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ered in an investigation of preterm birth. Because of the large number of potential exposures, an efficient and effective research program that will serve public health needs is required.

Various environmental chemical pollutants are recognized as risk factors for numerous diseases and pathophysiological responses, and with the recognition that these pollutants pose a risk, policies have been instituted to protect the public health. Lead is perhaps the most renowned toxicant for which such actions have been taken, with national policies and programs established to protect children from neurotoxicity by removing lead from gasoline and paint. However, the potential risk for preterm birth as a result of exposure to environmental pollutants is poorly understood. This lack of knowledge presents a potentially significant shortcoming in the design of public health preventive strategies.

The present discussion specifically considers epidemiological studies that have analyzed the associations between exposure to environmental chemical pollutants and preterm birth. For this review, preterm birth is defined as the delivery of a live infant before 37 completed weeks of gestation, unless an alternative definition was used by a particular study. Analyses and findings of associations between environmental chemical exposures and low birth weight are not discussed. Because differences in mean gestational age at birth may or may not be relevant for preterm birth, depending on whether or not the distribution is affected at the lower gestational ages, studies in which gestational age was used as a continuous variable are discussed only as they contribute to the understanding of preterm birth.

Studies for consideration were identified on the basis of a PubMed Boolean search that crossed various terms for preterm birth (e.g., “premature birth,” “prematurity,” and “preterm delivery”) or the term “birth outcomes” with general terms for toxicants (e.g., “air pollutants,” “water pollutants,” and “pesticides”) and terms for selected specific pollutants (e.g., “dioxin,” “polychlorinated biphenyls,” and “DDT” [dichlorodiphenyltrichloroethane]). If the results of the studies evaluated were adjusted for confounders, only the adjusted statistics were considered in the final review. If adjustments were not made for potential confounders, the crude odds ratio (OR), crude relative risk (RR; risk ratio), or the most relevant statistical result was considered. The results were considered significant if they achieved statistical significance in the particular study and the confidence interval of the statistical estimate of adjusted OR or RR did not include unity (1.0). Although an attempt has been made to be thorough, some studies in this area may have been overlooked.



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