Composition of fine particulate matter and risk of preterm birth: A nationwide birth cohort study in 336 Chinese cities.

BACKGROUND:Potential hazards of fine particulate matter (PM2.5) constituents on preterm birth (PTB) have rarely been explored in China. OBJECTIVE:To quantify the associations of PM2.5 constituents with PTB. METHODS:This study was based on a nationwide cohort of 3,723,169 live singleton births delivered between January 2010 and December 2015 in China. We applied satellite-based estimates of 5 PM2.5 constituents (organic carbon; black carbon; sulfate; ammonium; and nitrate). We used Cox proportional hazards regression models adjusted for individual covariates, temperature, humidity, and seasonality to evaluate the associations. RESULTS:During the entire pregnancy, each interquartile range (29 μg/m3) increase in PM2.5 concentrations was associated with a 7% increase in PTB risk [hazard ratio (HR): 1.07; 95% confidence interval (CI): 1.07-1.08). We observed the largest effect estimates on carbonaceous components (HR: 1.09; 95% CI: 1.08-1.10 for organic carbon and black carbon). Early pregnancy appeared to be the critical exposure window for most constituents. Women who were older, exposed to second-hand smoke, overweight or obese before pregnancy, conceived during winter, and living in northern China or rural areas were more susceptible. CONCLUSIONS:Carbonaceous components of PM2.5 were associated with higher PTB risk. Findings on characteristics of vulnerability underlined targeted protections on susceptible subgroups.