Preterm birth and air pollution: Critical windows of exposure for women with asthma

Background Ambient air pollutants may increase preterm birth (PTB) risk, but critical exposure windows are uncertain. The interaction of asthma and pollutant exposure is rarely studied. Objective We sought to assess the interaction of maternal asthma and air pollutant exposures in relation to PTB risk. Methods Electronic medical records for 223,502 US deliveries were linked with modified Community Multiscale Air Quality model outputs. Logistic regression with generalized estimating equations estimated the odds ratio and 95% CIs for PTB on the basis of the interaction of maternal asthma and particulate matter with aerodynamic diameter of less than 2.5 microns and particulate matter with aerodynamic diameter of less than 10 microns, ozone (O₃), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and carbon monoxide (CO) per interquartile range. For each gestational week 23 to 36, exposures among women who delivered were compared with those remaining pregnant. Three-month preconception, whole pregnancy, weeks 1 to 28, and the last 6 weeks of gestation averages were also evaluated. Results On assessing PTB by gestational week, we found that significant asthma interactions were sporadic before 30 weeks but more common during weeks 34 to 36, with higher risk among mothers with asthma for NO_x, CO, and SO₂ exposure and an inverse association with O₃ in week 34. Odds of PTB were significantly higher among women with asthma for CO and NO_x exposure preconception and early in pregnancy. In the last 6 weeks of pregnancy, PTB risk associated with particulate matter with aerodynamic diameter of less than 10 microns was higher among women with asthma. Conclusions Mothers with asthma may experience a higher risk for PTB after exposure to traffic-related pollutants such as CO and NO_x, particularly for exposures 3-months preconception and in the early weeks of pregnancy.