Exposure to air pollution linked to increased risk of stroke

1. Short-term exposure to air pollution was associated with an increased risk of hospital admission and mortality from stroke.

2. These associations held true for particulate matter and gaseous pollutants and were strongest in low and middle-income countries.

Evidence Rating Level: 1 (Excellent)      

Study Rundown: Stroke is undoubtedly a leading cause of disability and death around the world. The risk factors we have reasonable control over are well known- smoking, hypertension, and obesity. Environmental factors, however, have not been well studied. The purpose of this study was to assess the effect of air pollution exposure on the risk of stroke in order to inform improved policy decisions. In a systematic review and meta-analysis of current studies, the authors found that particulate air pollutants, specifically of sizes 2.5 micrometers or less (PM_2.5) and 10 micrometers of less (PM₁₀₎, were associated with increased admission to hospital for stroke and increased mortality from stroke. Gaseous pollutants such as nitrogen dioxide, sulfur dioxide, and carbon monoxide were also associated with increased hospital admission and mortality, while ozone showed only a weak association. The strongest links between stroke and pollution were found in low and middle-income countries.

This meta-analysis includes data from a pooled analysis of over 90 studies from over 6.2 million events incorporated across 28 countries, with over 20% of the studies stemming from low and middle-income countries. However, many of the included studies measured air pollution concentrations at remote monitoring sites, and so it may not be representative of true concentrations in more populated, urban areas. Additionally, while individual studies may have controlled for comorbidities, the meta-analysis failed to control for differences in inherent stroke risk and comorbidities between people in different geographic regions. Overall, these results validate an association between short-term exposure to gaseous and particulate air pollution and stroke risk, and will ideally inform policy interventions to improve public health.

Click to read the study in the BMJ

Click to read an accompanying editorial in the BMJ

Relevant Reading: Air pollution as an emerging global risk factor for stroke

In-Depth [systematic review and meta-analysis]: This study determined the impact of short-term air pollution exposure on hospital admission due to stroke and subsequent stroke mortality. Of 2748 initially identified articles, 94 were ultimately included in the meta-analysis for a total of 6.2 million events across 28 countries. Particulate air pollutants, specifically sizes PM_2.5 and PM₁₀, were associated with increased hospital admission and mortality due to stroke. Interestingly, a significant association was not found for PM_2.5-10. The relative risk of exposure per 10μg/m³ increase in PM_2.5 concentrations was 1.011 (CI_95% 1.011-1.012), and this association persisted when stratified by sex and age. For gaseous pollutants, similar results were obtained; per 10ppb nitrogen dioxide [1.014, CI_95% 1.009-1.019], sulfur dioxide [1.019, CI_95% 1.011-1.027], and carbon monoxide [1.015, CI_95% 1.004-1.026] had a significant relative risk increase in hospital admission or mortality due to stroke. Ozone, however, showed a weak association with a 1.001 (CI_95% 1.000-1.002) relative risk increase per 10ppb. These associations were all stronger in low and middle-income countries compared to high income countries.

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