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advantaged and of a nonwhite race were more likely to live in neighborhoods with more air pollution (Ponce et al., 2005; Woodruff et al., 2003). Because socioeconomic disadvantage and African American race are risk factors for preterm birth (see Chapter 4), geographical or ecological assessments of exposure to pollutants may be confounded by socioeconomic condition and race.

A variety of comparison strategies have been used. Many studies compared women who resided in a more polluted community with women who resided in a less polluted community. Some studies that used stationary air-monitoring station data to estimate exposures evaluated exposure-response relationships by analyzing the trends in preterm birth rates with respect to the exposure level on the basis of the woman’s residence. Another approach has been to study the same population over time, because air pollutant levels vary significantly due to short-term fluctuations and the seasons (Sagiv et al., 2005; Sram et al., 1996; Wilhelm and Ritz, 2005). Moreover, the air concentrations of certain pollutants are known to correlate with one another. Because of this potential confounding, some studies that monitored exposure to multiple air pollutants also analyzed or adjusted for copollutant effects (Sagiv et al., 2005; Xu et al., 1995).

Sulfur Dioxide and Particulates

The most consistent relationships between specific air pollutants and preterm birth have been reported for sulfur dioxide and particulates. Of particular relevance to the United States is a study of births in Vancouver, British Columbia, Canada, conducted between 1985 and 1998, which reported an association between sulfur dioxide air pollution and preterm birth with an adjusted OR of 1.09 (95% confidence interval [CI] = 1.01–1.19) per 5.0 parts per billion (ppb) (14.3 micrograms per cubic meter [μg/m3]) of sulfur dioxide (Liu et al., 2003). The mean daily concentration of sulfur dioxide during the study period was 4.9 ppb, with a maximum peak concentration (over 1 hour) of 128.5 ppb. Studies of women in the Czech Republic and Beijing, China, who were exposed to sulfur dioxide pollutant concentrations higher than usual for those in the United States reported concentration-dependent relationships between ambient air sulfur dioxide concentrations and preterm birth, with adjusted ORs of 1.27 (95% CI 1.16– 1.39) per 50-μg/m3 increase in the sulfur dioxide concentration (Bobak, 2000) and 1.21 (95% CI 1.01–1.45) for each ln natural log unit μg/m3 increase in the sulfur dioxide concentration (Xu et al., 1995). The rates of preterm births also increased among women living in the vicinity of a coalburning power plant in Croatia during a period of high-intensity power plant operation with higher sulfur dioxide emissions compared with those

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