (VOCs) and sunlight. Air quality policies have been in place to reduce surface ozone since the 1980s, through control of emissions of NOx and VOCs from sources such as vehicles, combustion, fuels and solvents. The 1990s saw reductions in peak concentrations and fewer severe ozone pollution events – a consequence of substantial reductions in emissions implemented internationally through agreements such as the Gothenburg protocol. Overall, since the UK is a net sink for ozone, changes in ozone concentration are controlled by the changes in the chemical and deposition sinks as well as changes in baseline concentration and local chemical production.The distribution of ozone across the UK shows highest concentrations over upland and rural locations, with annual average concentrations of >60 μg m-3 widespread over rural areas in Scotland, Northern Ireland, Wales, south-west England and the Pennines. Urban and suburban concentrations are typically ~15 μg m-3 lower than rural values, although this is very sensitive to local emissions from road transport, and in heavily-trafficked roadside locations ozone concentrations can locally be close to zero. Ozone typically reaches its highest concentrations in the UK during April and May, although anticyclonic weather patterns in summer can also lead to significantly elevated concentrations.Since 2000 there has been little observed change in UK rural ozone concentrations, or ozone in the air arriving from the wider Atlantic region to the UK. There have however been some upwards trends in suburban and urban ozone (average increases of the order 5 – 9 μg m-3 over the 20-year period 2000-2019). These upwards trends in suburban and urban ozone are a consequence of reductions in primary emissions of NO; the NO has the effect of locally suppressing ozone.All areas of the UK have been compliant with the EU target value for the protection of human health for at least the last 12 years (≤25 days per year with daily maximum 8-hour mean ozone concentration >120 μg m-3, averaged over 3 years). Substantial parts of the UK are however routinely not compliant with the EU long-term objective of zero exceedance of this metric, although the number and geographical extent of these exceedance days has generally been declining over the last 20 years. All areas of the UK have been compliant with the EU target value for the protection of vegetation for at least the last 12 years (AOT40 not exceeding 18000 μg m-3.hour per year, averaged over 5 years). The EU long-term objective for the protection of vegetation is lower (AOT40 The prediction of future surface ozone in the UK is complex and subject to multiple drivers. Wider hemispheric trends in ozone resulting from global trends in emissions, in particular of methane, will have a significant impact on UK concentrations, but the direction of change is unclear. Further UK reductions in NOx emissions are anticipated and these will likely lead to further increases in urban ozone but be beneficial more widely and reduce ozone at a transboundary scale. Projections indicate little change to UK VOC emissions in the coming decade and this will become a limiting factor for reducing ozone in an optimal manner. Climate change will have multiple effects on ozone precursor emissions, chemical production and loss, dispersion and deposition. Whilst climate change is likely to lead to periods of higher temperatures that are known to increase biogenic VOC emissions and to reduce the surface dry deposition of ozone, the net effect on ozone of all climate-change influences is not known.As other air pollutants in the UK continue to decline it seems likely that the importance of ozone will grow, especially as its contribution to climate provides further motivation for control measures.

There have been significant changes in the emissions of air pollutants from several sectors, but, with the exception of the transport sector which showed a marked decrease, availability of activity and emissions data for the lockdown period is still limited.- UK air quality has been negatively influenced by a significant change in meteorology between the weeks preceding and following the lockdown in addition to changes (both positive and negative) arising from actions in response to COVID-19.- The most pronounced changes in UK air quality during lockdown have been in the urban environment, notably for nitrogen oxides (NOx). Once weather effects are accounted for, mean reductions in urban NOx averaged over the lockdown period considered have been typically 30-40%, with mean NO2 reductions of 20-30%. In general, NOx and NO2 reductions have been greater at roadside than at urban background sites. These reductions would typically correspond to decreases in concentrations of 10-20 ug m-3 if expressed relative to annual averages.- Meteorological conditions have led to higher PM2.5 during lockdown than the average experienced in equivalent calendar periods from previous years. Analysis combining observations and models indicates however that PM2.5 concentrations were of the order 2 - 5 ug m-3 lower in Southern England than would have been expected under a business-as-usual emissions scenario. The changes to UK PM2.5 in terms of contributing sources and transboundary influences have yet to be determined.- Changes to population exposure to air pollution are variable and more uncertain than estimates of changes in ambient concentrations. Some urban locations have seen significant falls in NO2, and wider working from home has reduced travel exposure more generally in cities. In London, initial estimates of reduction in PM2.5 exposure compared to business-as-usual are in the range 5-24% depending on factors such as commuting mode.- Little is known about the impact of lockdown on indoor air quality, since homes are not routinely monitored in the UK. Whilst exposure to pollution in the workplace and during commuting will have likely reduced for many people, increased time spent on activities in the home such as cooking and cleaning may have increased emissions and concentrations of pollutants such as PM2.5 and Volatile Organic Compounds (VOCs).- Increased ozone has been observed at some urban monitoring stations, a result of lower local NO. Models suggest the responses in UK ozone for this summer compared to business-as-usual are variable, with no single direction of change, although there may be some modest increases in urban areas and in central and south-eastern parts of the UK.- Long-term exposure to air pollution is associated with increased morbidity and mortality from chronic diseases, some of which have also been identified as increasing the risk of severe COVID-19 symptoms. Given this, it would not be surprising if there was a link between exposure to air pollution (past or present) and the occurrence or severity of COVID-19 infection. Whilst several unpublished studies have examined this effect, and have reported associations with past exposure to both PM2.5 and NO2, there is currently no consensus on the pollutant responsible or the magnitude of any effect. Such studies require very careful control for confounding influences, and further work is needed before there can be confidence in their findings. - Very small amounts of RNA from SARS-CoV-2 have been observed in outdoor particulate matter but it is not yet known whether breathing air outdoors provides a significant route for transmission of live virus or infection. Whilst aerosol containing the virus can build up indoors in poorly ventilated rooms, dilution is rapid in an open outdoor environment, which is likely to reduce the dose of virus inhaled compared to indoors. The lifetime of active virus in the outdoor atmosphere has yet to be determined.

In this short report, the Defra Air Quality Expert Group (AQEG) has evaluated the Further Ambition scenario developed by The Committee on Climate Change (CCC) in 2019. See: https://www.theccc.org.uk/publication/net-zero-the-uks-contribution-to-stopping-global-warming/An initial assessment has been made of the possible impacts on air quality, primarily from the perspective of five pollutants for which either ambient standards or national emissions limits exist (PM, NOx, O3, VOCs, NH3). In this assessment AQEG has attempted to differentiate between uncertainty associated with the underlying science of a particular process or technology, for example ‘does an activity emit a particular type of pollutant, and if so in approximately in what quantities?’, and with the broader uncertainties that relate to the scale of uptake, implementation and mitigation.The assessment by AQEG is essentially identifying hazards rather than risks, and it is understood that for every action area within the Net Zero strategy some form of hazard mitigation is likely to be introduced, or the hazard managed through existing regulatory processes such as the permitting of emissions. In some cases, the technologies that are highlighted as part of a Net Zero pathway do not currently operate in the UK at industrial scale, notably for Carbon Capture and Storage (CCS), and there is potential for the emergence of unanticipated new hazards as a consequence of the materials used. In uncertain technology areas such as this, it is accepted that existing systems for risk assessment will be applied, and the hazards identified here may not catalyse into significant risk once mitigation is in place.The exact details of the realisation of each action are often critical to whether the air quality effects might be positive or negative. In some areas the selection of a specific technology pathway or implementation strategy could potentially lead to sub-optimal outcomes, and worsened air pollution emissions compared to plausible counterfactuals. AQEG aims to highlight those areas where air quality emissions and outcomes should be a factor that is considered in Net Zero design and specification, and to guide thinking around which realisations of individual Net Zero proposals would bring the greatest beneficial improvements in air quality.Encouragingly, for virtually all of the changes proposed on the CCC Net Zero pathway, positive, improved and better air quality outcomes can be envisaged.The report does not explicitly address the broader issue of societal health benefits arising from Net Zero, although in some cases we do identify where low carbon technologies may have different air quality impacts in different geographic areas. In this report we do not weight changes in different pollutants by their specific health outcomes, but view reductions in ambient concentrations of all major regulated pollutants as equally valuable. Follow-on work jointly with the DHSC Committee on the Medical Effects of Air Pollution (COMEAP) is anticipated to refine further which actions would bring the largest health benefits to the UK. Our approach to identifying possible air quality effects is restricted to those that would be experienced in the UK, but it is important to appreciate the enormous impacts of the manufacture, transportation and disposal of goods (including clean technologies) from outside of the UK. There is clearly significant potential for the export of air pollution emissions, and by extension adverse air quality, to other countries as part of the decarbonisation transition.

Volatile organic compounds (VOCs) are a broad class of air pollutants which act as precursors to tropospheric ozone and secondary organic aerosols (a component part of PM2.5). The National Atmospheric Inventory (NAEI) indicates that UK emissions of anthropogenic VOCs peaked around 1990 at 2,840 kt yr-1 and then declined to ~810 kt yr-1 in 2017. Notable has been success in reducing emissions from the tailpipe of gasoline vehicles and other evaporative losses of VOCs from fuels (including natural gas) during their production and distribution. Ambient observations of selected VOCs in the Defra Automated Hydrocarbon Network also show significant declines since the 1990s, including species that are emitted directly from fuel loss, such as alkanes and mono-aromatics, and VOCs that are by-products of incomplete combustion such as alkenes and ethyne. The rates of reduction in ambient concentrations slowed around 2010 and have now plateaued. Benzene and 1,3 butadiene have specific limit and target values in the UK and concentrations of these have been successfully reduced such that the UK has reported no exceedances in recent years. Whilst both emissions and concentrations of VOCs have fallen, further reductions in VOC emissions are anticipated for the UK to meet obligations under the National Emission Ceiling Directive in 2030 and UNECE Convention on Long-Range Transport of Air Pollution.The relative contribution to UK emissions from solvents is estimated to have increased over the past 20 years, in 2017 representing ~74% of national emissions. Notable has been a post-2000 growth in emissions of oxygenated VOCs, none of which are routinely measured in regulatory networks. Ethanol is now the largest VOC emitted by mass (~136 kt yr-1 in 2017 or ~16.8% of total UK emissions) followed by n-butane (52.4 kt yr-1) and methanol (33.2 kt yr-1). Alcohols more generally have grown in significance, representing ~10% of VOC emissions in 1990 rising to ~30% in 2017. The growth in ethanol is due to increased reported emissions from the whisky industries and in estimated domestic use of ethanol as a solvent, for example contained within personal care, car care and household products. For some simple hydrocarbons there have also been notable changes in the major contributing sources. N-butane for example is currently the second most abundant VOC in the UK inventory; in 1990 n-butane was emitted overwhelmingly from gasoline extraction and fugitive distribution losses (139.8 kt yr-1). In 2017 the largest anthropogenic source of n-butane in the inventory was from its use as a domestic aerosol propellant (25.5 kt yr-1), with the gasoline/fugitive losses having been reduced to 23.3 kt yr-1.Recent changes in the contributing VOC sources to emissions then impacts on the observational strategies to verify emission reduction policies. In 1992 UK national monitoring in the Defra Automated Hydrocarbon Network quantified 19/20 of the most abundant anthropogenic VOCs emitted (all non-methane hydrocarbons), but by 2017 monitoring captured only 13/20 species. To evaluate progress across Europe towards meeting the future VOC emissions targets requires a revision of ambient monitoring strategies. Adding ethanol, methanol, formaldehyde, acetone, 2-butanone and 2-propanol to existing non-methane hydrocarbon measurements would provide full coverage of the 20 most significant VOCs emitted on an annual mass basis in the UK.