COMEAP statement: response to publication of the World Health Organization Air quality guidelines 2021
Published 27 July 2022
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Summary
1. We welcome the World Health Organization’s (WHO’s) revised Air Quality Guidelines (AQGs), which we regard as suitable long-term targets to inform policy development in the UK. The guideline values should not be regarded as thresholds below which there are no impacts on health: continued reductions, even where levels are below the AQGs, are also likely to be beneficial to health.
WHO air quality guidelines
2. The World Health Organization (WHO) published revised Air Quality Guidelines (AQGs) for pollutants in ambient air in September 2021 (WHO, 2021). COMEAP welcomes this publication and the evidence reviews which have underpinned WHO’s considerations. We would make the following observations, which we hope will be useful to inform air quality policy in the UK.
3. The new AQGs for particulate matter (PM) and nitrogen dioxide (NO2) are substantially lower than the previous (2005) guidelines, (see table in Annexe A). The updated long-term (annual average) AQG for PM10 is 15µg/m3, for PM2.5 is 5µg/m3 and for NO2 is 10µg/m3.
4. The process adopted by WHO to review the epidemiological evidence and derive the new AQG values was systematic and, as far as possible, consistent across pollutants, following a pre-specified protocol developed by a WHO working group of experts in this research area. WHO commissioned systematic reviews and meta-analyses of the epidemiological evidence for health endpoints for which there is established evidence of associations with air pollutants. The systematic reviews were peer-reviewed and published in a special issue of the peer-reviewed journal Environment International. A wide range of international experts were involved in method development, systematic reviews, guideline development and external review, including some members of COMEAP.
5. For particulate matter (PM), nitrogen dioxide (NO2) and ozone (O3), long-term AQGs were developed based on evidence from studies of spatial variation in long-term average concentrations of air pollutants. The relative risks found in these studies of spatial variation in long-term average concentrations (often cohort studies) are typically larger than those reported from studies of effects associated with short-term temporal variations in concentrations (such as time-series studies). The WHO’s Guideline Development Group (GDG) chose the AQGs to represent long-term concentrations at which it was confident, from the available evidence, that there is an increase in adverse health effects.
6. WHO (2021) note that the existing evidence generally supports a linear, or supralinear, no-threshold relationship for the various pollutant-outcome pairs examined in the systematic reviews which underpin the AQGs. They explain that the concentrations used as the starting points for AQG development are not equivalent to thresholds of no effect; rather, they are levels below which there is less certainty about the existence of an effect. In our previous advice to the Department for Environment, Food and Rural Affairs (Defra) regarding PM2.5 (COMEAP, 2021), we noted that current epidemiological studies have not provided evidence of a threshold concentration below which no effects are observed in the studied populations. Nonetheless, we suggest that the apparent lack of a threshold for effect at the population level should not be interpreted to mean that there is no threshold for effect at an individual level: the level of exposure which can be tolerated without adverse effects would be expected to vary between individuals. It would also likely vary across the life-course for any given individual, depending upon factors such as age and health status.
7. Short-term (24-hour average) AQGs for particulate matter (PM), nitrogen dioxide (NO2) and ozone (O3), were derived from the long-term AQGs for these pollutants: as the 99th percentiles of daily concentrations observed in distributions with a mean equal to the long-term AQG. This is a different approach from that used for most of the previous (2005) short-term AQGs, which were based on a consideration of evidence of effects following short-term exposures, such as time-series studies or studies of controlled exposures.
8. For sulphur dioxide (SO2) and carbon monoxide (CO), for which long-term AQGs were not derived, short-term AQGs were developed which posed a similar level of risk to short-term AQGs recommended for the other pollutants. Previous (2005) AQGs for averaging times shorter than 24-hours were not covered by the 2021 update, and remain valid.
9. As part of its evaluation of the epidemiological evidence, the GDG considered studies which had used both single- and 2-pollutant models (for example, mutually adjusting for PM2.5 and NO2) and was satisfied that independent associations remained after adjustment for effects associated with concentrations of the other pollutant. The extent of possible mutual confounding is unclear. Even adjusted coefficients are likely to reflect the effects of correlated pollutants, to some extent, but this is difficult to quantify (COMEAP, 2018). We note that WHO were able to draw on some newer studies (Eum and others, 2019; Pappin and others, 2019) in which the associations of mortality with long-term average concentrations of NO2 remained robust to adjustment for PM2.5 in 2-pollutant models.
WHO good practice statements
10. The GDG also produced good practice statements (GPS) for some PM components: black carbon/elemental carbon (BC/EC), sand and dust storms (SDS) and ultra-fine particles (UFP) for which they considered that there was insufficient epidemiological data to recommend AQGs. Instead, they provide recommendations on monitoring, mitigation and future research. The good practice statements are not based on a similar peer-reviewed systematic review process as the AQGs. Where there is an absence of sufficient epidemiological evidence on which to base guideline recommendations for specific components and sources of PM (UFP, for example), COMEAP highlights the value of toxicological evidence in guiding future work in addition to supporting the case for causality.
11. The recommendations within the GPSs for sand and desert dust are likely to be of limited relevance to the UK situation. In relation to the other GPSs, we note that the UK has a network of BC/EC monitors, but that monitoring of UFP is more limited. We consider that increased UK monitoring of UFP would deliver several benefits, including an improved knowledge of population exposure, facilitation of epidemiological studies, and insights into changes over time, as may occur as a result of policy measures, such as emissions changes in the transition to Net Zero.
Conclusions
12. Overall, COMEAP’s view is that the WHO’s revised AQGs for pollutants in outdoor air are suitable as long-term targets to inform policy development. We stress that the AQG values should not be regarded as thresholds below which there are no impacts on health - the current evidence has not identified thresholds for effect at the population level, meaning that even low concentrations of pollutants are likely to be associated with adverse effects on health. Therefore continued reductions, even where concentrations are below the AQGs, are also likely to be beneficial to health.
COMEAP
July 2022
References
COMEAP (2018). ‘Associations of long-term average concentrations of nitrogen dioxide with mortality.’
COMEAP (2021). ‘Advice on health evidence relevant to setting PM2.5 targets’
Eum KD, Kazemiparkouhi F, Wang B, Manjourides J, Pun V, Pavlu V, Suh H (2019). ‘Long-term NO2 exposures and cause-specific mortality in American older adults.’ Environment International volume 124, pages 10 to 15. doi: 10.1016/j.envint.2018.12.060. Epub 2019 January 9. PMID: 30639903; PMCID: PMC7123874
Pappin AJ, Christidis T, Pinault LL, Crouse DL, Brook JR, Erickson A and others. (2019) ‘Examining the shape of the association between low levels of fine particulate matter and mortality across 3 cycles of the Canadian Census Health and Environment Cohort.’ Environmental Health Perspectives: volume 127, issue 10, page 107,008. doi: 10.1289/EHP5204.. PMID: 31638837; PMCID: PMC6867181
WHO (2021). ‘WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.’ License: CC BY-NC-SA 3.0 IGO
Annexe A
| Pollutant | Averaging time | 2005 air quality guideline | 2021 AQC level |
|---|---|---|---|
| PM2.5, µg/m3 | Annual | 10 | 5 |
| 24-hour* | 25 | 15 | |
| PM10, µg/m3 | Annual | 20 | 15 |
| 24-hour* | 50 | 45 | |
| O3, µg/m3 | Peak season** | - | 60 |
| 8-hour* | 100 | 100 | |
| NO2, µg/m3 | Annual | 40 | 10 |
| 24-hour* | - | 25 | |
| SO2., µg/m3 | 24-hour* | 20 | 40 |
| CO, mg/m3 | 24-hour* | - | 4 |
*99th percentile (3 to 4 exceedance days per year).
**Average of daily maximum 8-hour mean O3 concentration in the 6 consecutive months with the highest 6-month running-average O3 concentration.
Reproduced from: ‘WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide.’ Geneva: World Health Organization; 2021. Licence: CC BY-NC-SA 3.0 IGO