Written evidence from Tom Parkes (NIT0056)

Environment and Climate Change Committee: Lords Select Committee

Nitrogen inquiry

Written evidence submitted by Tom Parkes, Air Quality Programme Manager at London Borough of Camden

1. Questions from the Committee following oral evidence at 10.00 on 23 April 2025

1.1.            Information on indoor nitrogen pollution (from gas boilers and stoves)

Nitrogen air pollution exists in indoor environments as well as in the air outdoors. Major indoor sources of nitrogen oxides (NOx) include cooking with gas (hobs and ovens) and gas heating appliances such as boilers and room heaters.

Research from the United States estimated that 12.7% of childhood asthma in the U.S. is attributable to air pollution from the use of gas stoves (hobs) for cooking, which is prevalent in approximately one-third of households.[1]

In the United Kingdom, research indicates that slightly more than half of households cook using gas (53.9%) and that more than 36 million people are regularly exposed to the nitrogen and particulate matter (PM) air pollution that gas cooking releases indoors.[2] This research modelled that kitchens with gas cooking appliances would regularly contain concentrations of nitrogen dioxide (NO2) higher than the World Health Organization (WHO) 24-hour average guideline limit (25µg/m3) and the UK legal one-hour average limit value (200µg/m3). For comparison, the one-hour limit value has not been exceeded since 2022 at the Euston Road air quality monitoring site, historically one of the most polluted reference standard air quality monitoring locations in the UK. This contrast highlights the potential for gas cooking at home to regularly expose occupants to levels of NO2 air pollution higher than would be measured alongside a busy road in Central London.

Exposure to gas cooking-derived nitrogen air pollution indoors can be reduced by replacing gas hobs with induction or electric hobs. A small research project from the U.S. found that gas hob replacement led to a 51% and a 42% median reduction in NO2 concentration in the kitchen and bedroom areas, respectively, in participating households at three months after the intervention.[3]

Although there is a growing recognition of the influence of indoor exposures on the overall health burden associated with air pollution, there remains a lack of research to support effectively targeted actions to mitigate this impact.[4] This includes, for example: the need for research exploring the relative significance of indoor and outdoor environments and attributable disease burdens; the specific characteristics that exist in some households and indoor spaces that predispose some occupants to relatively higher concentrations of pollutants; and the efficacy of mitigation measures in reducing exposures in a way that is accessible for the most vulnerable in society.

Gas is also widely used in commercial kitchens and the resulting NOx air pollution represents a contributing factor in the overall air pollution health hazard for the 159,000 people working as chefs in the UK (in 2019, according to data from the Office for National Statistics (ONS)).

The evidence for the impact of gas heating on indoor concentrations of NOx is more limited than that for gas cooking, and the primary mechanism for heating-derived NOx and the products of combustion entering the living space is thought to be from malfunctioning boilers or from exhaust flues situated inappropriately near to openable windows or air intake ducts.[5]

The Air Quality Expert Group (AQEG) highlights that the emission of indoor NOx from boilers and cooking appliances may be larger if appliances are poorly maintained and, therefore, that this could place lower-income households at heightened risk of NOx exposure at home.[6]

Lower-income households are typically exposed to higher concentrations of air pollution indoors air home and occupants are more likely to experience health vulnerabilities that increase the severity of the additional health risks from this indoor air pollution exposure.[7] In this way, indoor air pollution is an influential component of health inequalities and the health gap. The health conditions that are understood to be causally associated (or likely to be causally associated) with exposure to air pollution cost society billions of pounds each year in health and social care costs and reduced productivity.[8],[9]

1.2.            Impact of the ULEZ

The Mayor of London introduced the London Ultra Low Emission Zone (ULEZ) in April 2019, covering the same area as London’s Congestion Charge Zone in Central London. Boundary roads, including Euston Road in Camden, were not part of the ULEZ area and vehicles not meeting the ULEZ standards could travel on boundary roads without being subject to the ULEZ charge.

In October 2021 the ULEZ expanded to include the entire area bounded by the North Circular and South Circular roads. In August 2023 the ULEZ expanded again to replicate the London Low Emission Zone (LEZ) for heavy vehicles, bringing all 32 boroughs and the City of London into the ULEZ charging area.

The Mayor of London has published progress reports that are compiled by the Greater London Authority (GLA) and Transport for London (TfL) and reviewed by independent experts. The latest report, published in March 2025, addresses the first full year of operation of the now London-wide ULEZ since the final expansion in August 2023.[10]

The March 2025 report finds that the ULEZ and LEZ have reduced the number of older, more polluting vehicles driven in London and that this is reducing Londoners’ exposure to air pollution, although the report notes that the ULEZ and LEZ cannot be solely attributable for the improvement in air quality in London and that other policies and technological changes will have contributed to the reduction in air pollution.

Specifically, the report finds that:

• Roadside NO2 concentrations across the whole of London were, on average, 27% lower than would be expected without the ULEZ and its expansions.
• In Central London, Inner London, and Outer London roadside NO2 concentrations are 54% lower, 29% lower and 24% lower, respectively, than would be expected without ULEZ.
• The reduction in exposure to NO2 is larger in deprived communities in London.
• The area outside London and the ULEZ area has also experienced a reduction in roadside NO2, with an average 14% reduction within 5km of the ULEZ boundary compared to a scenario without ULEZ.
• Emissions of NOx, PM2.5 and CO2 are estimated to be 24% lower, 29% lower and 2% lower, respectively, than would be expected without ULEZ.
• There appears to have been no negative impact on footfall or spend in London as a result of the ULEZ.

Recent research from the Institute for Policy Research (IPR) at the University of Bath has found that the London ULEZ and LEZ have “significantly reduced levels of key pollutants, leading to improvements in physical health and mental well-being, and a reduction in hospitalisations for respiratory problems.” The research includes a simple cost-benefit analysis and estimates savings of £963m in avoided healthcare costs and economic losses in Greater London as a result of the ULEZ and LEZ schemes. This is determined to have far outweighed the costs associated with the schemes.[11]

Local authority air quality monitoring data can also be used to help assess changes in air pollution that is, in part, the result of ULEZ. Camden Council has established an extensive air quality monitoring network to understand the geographical distribution of air pollution within the borough and to assess temporal changes and how the rates of change vary according to location. Monitoring data is crucial for understanding the scale of the problem of air pollution, for appraising the impact of interventions to reduce air pollution, and for building public awareness and empowering local action using relevant, local data.

Camden measures NO2 air pollution using diffusion tubes and automatic continuous monitoring devices (referred to as ‘reference standard’ monitors). These are approved methods for statutory monitoring and reporting. The Environment Agency also operates an air quality monitoring station in Camden as part of the national Automatic Urban and Rural Network (AURN). In 2024 there were four reference standard NO2 monitoring sites in Camden and 262 diffusion tube NO2 measurement locations.

Data from Camden’s Euston Road reference standard monitoring site show that annual mean NO2 concentrations at this location reduced by 66% from 2011 to 2024 (2011 was the year in which monitoring commenced on Euston Road). Road traffic statistics show a 43% reduction in all motor traffic between 2011 and 2023 (the latest year with available data) on this part of Euston Road. The corresponding change in annual mean NO2 over this same time period was a 63% reduction. Comparison of the changes in measured air quality and motor traffic indicate that although the reduction in traffic was a contributory factor, it does not explain all of the reduction in roadside NO2 air pollution and that a change in the emissions from vehicles, in part because of the influence of ULEZ, is also a factor.

The annual mean NO2 concentration measured at Euston Road in 2011 was 123µg/m3; more than three times higher than the 40µg/m3 legal limit. In 2024 the annual mean NO2 concentration was 42µg/m3, which still exceeds the legal limit but by only 5%.

There are 43 diffusion tube NO2 monitoring locations in Camden with data from 2018 (the final year prior to the introduction of the ULEZ in 2019) and 2024. Annual mean NO2 concentrations have reduced at all monitoring locations, by an average of 20.1µg/m3, representing an average 45.4% reduction and with a minimum reduction of 7.8µg/m3 (19%).

In 2024, 98.5% (258) of the 262 diffusion tube monitoring sites in Camden were compliant with the 40µg/m3 annual mean legal limit for NO2. In contrast, in 2018 only 38.4% of 159 diffusion tube monitoring sites (61) complied with the legal limit. There has been a consistent, widespread reduction NO2 concentrations throughout Camden. This reduction in outdoor NO2 air pollution correspondingly reduces the health hazard posed by this pollutant and the health risk associated with spending time outside, during time spent commuting, school travel, work, recreation, and for people experiencing homelessness. The reduction in outdoor air pollution also reduces the magnitude of ingress of pollution into homes and other indoors spaces.

Although not related to ULEZ but rather to the earlier London Low Emission Zone (LEZ) for heavy vehicles, research by Avila-Uribe et al. (2024) found a statistically and economically significant improvement in children’s test scores for students within the LEZ area when compared against students outside of the LEZ.[12] The researchers found that the improvement was larger in low-performing schools, indicating a mediatory impact on health disparities.

Over time, the gradual replacement of vehicles in UK cities will lead to an increasingly larger proportion of vehicles on the road that comply with the ULEZ standards (Euro 6 for diesel vehicles and Euro 4 for petrol vehicles). Modern diesel and petrol vehicles have effective emissions abatement technologies and electric vehicles produce no NO2 at all at the point of use, although there is still an emission of PM from brake and tyre abrasion and from the resuspension of particles deposited on the road surface.

Nevertheless, it is important to recognise that the London ULEZ and LEZ and other targeted (emissions based) road charging schemes are effective in accelerating the transition towards less polluting vehicles and improving air quality faster than would otherwise occur through gradual fleet replacement. This helps to reduce the cumulative exposure to air pollution for residents and people travelling, working and learning in emission control areas.

However, there is a limit to the absolute improvement in air quality that vehicle charging schemes can deliver unless the vehicle compliance standards change over time and schemes are designed to discourage driving regardless of vehicle emissions standards, with active travel and public transport encouraged instead. In London it can be anticipated that the rate of reduction in NO2 concentrations will slow down and attention may need to shift towards interventions that tackle the other major NO2 emissions sources that have become dominant as vehicle emissions have declined.

New research from the National Centre for Atmospheric Science and the University of York has identified that the combustion of natural gas in boilers accounted for 72% of NOx emissions between 2021 and 2023 in the area within a few kilometres of the BT Tower in Central London.[13] The research notes that there has been a shift from road transport being the dominant source of NOx emissions in city centres, and that tackling emissions from building heating could be effective in delivering further improvement in urban air quality.

Besides ULEZ-style vehicle charging schemes and sustainable transport programmes, funding schemes to support the decarbonisation of heating through electrification in public sector, commercial, and residential buildings would help to drive further progress on tackling urban nitrogen air pollution.

1.3.            The importance of measuring ozone and any findings

Ozone is not released directly from any source but is formed in the air from reactions of primary pollutants (NOx, methane and volatile organic compounds (VOC)) in sunlight. Ozone concentrations are typically slightly higher in rural areas than in urban areas, albeit dependent upon local emissions of precursor species. Ozone concentrations usually peak in late spring although summer heatwaves and settled weather can also lead to elevated concentrations. Urban ozone concentrations have been increasing since 2000 which is understood to be the result of reductions in urban NOx emissions, due to the chemical interrelationship between these pollutant species.[14]

Ozone is a respiratory irritant and damages human health and plants. Exposure to ozone can cause immediate effects including shortness of breath, coughing, inflammation of the airways, increased susceptibility to infection and asthma attacks. Long-term exposure

Ozone is a relatively long-lived air pollutant and can be transported long distances, while ozone pollution episodes can affect large areas and increase exposure for many people. Due to the lower spatial variability of ozone compared to NOx, the fact that ozone is not emitted directly, and also because of the strength of evidence for the health harms associated with NOx and PM air pollution (both emitted directly within local authority administrative areas), local authorities have focused their air quality programmes primarily on reducing emissions of and exposure to NOx and PM, and have targeted their monitoring efforts similarly. As a result, there are fewer ozone monitoring sites than there are for other pollutants.

However, ozone monitoring data for 2024 for the limited sites within London show that some measurement locations experienced many days with sustained high concentrations of ozone above the 100µg/m3 eight-hour average limit objective, whereas other sites within the same or neighbouring boroughs measured many fewer exceedances of this objective. This monitoring data highlights that within a city area there is a possibility that some communities and neighbourhoods may be exposed to much higher concentrations of ozone air pollution and for longer periods of time than others, and indicates the possibility of acute exposures at certain times of year which may disproportionately affect vulnerable groups and people working or spending time outdoors. For this reason, local authorities should not neglect ozone monitoring, although they will likely require grant funding to support additional monitoring costs.

Even in the absence of monitoring data it may still be important for the protection of public health for local authorities and healthcare professionals to seek to build public awareness about the health impacts linked to ozone exposure during spring and summer heatwaves, and the personal protective actions that can be taken to protect health and alleviate pressures on the healthcare system.

2. Additional written evidence

2.1.            Comparison of UK NO2 data and UK air quality legal limits

In 2004 there were 300 reference standard automatic air quality monitoring sites providing annual mean NO2 concentration data in the UK. 100 of these sites measured exceedances of the 40µg/m3 legal limit in 2004.

In 2024, only eight of 508 (1.6%) reference standard NO2 monitoring sites measured exceedances of the legal limit. Additionally, no monitoring sites failed to comply with the National Air Quality Objective for one-hour NO2 concentration not to exceed 200µg/m3 more than 18 times in a year, and across the entire monitoring network only 11 sites measured any one-hour periods higher than 200µg/m3 in 2024, with a grand total of 33 one-hour periods exceeding this short-term limit.

In contrast, in 2004 59 sites recorded one or more one-hour average NO2 concentration higher than 200µg/m3, with 17 sites breaching the maximum legally permissible 18 exceedances in a single year. In total across the entire monitoring network there were 5,654 one-hour periods with NO2 concentration higher than 200µg/m3 in 2004.

As noted in 1.2. above, monitoring data in Camden mirror this trend of large reductions in urban NO2 concentrations that have occurred in many parts of the UK.

The significant reduction in annual mean NO2 concentrations and in the number of short-term exceedances of the 200µg/m3 threshold is evidence that many of the measures to curb nitrogen air pollution in the UK have been effective.

However, it is now generally accepted that air pollution has the potential to adversely affect human health at any concentration, with the risk for health damage increasing with higher exposures but with a residual potential for health harm even at low concentrations. Additionally, some research has indicated a supra-linear relationship between air pollution exposure and health outcomes, meaning that an incremental increase in pollution exposure would be expected to lead to a larger adverse health outcome at low concentrations than at higher concentrations.[15],[16] This indicates that it is advantageous to continue to seek to reduce air pollution concentrations and population exposures even where legal limits have been achieved.

Considering the expense of reference standard air quality monitoring, it is conceivable that some local authorities may consider discontinuing monitoring if the legal limits for NO2 are met at their measurement sites. Further, it is possible that some local authorities may consider revoking Air Quality Management Areas (AQMA) for NO2 and, where this happens, there is a diminished expectation for the implementation of action plans to improve air quality.

In these circumstances it may become more challenging for local authorities to produce a strong policy rationale for continued emissions-reduction measures through, for example, planning controls and building design, construction management, sustainable travel, the electrification of heating, and cleaner standby power.

Compliance with the UK legal limit for outdoor NO2 does not mean that air pollution is not still damaging public health, or that local authorities, national government and other relevant public bodies should not take action to reduce population exposure. To sustain continued effort to further reduce air pollution – for the benefit of public health, health equity, the economy and public finances – it would be advantageous to revise down the legal limits for NO2 and other air pollutants.

Camden Council has voluntarily adopted the WHO annual mean guideline limits for NO2 and PM air pollution (10µg/m3 and 5µg/m3, respectively) with a deadline for achieving these levels at all monitoring sites no later than 2034.[17] This strategic commitment has provided a clarity of vision and secured the policy basis for ongoing action to protect public health from air pollution even where the legal limits are satisfied. 

Given the evidence for risks to human health at low concentrations of NO2 and PM air pollution, it would be advisable for government to consider adopting the WHO guideline limits for these pollutants and for the WHO interim targets to form a pathway towards compliance. This would help to ensure a supportive policy environment for ongoing action to reduce pollution and improve public health.

It is worth noting that the European Union introduced a revised Ambient Air Quality Directive in December 2024 which sets a new 20µg/m3 annual mean target for NO2 for Member States to meet by 2030. The new EU NO2 target is significantly more ambitious than the UK legal limit and leaves us with the laxest legal air quality targets in Western Europe.

Additionally, standards for indoor air quality could be introduced for the protection of public health in all environments, because indoor spaces can contain high levels of air pollution even in places with low levels of outdoor air pollution.[18]

2.2.            Emissions of nitrogen can lead to exposures to other health-damaging pollutants

In addition to nitrogen emissions posing a direct risk to health, nitrogen-containing species are precursors for other pollutants including ozone (as covered in 1.3.) and PM. Ozone and PM are more easily transported over large distances than NOx and it is therefore possible for NOx and other nitrogen species (including ammonia) emitted in one location to affect air quality and public health somewhere else.

The UK-AIR maps of current AQMAs for NOx and PM show that the regulatory focus on nitrogen and PM is centred on urban areas, even though a non-insignificant share of the PM air pollution in the air in these locations may be in the form of ‘secondary particulate’ from sources including nitrogen emitted from agriculture, shipping, aviation, diesel rail, intercity road transport, industrial activities and power generation.[19] These emissions sources may themselves not fall within an AQMA boundary or the administrative area of a local authority that has declared an AQMA and, consequently, not be subject to scrutiny or effort to reduce air quality impacts as part of a cohesive local strategic approach for protecting public health both locally and farther afield. For example, agricultural ammonia emissions have been determined to contribute to a larger share of urban fine particulate matter (PM2.5) concentrations than local urban sources in several UK cities including Leicester (where agriculture is responsible for 38% of PM2.5), Birmingham (32%) and London (25%).[20]

There is a need to:

• Recognise that emissions of airborne nitrogen influence not only the health and biodiversity risks associated with exposure to NOx air pollution, but also the risks associated with other pollutants for which nitrogen is a precursor (i.e. ozone and PM), and understanding that emissions in one location can damage public health elsewhere.
• Consider revising the way the Local Air Quality Management (LAQM) regime is applied to more effectively ensure that action is taken on sources of nitrogen and other air pollutants regardless of whether or not there is measured population exposure to nitrogen or a declared AQMA in a given locale. This could include revising down the legal limits for air pollution to align fully or more closely with the WHO guidelines.
• Ensuring all local authorities and relevant public bodies are afforded a suitable opportunity to engage with and contribute to the development of strategies, policies, legislation, consultations and other measures to reduce nitrogen and other air pollutants from all emission sources and sectors, regardless of whether or not those sources and sectors are relevant emissions sources within the administrative boundaries of all local authorities. For example, ensuring all local authorities have an opportunity to participate in the consideration of reducing ammonia emissions from agriculture because of the significant contribution of ammonia emissions to air pollution even in urban areas without any agricultural activity.

END OF WRITTEN EVIDENCE

16/05/2025