Green et al. were more likely to be younger, report smoking during their pregnancy, be of lower SES, have recently experienced two or more stressful life events, and report working at time of interview. Table 3 shows the AORs for SAB and traffic exposure using several traffic metrics. Among the first three metrics listed, maximum AADT (daily maximum traffic) within 50 m still showed the strongest association with SAB, although not statistically significantly, in mod- els without the temporal adjustment (AOR for 90th vs. 75th percentile = 1.18; 95% CI, 0.87– 1.60). Maximum traffic within 100 m showed stronger associations when the traffic counts were scaled using NO2 levels during the first (AOR = 1.23; 95% CI, 0.89–1.70) and second (AOR = 1.24; 95% CI, 0.89–1.71) months of pregnancy. The odds of SAB did not differ by distance to an arterial road. Although maxi- mum weighted traffic within 150 m was not associated with SAB in the analysis without the temporal adjustment, adjusting for NO2 levels during the first month of pregnancy increased the association between SAB and that traffic metric (Table 3). There was no difference in the temporally adjusted data for any of the traffic metrics when the analysis was limited to study subjects who lived within 10 km of an air pollution monitor (data not shown). A priori hypotheses regarding variables as possible effect modifiers were also assessed using the traffic metric (maximum traffic within 50 m) with the strongest association with SAB in the adjusted analyses. No effect modifica- tion was found by maternal age, employment status, season of conception, reported use of air conditioning, time spent commuting, or regional site. We did not see effect modifica- tion by region, even though the background levels of carbon monoxide, NO2, ozone, and particulate matter < 10 µm in aerodynamic diameter varied among the regions, with the San Bernardino area recording higher levels for most pollutants. This suggests that local varia- tions in levels of traffic-related pollutants were more important than regional differences. However, we did observe effect modifi- cation by race/ethnicity and smoking status, and there was a suggestion of interaction for gestational age of SAB. The AOR for African Americans in the highest traffic exposure cat- egory compared to the lowest was 3.11 (95% CI, 1.26–7.66) (Table 4), whereas among whites (and other racial/ethnic groups), traffic was not significantly associated with SAB. The excess risk of SAB in relation to traffic was about doubled in African Americans com- pared with whites. Furthermore, the test for interaction by race/ethnicity found that this racial/ethnic difference was statistically signifi- cant (RERI = 2.2; 95% CI, 0.7–3.6). Results were similar when the analysis was adjusted for NO2 levels during the first and second months of gestation. 0.91 (0.68–1.21) 1.18 (0.87–1.60) 1.04 (0.78–1.38) 1.16 (0.84–1.59) 1.02 (0.76–1.36) 1.17 (0.85–1.62) Table 3. Effects of traffic exposure on SAB using different traffic metrics. Traffic metric No. Maximum daily traffic within 50 mb [percentile (range)] AORa (95% CI) AORa after traffic scaled using NO2 0–29 days after LMP (95% CI) AORa after traffic scaled using NO2 30–59 days after LMP (95% CI) Among women who were nonsmokers, significantly increased odds of SAB were observed in the highest traffic exposure group (AOR = 1.47; 95% CI, 1.07–2.04). Results were unchanged after the temporal adjust- ments. However, among women who did report smoking at some point in their preg- nancy, a significantly negative association was found (AOR = 0.36; 95% CI, 0.14–0.93) when examining the highest versus the lowest traffic exposure groups. These differences in the effect of traffic between smoking groups were statistically significant (RERI for non- smokers = 0.95; 95% CI, 0.43–1.46). The effect estimates using maximum traffic at 50 m as the exposure metric were slightly higher for pregnancy losses at ≤ 10 weeks (AOR = 1.35; 95% CI, 0.87–2.10) versus later losses (AOR = 1.08; 95% CI, 0.73–1.60). Discussion To our knowledge, this is the first published study of the effect of residential traffic expo- sure on the risk of SAB. We found a slightly increased risk of SAB among women residing within 50 m of a road with an AADT above the 90th percentile (or ≥ 15,200 vehicles in this study) compared with women residing near roads with maximum AADT below the 75th percentile (about 1,000 vehicles). Temporal adjustment for average NO2 levels during the first or second month of gesta- tion did not change the magnitude of the association for that particular traffic metric. The increase in risk associated with traffic was stronger in African-American women than in whites and in nonsmokers than in smokers. Comparison with previous studies. Although the present study is the first to evalu- ate the association between traffic exposure and SAB, two previous studies conducted in Europe examined the association between SABs before 26 weeks of gestation, which were ascertained from hospital records, and indus- trial air pollution. Although the Norwegian study (Hansteen et al. 1996) failed to find an association, the Finnish study (Hemminki and Niemi 1982) found an increased rate of SAB in areas with a higher mean annual level of hydro- gen sulfide (> 4 µg/m3). A time-series study in Brazil (Pereira et al. 1998) found a strong association between stillbirth and both NO2 as an individual pollutant and an index that com- bined NO2, CO, and sulfur dioxide. SAB has also been associated with ETS, which contains many of the same chemicals as traffic exhaust (George et al. 2006; Windham et al. 1999). Biological mechanisms. There are bio- logically plausible mechanisms by which the association between SAB and emissions from vehicular traffic could be causal. Researchers in Tehran found an association between ambient CO and both carboxyhemoglobin (COHb) and nucleated red blood cells, a sign of fetal ovlume 117 | number 12 | December 2009 • Environmental Health Perspectives Maximum weighted traffic within 150 m (percentile) NA, not applicable. aAdjusted for maternal age, race, employment status, SES, stressful life events (0–1 vs. ≥ 2 in the preceding 6 months), and maternal smoking (at LMP or since). bLocal roads were assigned a value of zero. c Arterial road was defined by functional class rather than by daily traffic; therefore, NO2 adjustment could not be computed for this traffic metric. Maximum daily traffic within 100 mb [percentile (range)] Maximum daily traffic within 150 mb [percentile (range)] 0–74th (0–1,089) 75–89th (1,090–15,199) ≥ 90th (≥ 15,200) 0–49th (0) 50–74th (1–11,499) 75–89th (11,500–25,799) ≥ 90th (≥ 25,800) 0–49th (0–5,269) 50–74th (5,270–17,999) 75–89th (18,000–33,199) ≥ 90th (≥ 33,200) Distance to nearest arterial (m)c 0–50 51–100 101–200 201–300 > 300 0–49th 50–74th 75th–89th ≥ 90th 1942 3,734 741 504 2,530 1,203 746 500 2,488 1,233 757 501 882 783 1,196 868 1,250 2,489 1,245 747 498 1 1 1 1 1 0.97 (0.76–1.24) 1.10 (0.84–1.46) 1.11 (0.80–1.54) 1.02 (0.80–1.29) 0.88 (0.66–1.18) 0.99 (0.71–1.38) 1.00 (0.74–1.34) 0.93 (0.68–1.26) 0.89 (0.67–1.18) 1.00 (0.75–1.35) 1.07 (0.85–1.36) 1.05 (0.79–1.40) 1.08 (0.78–1.50) 1 1 1 1.00 (0.78–1.28) 1.06 (0.79–1.42) 1.23 (0.89–1.70) 0.91 (0.71–1.17) 1.08 (0.81–1.43) 1.05 (0.75–1.48) 1 1 1 0.98 (0.76–1.27) 0.97 (0.72–1.32) 1.24 (0.89–1.71) 0.97 (0.76–1.25) 0.96 (0.71–1.29) 1.05 (0.74–1.47) 1 1.04 (0.82–1.34) 1.02 (0.76–1.38) 1.21 (0.87–1.67) 1 1.06 (0.83–1.36) 0.96 (0.71–1.30) 1.13 (0.81–1.58)