Where there’s smoke . . .BMJ 2014; 348 doi: http://dx.doi.org/10.1136/bmj.g40 (Published 21 January 2014) Cite this as: BMJ 2014;348:g40
- Michael Brauer, professor, school of population and public health,
- G B John Mancini, professor, division of cardiology, department of medicine
Air pollution has received much attention in the past year. The Global Burden of Disease Study estimated that 3.2 million deaths a year are attributable to particulate matter in outdoor air,1 the International Agency for Research on Cancer classified polluted outdoor air as carcinogenic,2 and we witnessed extreme episodes in Beijing and Shanghai. While effects on respiratory health have long been recognised, it is the impacts on cardiovascular disease3 that are responsible for most of the disease burden attributable to air pollution. Two linked papers provide new insight into the role of air pollution on cardiovascular disease and subsequent impacts on population health.4 5
Perhaps nowhere are the health impacts of outdoor air pollution more acutely felt than in China, where air pollution is the fourth most important risk factor for disease burden6 and is responsible for 1.2 million deaths each year. In one linked paper (doi:10.1136/bmj.f7139), Guo and colleagues applied the well known time series methods to age of death and life table statistics to assess the relation between daily pollution levels in Beijing and years of life lost.4 The results suggest that air pollution has a substantial impact, but it is not possible from this study to directly assess the magnitude of life shortening attributable to air pollution in Beijing. Cohort studies conducted in North America and Europe suggest that a 30 µg/m3 difference in long term exposure to particulate matter <2.5 μm in aerodynamic diameter (PM2.5; about a third of the interquartile range encountered throughout China) is associated with a reduction in life expectancy of about two years.7 Deriving such estimates for China will require analysis in cohort studies of the Chinese population, an important research priority.
The linked paper by Cesaroni and colleagues (doi:10.1136/bmj.f7412) reports on the relation between long term exposure to air pollution and incidence of myocardial infarction and unstable angina in a meta-analysis of 11 cohort studies from five European countries.5 As part of the ESCAPE project, their analysis complements recent reports from this extensive collaboration on associations between chronic exposure to air pollution and natural mortality8 and lung cancer.9 As one of the largest studies on cardiac events in relation to air pollution, the finding that this association is not dominated by fatal events suggests that cardiovascular disease events attributable to particulate matter are underestimated in more traditional analyses that consider only deaths. The study is notable for its size. The authors analysed data from more than 100 000 people in heterogeneous cohorts with a standardised statistical approach including assignment of exposure at the individual level. The results are supported by the high level of adjustment for important covariates, including community noise exposure, and multiple sensitivity analyses.
While this study is a noteworthy addition to the literature, several important questions remain unanswered. For example, the analysis by Cesaroni and colleagues compares only exposure differences within cities caused by local sources of air pollution largely related to traffic.5 As they did not carry out a pooled analysis, assessment of comparisons between cities in exposure that could be driven by sources other than traffic cannot be evaluated. Understanding the relative role of local and regional pollution, specific sources of pollutants, and mixtures has important policy implications. The focus on long term exposures and the observation of an association with increased acute cardiac events suggests that air pollution can be a trigger for acute events, as reported previously.10 Whether air pollution also encourages the progression of atherosclerosis is still unclear but is the subject of intense investigation.11
The study by Cesaroni and colleagues has specific relevance to the management of air quality in Europe. Particulate air pollution was associated with cardiac events even after they excluded exposures below the 25 µg/m3 European Union limit value for fine particles (PM2.5).5 Significant effects were also discernible for exposure levels only slightly above the 10 µg/m3 World Health Organization (WHO) air quality guideline. Despite this evidence of effects on cardiac events at relatively low levels, nearly 90% of the world’s population lives in locations where the WHO guidelines are exceeded.12 Indeed, in the analysis of Guo and colleagues the mean PM2.5 concentration over a five year period in Beijing was more than 10 times the WHO guideline value.4
The important impact of air pollution on cardiovascular disease highlighted by these two papers supports efforts to meet existing and even more stringent air quality standards to minimise cardiovascular morbidity and mortality. A specific focus on the mitigation of other widely recognised risk factors for cardiac events in areas where poor air quality presents an additional risk might also be warranted. For example, there are currently no focused interventions to enhance smoking cessation in highly polluted areas or to provide more specific guidance as to how to safely achieve targets for aerobic exercise in such areas. As particulate air pollution can trigger cardiovascular events, there may also be a need for more deliberate assessment of this risk, in conjunction with other traditional risk factors for cardiovascular disease, to ensure that treatments that might prevent events are being used. People with or at risk of cardiovascular disease who live in highly polluted areas might warrant more aggressive use of primary and secondary preventive therapies, including antiplatelet agents, lipid lowering agents, and treatments for hypertension or diabetes, all known to prevent cardiovascular events. Indeed, the relative effectiveness of such approaches in highly polluted compared with cleaner areas is unknown but potentially important to public health. These two new studies can help direct further research to evaluate interventions to improve air quality and other risk factors for cardiovascular disease, with the ultimate goal of reducing the global burden of cardiovascular disease.
Cite this as: BMJ 2014;348:g40
Competing interests: We have read and understood the BMJ Group policy on declaration of interests and declare the following interests: MB has collaborated with Pershagen, Hoek, and Brunekreef on topics unrelated to this article.
Provenance and peer review: Commissioned; not externally peer reviewed.