High Temperature Associations with Preterm Birth and Mediation by Air Pollutants in Detroit, Michigan, 1991-2001.

Abstract

Introduction: Preterm births (PTBs) cause significant infant health risks, and several studies have found associations between high temperatures and PTB. To understand this association in Detroit, Michigan and the extent to which temperature effects are mediated by air pollution, we evaluated both the total and natural direct effects (independent of particulate matter, ozone, and nitrogen dioxide) of two-day mean apparent temperature (AT) on PTB for the warm months (May to September), 1991-2001. We evaluated effect modification by maternal age, race, education, smoking status, and prenatal care.Methods: We used a time-stratified case-crossover design with splines of AT and wind speed (National Center for Environmental Information), solar radiation (National Renewable Energy Laboratory), and citywide average precipitation (Oregon State University’s Parameter-elevation Regression on Independent Slopes Model) to estimate total effects. To accommodate multiple mediators and exposure-mediator interactions, AT inverse odds weights, predicted by meteorological and air pollutant (Environmental Protection Agency AirData) covariates, were included in a subsequent case-crossover model to estimate direct effects.Results: At 24.9 deg. C (95th percentile of warm-season AT) vs. 18.6 deg. C (50th percentile of warm-season AT), 18.7% (95% CI: 1.3%, 31.4%) of PTBs were attributable to the total effects of AT and 11.1% (95% CI: 3.5%, 17.5%) to direct effects. Relative excess risks of interaction were null for each potential modifier.Conclusion: This additional evidence of a direct association between high temperature and PTB motivates public health interventions to reduce extreme heat exposures among pregnant women.