American Journal of Epidemiology Advance Access originally published online on August 4, 2007
American Journal of Epidemiology 2007 166(9):1045-1052; doi:10.1093/aje/kwm181
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ORIGINAL CONTRIBUTIONS |
Ambient Air Pollution and Preterm Birth in the Environment and Pregnancy Outcomes Study at the University of California, Los Angeles
1 Department of Epidemiology, School of Public Health, University of California, Los Angeles, CA
2 Department of Epidemiology, University of Michigan, Ann Arbor, MI
Correspondence to Dr. Beate Ritz, Department of Epidemiology, School of Public Health, University of California, Los Angeles, P.O. Box 951772, Los Angeles, CA 90095 (britz{at}ucla.edu).
Received for publication March 21, 2007. Accepted for publication May 18, 2007.
The authors conducted a case-control survey nested within a birth cohort and collected detailed risk factor information to assess the extent to which residual confounding and exposure misclassification may impact air pollution effect estimates. Using a survey of 2,543 of 6,374 women sampled from a cohort of 58,316 eligible births in 2003 in Los Angeles County, California, the authors estimated with logistic regression and two-phase models the effects of pregnancy period-specific air pollution exposure on the odds of preterm birth. For the first trimester, the odds of preterm birth consistently increased with increasing carbon monoxide exposures and also at high levels of exposure to particulate matter less than or equal to 2.5 µm in diameter (>21.4 µg/m3), regardless of type of data (cohort/sample) or covariate adjustment (carbon monoxide exposures of >1.25 ppm increased the odds by 21–25%). Women exposed to carbon monoxide above 0.91 ppm during the last 6 weeks of pregnancy experienced increased odds of preterm birth. Crude and birth certificate covariate-adjusted results for carbon monoxide differed from each other. However, further adjustment for risk factors assessed in the survey did not change effect estimates for short-term pollutant averages appreciably, except for time-activity patterns, which strengthened the observed associations. These results confirm the importance of reducing exposure misclassification when evaluating the effect of traffic-related pollutants that vary spatially.
air pollution; confounding factors (epidemiology); misclassification; premature birth; two-phase regression; vehicle emissions
Abbreviations: CI, confidence interval; EPOS, Environment and Pregnancy Outcomes Study; OR, odds ratio; PM2.5, particulate matter less than or equal to 2.5 µm in diameter (PM10 and PM0.1 defined analogously)
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