Meta-analysis of the association between low concentration PM_2.5 and cardiovascular mortality in the United States and Canada

The adverse effects of fine particulate matter (PM_2.5), including cardiovascular outcomes, are well established. This review and meta-analysis investigates the association between long-term exposure to low concentration PM_2.5 (<12 µg/m³) and CVD mortality in U.S. and Canadian populations. We conducted a literature search and completed random effect meta-analyses. Twenty-four studies were reviewed, with 12 from each of the U.S. and Canada. Fifteen of eighteen studies that reported hazard ratios (HRs) for total CVD mortality reported statistically significant positive associations with low concentration PM_2.5. For cause-specific CVD mortality, more consistent results were shown for ischemic heart disease (IHD) mortality, with all eleven studies reporting statistically significant associations (HR = 1.09 to 2.48). Only three of 12 studies evaluating cerebrovascular mortality reported statistically significant associations (HR = 1.10 to 1.27). Studies that restricted analyses to participants with mean exposures <12 µg/m³ found statistically significant associations between PM_2.5 and at least some of the CVD mortality outcomes of interest. However, the shape of the concentration-response functions varied widely. Only six studies controlled for at least one additional air pollutant, and multi-pollutant models generally showed an attenuated impact of PM_2.5. Despite existing gaps in understanding the association between low concentrations of PM_2.5 and cardiovascular mortality, this review highlights the critical importance of ongoing efforts to improve air quality for public health benefits. Continued focus on understanding the shape of the concentration-response function for PM_2.5, the impact of co-pollutants on observed effects, and how particle composition may impact effect estimates, is recommended.