of vulnerability have been inconsistent across studies (Slama et al. 2008). The International Collaboration on Air Pollution and Pregnancy Outcomes has recently emerged as an orga- nized effort to streamline the methods used and potentially reconcile some of the apparent differences in results across studies (Woodruff et al. 2010). The magnitude of birth weight reduc- tions we observed was modest, ranging from a decrease of 3.9 g to 16.5 g per IQR increase in pollutant concentration. In comparison, maternal active smoking during pregnancy is thought to reduce birth weight by an average of 250 g (U.S. Department of Health and Human Services 2001), and maternal passive smoking is thought to reduce birth weight by an average of 30 g (U.S. Department of Health and Human Services 2006). Although the observed reductions have little clini- cal significance for an individual infant, on a population level a downward shift in the birth weight distribution would result in an increased number of infants at low birth weight (< 2,500 g). Although we scaled the pollutant estimates to an IQR increase in pol- lutant concentration, the range of pollutant concentrations extends well beyond the IQR. In addition, observed associations may have been biased toward the by measurement error, particularly for spatially heterogeneous pollutants such as NO2, CO, and PM2.5 ele- mental carbon. One possible explanation for seemingly contradictory results for PM and fetal growth across geographic locations is the difference in chemical composition and corresponding toxicity of PM across study sites (Parker and Woodruff 2008). Because of limited moni- toring data, previous investigators have been unable to assess measures of reduced fetal growth in relation to specific PM2.5 compo- nents. A major strength of this study is the availability of daily measurements of PM2.5 component data between 1998 and 2004. PM2.5 elemental carbon and PM2.5 water- soluble metal concentrations in the third trimester were significantly associated with Air pollution and birth weight reduced birth weight, whereas other compo- nents of PM were not. In the distributed lag analyses, PM2.5 organic carbon concentrations throughout pregnancy were associated with decreased birth weight. The strongest asso- ciation we observed was with third-trimester PM2.5 water-soluble metals, a sum of chro- mium, copper, iron, manganese, nickel, and vanadium. These metals have been shown to accumulate in fetal tissues and at high lev- els may have direct effects on fetal growth (Domingo 1994). Alternatively, our water- soluble metals index may be acting as a sur- rogate for other unmeasured pollutants from the same sources—for example, metal refining (chromium, iron, manganese), fuel oil com- bustion (nickel, vanadium), or motor vehicles (copper from brake linings). Results of our analysis also suggested effect modification by race and ethnic- ity. Associations between birth weight and third-trimester CO, O3, PM10, PM2.5–10, and PM2.5 were significantly stronger in Hispanics and non-Hispanic African Americans than Table 4. Mean change in birth weight (Δg) and 95% confidence intervals (CI) for ambient air pollution levels in the third trimester for full-term births in five-county Atlanta by race and ethnicity. Pollutant 1-hr maximum concentration CO (ppm)* NO2 (ppb) SO2 (ppb) 8-hr maximum O3 (ppb)* 24-hr PM concentration (μg/m3) PM10* PM2.5* PM2.5–10* PM2.5 sulfate PM2.5 nitrate PM2.5 elemental carbon PM2.5 organic carbon PM2.5 water-soluble metals n 183,946 183,946 183,946 183,946 144,761 100,562 88,135 79,275 75,193 100,446 100,446 78,715 Non-Hispanic white Δg (95% CI) –5.6 (–13.3 to 2.1) –4.6 (–9.3 to 0.1) –5.2 (–9.2 to –1.2)** 4.3 (–5.1 to 13.6) 3.5 (–2.2 to 9.1) 0.1 (–6.0 to 6.2) 2.3 (–2.9 to 7.5) –5.2 (–16.3 to 5.9) –10.3 (–21.0 to 0.3) –5.5 (–12.8 to 1.7) –3.4 (–9.5 to 2.8) –14.7 (–27.1 to –2.3)** n 164,731 164,731 164,731 164,731 135,077 97,861 86,225 77,894 74,170 97,727 97,727 77,418 Non-Hispanic black Δg (95% CI) –6.6 (–14.3 to 1.2) –3.9 (–8.7 to 0.8) –1.9 (–5.9 to 2.2) –7.6 (–17.1 to 1.8) –5.5 (–11.2 to 0.2) –6.3 (–12.5 to –0.2)** –5.4 (–10.6 to –0.2)** –11.7 (–22.9 to –0.6)** –2.7 (–13.3 to 7.9) –7.9 (–15.2 to –0.7)** –3.2 (–9.3 to 3.0) –17.4 (–29.8 to –5.0)** n 57,950 57,950 57,950 57,950 53,352 45,058 40,776 37,508 36,234 45,022 45,022 37,244 Hispanic Δg (95% CI) –14.4 (–23.4 to –5.5)** –5.8 (–12.4 to 0.7) –5.7 (–11.4 to –0.1)** –11.1 (–22.1 to –0.0)** –12.1 (–19.6,–4.7)** –10.2 (–17.6 to –2.7)** –8.6 (–14.8 to –2.5)** –9.4 (–22.0 to 3.2) –2.1 (–1 3.8 to 9.4) –8.6 (–17.1 to –0.2)** –3.9 (–11.8 to 3.9) –18.7 (–32.2 to –5.1)** Change in birth weight and 95% CIs correspond to an IQR increase in third-trimester pollutant concentration reported in Table 2. Models controlled for long-term trends, seasonal trends, gestational week, sex, and maternal marital status, education, age, race/ethnicity, reported tobacco use, and parity. *p < 0.05 for chunk test of interaction terms. **p < 0.05 for pollutant beta. Table 5. Mean change in birth weight (Δg) and 95% confidence intervals (CI) for ambient air pollution levels in selected gestational windows for births with a maternal residential address within 4 miles of a monitor. First month of gestation Third trimester n monitor Pollutant na Δg (95% CI) na Δg (95% CI) capture areasb 1-hr maximum concentration CO (ppm) 45,985 –5.2 (–10.0 to –0.5)* 44,226 1.8 (–6.5 to 10.3) NO2 (ppb) 48,112 –1.6 (–5.7 to 2.5) 46,251 3.6 (–3.7 to 11.0) 8-hr maximum O3 (ppb) 29,281 6.8 (–13.3 to 26.9) 26,054 5.8 (–27.1 to 38.7) 24-hr PM concentration (μg/m3) PM10 16,512 2.2 (–11.3 to 15.6) 16,384 15.9 (–8.6 to 40.5) PM2.5 37,354 –4.2 (–13.6 to 5.2) 35,026 –2.5 (–20.4 to 15.3) PM2.5–10 11,259 –0.6 (–15.0 to 13.9) 12,570 21.0 (2.6 to 39.4)* PM2.5 sulfate 10,518 –1.8 (–23.5 to 20.0) 11,360 –11.8 (–54.4 to 30.9) PM2.5 nitrate 10,515 –5.2 (–30.8 to 20.5) 10,822 –3.0 (–44.2 to 38.2) PM2.5 elemental carbon 12,918 –2.3 (–14.7 to 10.2) 14,292 4.6 (–23.2 to 32.4) PM2.5 organic carbon 12,918 –4.4 (–16.6 to 7.9) 14,292 2.7 (–18.2 to 23.5) PM2.5 water-soluble metals 10,576 14.5 (–9.7 to 38.6) 11,290 –8.4 (–55.5 to 38.8) Change in birth weight and 95% CIs correspond to an IQR increase in pollutant value for each exposure window reported in Table 2. Models control for long-term trends, seasonal trends, gestational week, sex, and maternal marital status, education, age, race/ethnicity, reported tobacco use, and parity. aNumber of births in pooled analysis. bOnly monitors with daily measurements were used. *p < 0.05. 2 6 1 1 1 1 1 1 3 4 3 Environmental Health Perspectives • volume 119 | number 5 | May 2011 735