Research and analysis

COMEAP advice note

Published 27 July 2022

Summary

1. The Committee on the Medical Effects of Air Pollutants (COMEAP) welcomes the Department for Environment, Food and Rural Affairs (Defra)’s proposals to reduce levels of air pollution and to introduce stricter targets for fine particulates in air (PM_2.5). In particular, we think the population exposure reduction target (PERT) will be important in maximising benefits to public health.

2. COMEAP provided health advice to Defra which helped shape the targets, but we did not set the values of the proposed targets. We regard the targets as health-informed – as information on the health effects of air pollution was taken into account when developing the targets – but not health-based, as the target values have not been derived directly from the health evidence. The development of the targets has also focused on the requirement (in the Environment Act 2021) to set targets that can be met.

3. In this statement, we raise specific points on the evidence provided by Defra in its report on how the targets were developed and in its accompanying impact analysis. We also make some recommendations, which we hope will be helpful to Defra. We:

• emphasise the health benefits of continued reductions in PM_2.5 levels and recommend that Defra should consider the role of local authorities and others in continuing to improve air quality, even where the legally binding targets are met
• think that interim targets will play an important role in ensuring early progress to maximise health benefits, and we encourage Defra to publish these soon
• comment on the importance of considering inequalities and recommend using monitoring and/or modelling to assess whether disparities in exposure are reduced in future
• reiterate our advice to examine the relationship between long- and short-term levels of PM_2.5, to ensure that the targets for long-term levels protect people from effects of short-term elevated exposures
• mention the importance of reducing other pollutants, such as nitrogen dioxide (NO₂) and ozone (O₃), in addition to PM_2.5 and recommend continuing to assess their levels to ensure that there are no unintended consequences of the focus on reducing PM_2.5 levels
• note the benefits suggested by the government’s impact assessment and recommend that the costs and benefits of detailed plans to deliver compliance with the targets are assessed using the impact pathway approach
• highlight the importance of continuing to engage internationally to ensure that pollution from other countries does not prevent the targets being met

Introduction

4. COMEAP has previously provided advice to Defra regarding the health evidence relevant to setting targets for PM_2.5 concentrations under the Environment Act 2021. Since our most recent advice (published in March 2022) Defra has launched a consultation on proposed targets.

5. A report detailing the evidence evaluated by Defra in developing the PM_2.5 targets, and an impact analysis of the types of policies and interventions that might need to be implemented to achieve them, have also been published. COMEAP discussed Defra’s evidence report and impact analysis at a meeting held on 10 June 2022. Defra officials attended the meeting to address any questions or requests for clarification. The statement also reflects members’ comments made, since the meeting, via correspondence and a short discussion.

6. Defra’s Environment Act Targets consultation asks whether respondents agree or disagree with the level of ambition of the targets. Such views fall outwith COMEAP’s remit, which is to provide advice on the evidence linking air pollutants with health effects. Nonetheless, we think it will be useful to draw aspects of the health evidence to Defra’s attention, for consideration along with responses to the consultation. This statement summarises these aspects, which are raised in response to the published evidence report and impact analysis.

Background

7. The Environment Act 2021 requires 2 air quality targets to be set, one of which must be a maximum annual average concentration of PM_2.5 in outdoor air. Defra has proposed that the second target should also be for PM_2.5, with the focus being on reducing overall population exposure to PM_2.5.

8. COMEAP has previously provided advice1 to Defra regarding the health evidence relevant to developing health-based targets for PM_2.5:

• initial advice following a workshop in July 2020 (published in July 2021)
• detailed advice in March 2021 (published in July 2021)
• an update to previous advice (published in March 2022) in the light of revised World Health Organization (WHO) Air Quality Guidelines

9. Our advice included that:

• a focus on reducing long-term average concentrations of PM_2.5 is appropriate
• newer evidence indicates that PM_2.5 pollution can have harmful effects on people’s health at lower concentrations than had been studied previously
• continuing to reduce concentrations to, or below, the WHO’s new air quality guideline (5µg/m³) would benefit public health
• there is less evidence for benefits of reductions below this level, although the available studies have not indicated a threshold of effect below which there is no harm
• reducing exposure of the whole population would achieve the greatest overall public health benefit
• some individuals or groups are more at risk, but it might be difficult to reflect this in a national targets framework
• reducing air pollution to low levels is likely to be challenging, and cost-benefit assessments may play a role in defining targets
• the health benefits of reducing other pollutants, such as nitrogen dioxide (NO²) and ozone (O³), should not be overlooked

10. As well as COMEAP’s advice, Defra has drawn on advice received from the Air Quality Expert Group (AQEG) and during a series of technical workshops. The Environment Act 2021 requires that any targets set can be met, so Defra has also commissioned modelling of the reductions in concentrations that can be achieved by different packages of interventions, along with cost-benefit assessments. A detailed evidence report, and a summary impact analysis were published by Defra in May 2022. These explain the approaches, and analyses, underpinning the proposed targets.

11. The air quality targets proposed in the public consultation published on 16 March 2022 are:

• annual mean concentration target (AMCT, ‘concentration target’) – a target of 10 micrograms per cubic metre (μg/m³) to be met across England by 2040
• population exposure reduction target (PERT, ‘exposure reduction target’) – a 35% reduction in population exposure across England by 2040 (compared to a base year of 2018)

12. Legal compliance with the targets will be assessed using measurements at monitors within Defra’s Automatic Urban and Rural Network (AURN). The concentration target is to be achieved at all monitors in England in the AURN by 2040. Compliance with the exposure reduction target will be assessed using AURN monitoring sites across England in 2040 considered to be in locations representative of typical concentrations experienced by the population within a region. These are likely to comprise ‘urban background’ sites and some ‘suburban’ sites.

13. Defra considered 3 possible scenarios in assessing PM_2.5 concentrations that might be achievable across England: ‘medium’, ‘high’ and ‘speculative’, in addition to a ‘baseline’ (business as usual) scenario. The 3 scenarios represent packages of additional possible interventions that could be implemented in the future, ranging from well-established mitigation methods to (in the ‘speculative’ scenario) technologies that have not yet been fully developed or implemented at scale. The speculative scenario also included policies that were considered to represent behaviour changes that were too dramatic to be practicable. Emissions trajectories for each scenario were produced and combined with air quality modelling to produce feasibility curves of how likely it was that different concentrations would be reached by certain dates. The proposed targets correspond to the ‘high’ scenario. The impact analysis indicates that this scenario will be cost-beneficial.

Overview comments

14. COMEAP welcomes Defra’s proposals to reduce levels of air pollution and to introduce new, stricter, legally binding targets for PM_2.5.

15. We note that Defra has drawn on advice from COMEAP in the formulation of the targets. For example, specifying PM_2.5 (mass concentration) as the metric, and focusing on reducing long-term average concentrations. This reflects the evidence that associations with long-term average concentrations of PM_2.5 suggest a greater effect on public health than those seen in studies of short-term fluctuations, and that the available studies do not allow firm conclusions to be drawn about the relative toxicity of particles of different composition or from different sources.

We consider it important that the proposals include a population exposure reduction target which will help to ensure that health benefits across the whole population are maximised, as well as a maximum allowable annual mean concentration target to ensure that the highest concentrations are reduced. Epidemiological studies have not identified a threshold of effect for PM_2.5 exposure, making it important to focus on continuous improvement, even if levels are below a legal concentration target.

16. We were not specifically consulted on the targets themselves, nor involved in their derivation. We note that neither the concentration target of 10µg/m³ PM_2.5, nor the exposure reduction target, are derived directly from the available health evidence or a cost-benefit assessment. Therefore we regard the proposed targets as health-informed, rather than health-based, targets. Nonetheless, the aim is clearly to benefit public health. We appreciate that the Environment Act 2021 requires that any targets set can be met, and that achievability has therefore been a key concern in proposing numerical values for the concentration and exposure reduction targets. It has not been our role to advise on achievability, nor how this requirement is interpreted.

17. Although we consider that a focus on reducing long-term average concentrations of PM_2.5 is appropriate, the health benefits of reducing concentrations of other pollutants (for example, NO₂ and O₃) should not be overlooked. The impact analysis includes valuation of the expected health, ecosystem and climate benefits that would arise from interventions implemented to reduce PM_2.5 concentrations to meet the proposed targets. It indicates that this value would substantially exceed the anticipated costs. The methods used include valuation of benefits arising from reductions in emissions and concentrations of pollutants other than PM_2.5. In particular, we draw attention to important co-benefits identified in the analysis, such as reductions in carbon dioxide emissions, which will help to mitigate climate change.

18. Aspects relevant to these points are discussed in more detail in subsequent sections of this statement.

Reduction in PM_2.5 concentrations

19. Defra’s evidence report indicates that, in 2018, the highest measured PM_2.5 concentration was 16μg/m³ and that 46% of monitoring sites in England measured over 10μg/m³. The UK’s current concentration limit value is 20μg/m³. (see UK Statutory Instrument 2020 No 1313 The Environment (Miscellaneous Amendments) (EU Exit) Regulations 2020.) Defra’s proposed targets therefore represent an important tightening in legal limits and a reduction in expected population exposure.

20. COMEAP considers the exposure reduction target to be of particular importance. This is because the introduction of concentration targets in isolation could result in policies focused on introducing measures to address the concentrations at a relatively small number of hotspot locations, rather than on wider interventions which would benefit the whole population. It could also mean that concentrations are allowed to increase at locations where concentrations are lower than the concentration target. We therefore wish to emphasise the importance of reducing population exposure and achieving the PERT. We feel that this should be given a high priority, and would reinforce that it should not be regarded as less important than achieving the concentration target.

21. We have previously observed (in COMEAP 2022. ‘Advice on health evidence relevant to setting PM_2.5 targets: update’) that the current WHO air quality guideline for PM_2.5 is 5µg/m³, and that the available evidence has not identified a population threshold for health effects for PM_2.5.

Taken at face value, the proposed concentration target of 10µg/m³ therefore seems high. However, we acknowledge the significance of the proposal that all types of monitors within the AURN will be used to assess compliance with this target. This includes monitors at kerbside locations, which usually experience concentrations of PM_2.5 considerably higher than those representative of exposure of most of the population.

(The kerbside monitors are located in accordance with the requirements of the EU Ambient Air Quality Directive, which specifies that traffic-orientated sampling probes shall be at least 25 metres from the edge of major junctions and no more than 10 metres from the kerbside.)

Difficulties in achieving reductions at these types of locations is the primary reason that Defra does not expect to meet the concentration target until 2040, under the ‘high’ scenario.

Some people, especially in inner-city areas, live alongside busy roads. Therefore, although concentrations measured at kerbside monitors are not representative of exposure of the majority of the population, they may be relevant to the exposure of some of the most deprived people in our society.

22. The available evidence indicates that continuing to reduce PM_2.5 concentrations below 10µg/m³ will be beneficial to public health. It will therefore be important to encourage local authorities and other bodies to continue to exert effort to improve air quality, even if the concentration target has been met.

Interim targets

22. Defra’s evidence report explains that 5-yearly interim targets will be set to ensure suitable progress is made towards meeting the targets over the coming years. There are currently no published proposals for what these targets might be. Our view is that the interim targets are likely to play an important role in ensuring continuous progress in reducing PM_2.5 exposure, as well as evaluating whether the policies implemented are having the expected impact.

22. We emphasise that it will be beneficial to get as much exposure reduction as possible, as early as possible, to maximise the public health benefits.

Consideration of inequalities

23. We previously recommended that Defra should investigate whether proposed or implemented interventions reduced inequalities in exposure or had undesirable consequences for inequalities (for example, by increasing concentrations of pollutants in areas of socioeconomic deprivation). We welcome the findings of modelling undertaken for Defra, summarised in the detailed evidence report, which indicates that improvements in disparities in exposure to PM_2.5 would be expected for all 3 of the scenarios evaluated.

24. We note that the evidence report states that consideration will be given to locating a proportion of monitoring in areas of deprivation. We think it will be important to use monitoring and/or modelling to assess whether the expected reductions in disparities in exposure have been achieved, and to ensure that there are no unintended consequences for areas of greater socioeconomic deprivation. This is particularly important as there is evidence to suggest that the health effects of air pollution may be more severe in deprived communities.

Reliance on long-term average targets

25. Defra’s evidence report mentions COMEAP’s advice that, although short-term exposures to PM2.5 also affect health, the relative stability in frequency distributions of daily average concentrations suggests that policies to reduce long-term average concentrations would also reduce elevations in short-term concentrations. COMEAP’s advice also mentioned that the relationship between long-term (annual) and short-term (daily) average concentrations will likely be dependent on the air pollution climate, and the policies being pursued, at the time.

26. We therefore indicated a need to evaluate the relationship between long-term and short-term average concentrations, and the extent to which policies to reduce long-term average concentrations would also reduce short-term (such as 24-hour) average concentrations. We welcome Defra’s intention (expressed during our discussion in June 2022) to examine this aspect. As well as using modelling and existing data, we also think that future monitoring data should be used to evaluate this relationship. This would serve as a check that no unforeseen consequences arise from policies put in place to achieve the long-term concentration targets.

Other pollutants

27. It can be anticipated that most measures implemented to achieve reductions in PM_2.5 (for instance those to reduce sources of combustion) would also be likely to reduce concentrations of other pollutants such as NO₂. Nonetheless, actions targeted at reducing PM_2.5 concentrations might not be optimal to achieve reductions in other pollutants. We suggest that modelling and monitoring of concentrations of other pollutants should continue, to establish that there are no unintended consequences of policies implemented to achieve the PM_2.5 targets.

Cost-benefit assessment

28. The impact analysis indicates a clear net benefit of achieving the proposed targets, which are based on Defra’s ‘high’ scenario. It therefore seems likely that targets defined on the basis of a more ambitious scenario of interventions might still be cost-beneficial. This suggests that using a cost-benefit approach to develop targets – that is, basing them on a package of interventions for which the anticipated costs equalled the monetary of value of the expected benefits – might have generated stricter targets than the proposed concentration and exposure reduction targets. Therefore, we regard the proposed targets as health-informed, rather than health-based.

29. We note that the cost-benefit assessment used damage costs, rather than the full impact pathway approach. The version of damage costs used (based on a one-year change in concentrations rather than changes in emissions) lessens the issue of non-linearities, for example in the relationship between NO_x emissions and NO₂ concentrations. Nonetheless, it does not take into account factors that would be included in a full life-table analysis, such as ongoing changes in population size and age distribution. We encourage Defra to use the full impact pathway approach in future analyses of air pollution policies, such as detailed delivery plans for achieving the PM_2.5 targets.

30. COMEAP9 previously indicated that, although a focus on reducing long-term average concentrations of PM_2.5 is appropriate, the health benefits of reducing concentrations of other pollutants (for example, NO₂ and O₃) should not be overlooked. We recommended that these should be included in cost-benefit analyses undertaken to support policy development. Health benefits associated with expected reductions in concentrations of PM_2.5 and NO₂ and in emissions of O₃ precursors, and other benefits linked to reduced emissions of PM_2.5, nitrogen oxides (NO_x), ammonia (NH₃), sulphur dioxide (SO₂) and volatile organic chemicals (VOCs), were included in Defra’s impact assessment.

Integrated action across emissions sources

31. Emissions from a wide range of different sources contribute to ambient PM_2.5 concentrations. An integrated range of measures across many sectors will therefore be needed to achieve reductions. The scenarios considered in Defra’s evidence report include measures to address emissions from agriculture, aviation, construction and non-road mobile machinery, industry and manufacturing, domestic and commercial combustion, rail, road traffic technology, shipping, and urban mobility.

32. The report also explains that approximately 13% of ambient PM_2.5 in the UK is imported from Europe and 6% arises from international shipping. This suggests that international, particularly European, co-ordination and collaboration will also be important in reducing concentrations within the UK. We would highlight that it will be important for Defra to continue to influence the international agenda to achieve this.

Uncertainties

33. We recognise that there are considerable uncertainties inherent in all stages of the evidence to support the target proposals. These include:

• the policies and interventions that will be put into place to achieve the reductions
• the projection of the emissions trajectories (especially for emissions from domestic combustion and non-exhaust traffic emissions from vehicles, such as tyre wear)
• uncertainties inherent in the numerical modelling of future concentrations
• the technical capability of monitoring equipment, particularly at low concentrations
• the siting of additional monitors as laid out within the proposals and for the purposes of Local Air Quality Management, recognising that local networks might give different measurements from the AURN

These may make it difficult for the targets to be achieved and/or for compliance to be demonstrated. There are also uncertainties in estimating and valuing the health (and other) benefits of the predicted reductions in air pollutants, and in the costs of implementing the interventions needed to achieve them.

Recommendations

36. We consider that proposals for interim targets – both concentration and exposure reduction targets – should be published soon. We would emphasise the public health benefits of maximising exposure reduction early in the timeline.

37. We recommend that monitoring and/or modelling is used to ensure that there are no unintended consequences for areas of greater socioeconomic deprivation and to confirm that disparities in exposure are reduced.

38. We recommend that Defra should evaluate the current relationship between the distributions of short-term average concentrations and annual mean concentrations, to assess whether the annual concentration target provides adequate protection against short-term elevations in exposure. Modelling and future monitoring data should also be evaluated, to ensure that no unforeseen consequences arise from policies put in place to achieve the long-term concentration targets.

39. We recommend that modelling and monitoring of other pollutants should continue, to ensure that there are no unintended consequences from policies aimed at reducing PM_2.5. The health consequences of changes in concentrations of other pollutants should continue to be included in cost-benefit assessments of interventions proposed to achieve PM_2.5 targets.

40. We think that it would be useful for Defra to continue to develop the role of local authorities and other bodies in achieving these targets. Consideration should also be given as to how to achieve improvements below 10µg/m³, as further reductions in PM_2.5 concentrations will result in continued public health benefits.

41. Cross-sectoral working to target multiple sources will be needed to achieve reductions in PM_2.5 concentrations. We consider that it will also be important for Defra to continue to influence the international agenda to achieve the reductions in transboundary PM_2.5 which will be necessary to achieve the targets.

42. We encourage Defra to use the full impact pathway approach to cost-benefit assessment in analyses of air pollution policies, such as detailed delivery plans to achieve the PM_2.5 targets.

COMEAP

July 2022