Assessing health effects of air quality actions: what's next?

In The Lancet Public Health, Ian Mudway and colleagues1Mudway IS Dundas I Wood HE et al.Impact of London's low emission zone on air quality and children's respiratory health: a sequential annual cross-sectional study.Lancet Public Health. 2018; (published online Nov 14.)http://dx.doi.org/10.1016/S2468-2667(18)30202-0PubMed Scopus (58) Google Scholar report an evaluation of the effect of the low emission zone (LEZ) in London, UK, on air quality and children's respiratory health. The LEZ was implemented in stages during 2008 and 2012, requiring vehicles entering Greater London to meet certain emissions standards or pay daily charges if they did not. The study team evaluated the health effect of the LEZ indirectly, by examining the association between air pollution exposure and respiratory health in school children living within the LEZ using a sequential cross-sectional study design over the period 2009–14. The study has many notable strengths, such as the well characterised study population, highly resolved exposure estimates based on home and school addresses, and high-quality respiratory health data, including lung function with and without bronchodilation. They documented small reductions in NO2 and NOx in central London during implementation of the LEZ, but the effect on children's respiratory health was less clear. Interest in assessing the health effects of air quality interventions has grown in response to questions about the benefit of further tightening of air pollution regulations.2Boogaard H van Erp AM Walker KD Shaikh R Accountability studies on air pollution and health: the HEI Experience.Curr Environ Health Rep. 2017; 4: 514-522Crossref PubMed Scopus (35) Google Scholar New research has strengthened the evidence for adverse health effects of air pollution at concentrations at and below current ambient air quality standards, supporting the case for further regulatory action.3Beelen R Hoek G Raaschou-Nielsen O et al.Natural-cause mortality and long-term exposure to particle components: an analysis of 19 European cohorts within the multi-center escape project.Environ Health Perspect. 2015; 123: 525-533Crossref PubMed Scopus (114) Google Scholar, 4Di Q Wang Y Zanobetti A et al.Air pollution and mortality in the Medicare population.New Engl J Med. 2017; 376: 2513-2522Crossref PubMed Scopus (801) Google Scholar, 5Pinault LL Weichenthal S Crouse DL et al.Associations between fine particulate matter and mortality in the 2001 Canadian Census Health and Environment Cohort.Environ Res. 2017; 159: 406-415Crossref PubMed Scopus (110) Google Scholar Studies assessing the health effects of air quality actions are appealing since they are the closest epidemiological equivalent to controlled experimental studies, and thus might provide evidence for causal associations. This apparent advantage does not imply, however, that accountability studies are less susceptible to confounding that might bias results. We previously described the general challenges encountered in such studies.2Boogaard H van Erp AM Walker KD Shaikh R Accountability studies on air pollution and health: the HEI Experience.Curr Environ Health Rep. 2017; 4: 514-522Crossref PubMed Scopus (35) Google Scholar, 6van Erp AM Cohen AJ HEI's research program on the impact of actions to improve air quality: interim evaluation and future directions. Communication 14. Health Effects Institute, Boston, MA2009Google Scholar The study by Mudway and colleagues1Mudway IS Dundas I Wood HE et al.Impact of London's low emission zone on air quality and children's respiratory health: a sequential annual cross-sectional study.Lancet Public Health. 2018; (published online Nov 14.)http://dx.doi.org/10.1016/S2468-2667(18)30202-0PubMed Scopus (58) Google Scholar shows some of the challenges in doing such research, and provides possible directions for future research. One common challenge is sufficient statistical power. The air quality effects of the LEZ in London were estimated to be small in an earlier analysis by the same study team,7Kelly FJ Armstrong B Atkinson R et al.The London low emission zone baseline study. Research report 163. Health Effects Institute, Boston, MA2011Google Scholar reducing the statistical power for a direct evaluation of health effects. The analysis of air quality monitoring data in their current study during 2006–14 documented reductions of about 1 μg/m3 of NO2 per year, with no change in measured particulate matter with a diameter of less than 10 μm (PM10) and less than 2·5 μm (PM2·5). The relevant exposure contrast deployed in the analyses were relatively small, in particular for PM10 and PM2·5 (IQR <1 μg/m3). Statistical power issues are also apparent when looking at their main results, with some estimates significant only when combined over all 5 years. These limitations are likely to reflect a well known weakness of a cross-sectional design; a longitudinal design would have been preferred, as the authors note. The detailed exposure assessment at 20 × 20 m grid point resolution for several pollutants (NO2, NOx, PM10 and PM2·5) is a notable strength of the study. A key feature was the use of novel modelling of exposures that allowed differentiation between effects of short-term and long-term exposures. Ideally, such analysis should be done in joint models, if correlations are not too high. Mudway and colleagues also pursued multipollutant modelling, but, given the high correlations among pollutants at each timepoint, they reported that those analyses were not informative. Advanced statistical methods are clearly needed to investigate the health effects of air pollution mixtures in future research.8Dominici F Peng RD Barr CD Bell ML Protecting human health from air pollution: shifting from a single-pollutant to a multi-pollutant approach.Epidemiology. 2010; 21: 187-194Crossref PubMed Scopus (299) Google Scholar, 9Health Effects InstituteDevelopment of statistical methods for multipollutant research. Research report 183, parts 1 and 2. Health Effects Institute, Boston, MA2015Google Scholar The absence of preintervention health data prevented the study team from a direct evaluation of the health effect of the LEZ. Prospective studies are potentially the most informative to overcome such obstacles, but usually require that investigators identify proposed actions in advance and begin work early enough to capture stable estimates of baseline conditions. This task is not easy, and opportunities to do such prospective research remain relatively scarce. Another important aspect worth discussing is that the study1Mudway IS Dundas I Wood HE et al.Impact of London's low emission zone on air quality and children's respiratory health: a sequential annual cross-sectional study.Lancet Public Health. 2018; (published online Nov 14.)http://dx.doi.org/10.1016/S2468-2667(18)30202-0PubMed Scopus (58) Google Scholar lacked a control population unaffected by the intervention, as acknowledged by the authors. Use of appropriate comparison populations unaffected by the intervention, simulation, and sensitivity analyses to evaluate choices of reference populations and of statistical models adjusting for background trends, remain important in future intervention studies.2Boogaard H van Erp AM Walker KD Shaikh R Accountability studies on air pollution and health: the HEI Experience.Curr Environ Health Rep. 2017; 4: 514-522Crossref PubMed Scopus (35) Google Scholar In a Cochrane systematic review (unpublished) of interventions to reduce PM air pollution and their effects on health, studies without a control population were only used as so-called supporting evidence, highlighting the importance of such comparisons in intervention research.10Burns J Boogaard H Turley R et al.Interventions to reduce ambient particulate matter air pollution and their effect on health.Cochrane Database Syst Rev. 2014; 1 (CD010919.)Google Scholar New promising methods have been developed, including use of causal inference methods and counterfactual methods.11Zigler CM Kim C Choirat C et al.Causal inference methods for estimating long-term health effects of air quality regulations. Research report 187. Health Effects Institute, Boston, MA2016Google Scholar, 12Russell AG Tolbert P Henneman L et al.Impacts of regulations on air quality and emergency department visits in the Atlanta Metropolitan Area, 1993–2013. Research report 195. Health Effects Institute, Boston, MA2018Google Scholar Such approaches would enhance the attribution of the changes in air quality and health directly to an intervention and should continue to be further explored. Mudway and colleagues also place much emphasis on NO2, in part because the annual exposure of many school children in the study remained above the EU NO2 limit value of 40 μg/m3. The evidence for effects of NO2, which in cities originates largely from vehicle exhaust and is considered a marker for traffic-related air pollution, has strengthened.13US EPAIntegrated Science Assessment (ISA) for oxides of nitrogen—health criteria (final report, 2016). EPA/600/R-15/068. US Environmental Protection Agency, Washington, DC2016Google Scholar The Committee on the Medical Effects of Air Pollutants in the UK have released a comprehensive report to quantify the independent effects of NO2 for health and burden assessments.14COMEAPAssociations of long-term average concentrations of nitrogen dioxide with mortality. Public Health England, London2018Google Scholar A key question that remains largely unresolved is whether NO2 is a causal agent or only an indicator of traffic-related air pollution, given that correlations in space and time between concentrations of NO2 and other traffic-related air pollutants are often high. The study by Mudway and colleagues adds to this quandary, and shows that more stringent measures to improve urban air quality and children's health might be needed. We declare no competing interests. Impact of London's low emission zone on air quality and children's respiratory health: a sequential annual cross-sectional studyWithin London's LEZ, a smaller lung volume in children was associated with higher annual air pollutant exposures. We found no evidence of a reduction in the proportion of children with small lungs over this period, despite small improvements in air quality in highly polluted urban areas during the implementation of London's LEZ. Interventions that deliver larger reductions in emissions might yield improvements in children's health. Full-Text PDF Open Access

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