The acute lag effects of elevated ambient air pollution on stillbirth risk in Ulaanbaatar, Mongolia

Ulaanbaatar city (UB), the capital and the home to half of Mongolia’s total population, has experienced extreme seasonal air pollution in the past two decades with levels of fine particulate matter with an aerodynamic diameter less than 2.5 micrometers (PM 2.5 ) exceeding 500 μ g/m 3 during winter. Based on monitoring data, (PM 2.5 ), sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), and carbon monoxide (CO) exposures were estimated for residential areas across UB using Random Forest models. We collected individual-level data on 1093 stillbirths from UB hospital records (2010-2013) and a surveillance database (2014-2018). Using a time-stratified case-crossover design, we investigated whether short-term increases in daily ambient air pollutants with different exposure lags (2 to 6 days) before delivery were associated with stillbirth. We estimated associations using conditional logistic regression and examined individual-level characteristics for effect modification. During the cold season (Oct-Mar) we observed significantly elevated relative odds of stillbirth per interquartile range increase in mean concentrations of PM 2.5 (odds ratio [OR]=1.35, 95% confidence interval [CI]=1.07-1.71), SO 2 (OR=1.71, 95% CI=1.06-2.77), NO 2 (OR=1.30, 95% CI=0.99-1.72), and CO (OR=1.44, 95% CI=1.17-1.77) 6 days before delivery after adjusting for apparent temperature with a natural cubic spline. The associations of pollutant concentrations with stillbirth were significantly stronger among those younger than 25, nulliparous, and without comorbidities or pregnancy complications during stratified analyses. There was a clear pattern of increased risk for women living in areas of lower socioeconomic status. We conclude that acute exposure to ambient air pollution before delivery may trigger stillbirth, and this risk is higher for certain subsets of women.