Main air pollutants and myocardial infarction: A systematic review and meta-analysis

Context: Short-term exposure to high levels of air pollution may trigger myocardial infarction (MI), but this association remains unclear. Objective: To assess and quantify the association between short-term exposure to major air pollutants (ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulate matter ≤10 μm [PM₁₀] and ≤2.5 μm [PM _2.5] in diameter) on MI risk. Data Sources: EMBASE, Ovid MEDLINE in-process and other nonindexed citations, and Ovid MEDLINE (between 1948 and November 28, 2011), and EBM Reviews- Cochrane Central Register of Controlled Trials and EBM Reviews-Cochrane Database of Systematic Reviews (between 2005 and November 28, 2011) were searched for a combination of keywords related to the type of exposure (air pollution, ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, PM₁₀, and PM_2.5) and to the type of outcome (MI, heart attack, acute coronary syndrome). Study Selection: Two independent reviewers selected studies of any study design and in any language, using original data and investigating the association between short-term exposure (for up to 7 days) to 1 or more air pollutants and subsequent MI risk. Selection was performed from abstracts and titles and pursued by reviewing the full text of potentially eligible studies. Data Extraction: Descriptive and quantitative information was extracted from each selected study. Using a random effects model, relative risks (RRs) and 95% CIs were calculated for each increment of 10 μg/m³ in pollutant concentration, with the exception of carbon monoxide, for which an increase of 1 mg/m³ was considered. Data Synthesis: After a detailed screening of 117 studies, 34 studies were identified. All the main air pollutants, with the exception of ozone, were significantly associated with an increase in MI risk (carbon monoxide: 1.048; 95% CI, 1.026-1.070; nitrogen dioxide: 1.011; 95% CI, 1.006-1.016; sulfur dioxide: 1.010; 95% CI, 1.003- 1.017; PM₁₀: 1.006; 95% CI, 1.002-1.009; and PM_2.5: 1.025; 95% CI, 1.015-1.036). For ozone, the RR was 1.003 (95% CI, 0.997-1.010; P=.36). Subgroup analyses provided results comparable with those of the overall analyses. Population attributable fractions ranged between 0.6% and 4.5%, depending on the air pollutant. Conclusion: All the main air pollutants, with the exception of ozone, were significantly associated with a near-term increase in MI risk.