Smog and risk of overall and type-specific cardiovascular diseases: A pooled analysis of 53 cohort studies with 21.09 million participants

The reported associations of smog with the risk of cardiovascular disease (CVD) and CVD subtypes were inconsistent. We systematically searched the Pubmed (Medline) and Embase databases (from the inception to April 25, 2018) to identify the cohort studies investigating the association between smog and CVD and specific types of CVD. We conducted a meta-analysis for different types of air pollutants (PM2.5, PM10, NO2, and O-3) in smog with the risk of specific types of CVD separately. We summarized the study-specific effect estimates using both the fixed effect model and the random effect model. The meta-analysis included 35 publications with 53 cohort studies. Overall, the associations between per 10 mu g/m(3) increase in PM2.5 exposure and risk of CVD events, stroke events, ischemic heart disease(IHD) events were significant, with relative risks (RRs) of 1.11 (95% confidence interval: 1.07-1.15), 1.12 (95% CI: 1.08-1.16) and 1.14(95% CI: 1.08-1.21), respectively. PM2.5, PM10, NO2, and O-3 exposure were associated with an increased risk of CVD mortality, with RRs of 1.11 (95% CI: 1.07-1.15), 1.09 (95% CI: 1.02-1.16), 1.23 (95% CI: 1.15-1.31) and 1.03 (95% CI: 1.02-1.05), respectively. Compared with PM10, NO2, and O-3 exposure, PM2.5 exposure had a greater risk of stroke incidence and IHD incidence (RR 1.12, 95% CI 1.05-1.19 for stroke incidence; 1.19, 1.09-1.30 for IHD). However, no clear evidence for the associations of PM10 exposure with risk of CVD incidence, stroke incidence, and IHD incidence was observed. This meta-analysis confirms the evidence that PM2.5 exposure was significantly associated with increased risk of CVD, stroke, and IHD. PM2.5, PM10, NO2, and O-3 exposure were separately associated with an increased risk of CVD mortality. There was a stronger association between PM2.5 exposure and the risk of stroke and IHD incidence. It urgently needs well-designed studies to further to elaborate the biological and epidemiological mechanisms that link smog with CVD. Main findings: Compared with PM10, NO2, and O-3 exposures, PM2.5 exposure was positively associated with increased risk of stroke and IHD incidence. For air pollutants and CVD events, the association of NO2 with the risk CVD mortality is more significant.