Increases in Birth Weight Associated With the 2008 Beijing Olympic Air Pollution Reduction: Results from a Natural Experiment

Abstract

Background: Previous studies have reported decreased birth weight associated with increased ambient air pollutant concentrations during pregnancy. However, it is not clear in which trimester or month air pollution exposure has the strongest effects on birth weight. Aims: Using the natural experiment of the drastic, but temporary, air pollution declines during the 2008 Beijing Olympics, we evaluated whether having specific months of pregnancy (i.e. 1st, 2nd…8th) during the 2008 Olympic period resulted in larger birth weights, compared to the same dates in 2007 or 2009. Methods: Using n=18,835 term births (37-42 weeks gestation) to mothers residing in 4 urban districts of Beijing (Xicheng, Chaoyang, Fengtai, Haidan), we estimated the change in birth weight associated with having individual months of pregnancy (months 1-8) during the 2008 Olympic period (8/8/08 – 9/24/08) compared to pregnancies with that month during the same dates in 2007 (8/8/07 – 9/24/07) or 2009 (8/8/09 – 9/24/09). We also estimated the change in birth weight associated with interquartile range increases (IQR) in mean ambient PM2.5, SO2, NO2, and CO concentrations during each pregnancy month. Results: We found that the birth weight of babies with their 8th month of pregnancy during the 2008 Olympics were on average 22.6g larger (95% CI = 5.4g, 39.8g) than babies having the 8th month in 2007 or 2009. IQR increases in PM2.5 (19.2 µg/m3), SO2 (1.77 ppb), CO (0.3 ppm), and NO2 (13.3 ppb) concentrations during the 8th month of pregnancy were associated with birth weight decreases of -15.4g (95% CI = -29.5g, -1.3g), -28.4g (95% CI = -41.3g, -15.5g), -24.1g (95% CI = -35.5g, -12.7g), and -24.7 (95% CI = -61.4g, 12.1g), respectively. Conclusions: This natural experiment suggests that even a short term decrease in air pollution late in pregnancy, in a normally heavily polluted city, can have impacts on the birth weight of babies. Further work is needed to understand what mechanisms mediate this response.