Significance of Overall Findings for Air Pollution

It should be noted that the adjusted ORs or RRs in all of the studies were relatively low (less than 1.68), with many estimates being much lower. The relatively small increased risk of preterm birth reflected by the relatively low ORs or RRs may represent a truly marginal increased risk or may be a consequence of the exposure assessment complexities discussed earlier. In addition, several of the studies analyzed multiple air pollutants, and the tendency to focus on positive findings should be balanced by consideration of the entire context of the study. Nonetheless, given the number of studies that have found significant relationships between exposures to air pollution and preterm birth, the epidemiological findings suggest that air pollution contributes to a woman's risk for preterm birth. Moreover, several studies reported exposure-response relationships of significance, particularly for sulfur dioxide and particulates (Bobak, 2000; Liu et al., 2003; Maroziene and Grazuleviciene, 2002; Ritz et al., 2000; Sagiv et al., 2005; Xu et al., 1995). Inconsistent findings that were regionally dependent were reported for carbon monoxide, and contradictory results were reported for nitrous oxides. Consequently, conclusions about the risk for preterm birth from ambient air exposures to carbon monoxide and nitrous oxides cannot be made at this time. Moreover, the two studies that evaluated ozone found that ozone exposure has no significant association with preterm birth, suggesting that ozone may not be a significant contributor to preterm birth. Likewise, the contradictory findings do not allow conclusions regarding a critical gestational stage for exposure to air pollutants and an increased risk of preterm birth.

Finally, the analysis by Xu et al. (1995) deserves further comment. Those investigators found that exposure to sulfur dioxide and total suspended particles modified the distribution of gestational age at birth, such that the largest effects were observed with exposures at the youngest gestational ages. Those researchers calculated an attributable risk (the proportion of cases of preterm delivery in the sample attributable to air pollution) of 33.4 percent. This high attributable risk indicates that exposure to elevated levels of sulfur dioxide and particulate air pollution poses a significant public health concern for pregnant women. Although the concentrations of sulfur dioxide and particulate air pollution that Xu and colleagues (1995) measured in Beijing air are not anticipated in the United States, the study nonetheless illustrates the potential and preventable impacts that air pollutants can have on preterm birth.

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Gaining Weight 101

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