Effect of long-term exposure to air pollution on type 2 diabetes mellitus risk: a systemic review and meta-analysis of cohort studies

Objective: To assess the effect of long-term exposure to air pollution on type 2 diabetes risk, a meta-analysis of prospective cohort studies was performed. Methods: Literature search was conducted with Pubmed, Embase, and Web of Science for prospective cohort studies investigating the association of type 2 diabetes risk with increments in particulate matter (PM, diameter <2.5 mu m (PM2.5) or <10 mu m (PM10)) or nitrogen dioxide (NO2). We used a random-effects model to calculate the overall relative risk (RR) with 95% CI. Results: Of 808 identified articles, ten cohort studies were finally included, which involved a total of 2 371 907 participants and 21 095 incident cases of type 2 diabetes. Elevated risk of type 2 diabetes was significantly associated with long-term exposures to high levels of PM2.5 (RR=1.28, 95% CI 1.06-1.55, P=0.009, I-2=83.5%), PM10 (RR=1.15, 95% CI 1.02-1.30, P=0.022, I-2=0%), and NO2 (RR=1.12, 95% CI 1.02-1.23, P=0.015, I-2=63.5%). When using standardized risk estimates, the RRs of type 2 diabetes were significant for increments in concentrations of PM2.5 (1.39 per 10 mu g/m(3) increment, 95% CI 1.14-1.68, P=0.001), PM10 (1.34 per 10 mu g/m(3) increment, 95% CI 1.22-1.47, P < 0.001), and NO2 (1.11 per 10 mu g/m(3) increment, 95% CI 1.07-1.16, P < 0.001). No obvious evidence of publication bias was observed. Conclusion: Long-term exposure to high levels of main air pollutants is significantly associated with elevated risk of type 2 diabetes mellitus.