Int. J. Environ. Res. Public Health 2012, 9 283 of preterm births attributed to the upper quartile of first trimester O3 exposure was 7.8% (corresponding to 129 preterm births) for this exposure category. The upper quartile of last week NO2 exposure has a mean value of 16.9 µg/m3 over the lowest quartile, which corresponds to an OR of 1.11. The estimated fraction of preterm births attributable to the highest quartile of late pregnancy NO2 exposure was 5.2% (corresponding to 85 preterm births) for this exposure category. Heavy smokers had a reduction in duration of gestation estimated to approximately 0.19 [95% CI = (−0.23, −0.14)] weeks, while moderate smokers had no reduction. The upper quartile of first trimester O3 exposure has a mean value of 27.7 μg/m3 over the lowest quartile, which corresponds to 0.13 [95% CI = (−0.21, −0.05)] weeks shorter gestation (using the adjusted slope). However, this is a mean value, but the real effect is likely not evenly distributed. The average reduction in duration of gestation for women exposed to levels within the highest quartile of last week NO2 compared to the lowest quartile was 0.068 [95% CI = (−0.096, −0.040)] weeks. 5. Conclusions The results found in this study indicate that there is an early (first trimester) effect of O3 in accordance with suggested pathways for the early exposure window. The effect of O3 was smallest among women who conceived during the autumn when the vitamin D levels in Scandinavian women should be the highest. We observed an adverse effect of exposure to elevated NO2 levels in late gestation on preterm delivery. However, no spatial variation in NO2 was modelled. There remains a need to study these associations further with subject specific exposure assessments. However, the total size of these effects supports improvements in air quality also in a moderately polluted region. Conflict of Interest Acknowledgements References The authors declare no conflict of interest. This study was supported by CMF (Centre for Environmental Research) in Umeå and the Umeå SIMSAM Node. We acknowledge Anders Bucht for his toxicological expertise, Xavier de Luna for his statistical advice and Adrian Lowe for valuable help with the manuscript. 1. 2. 3. Maisonet, M.; Correa, A.; Misra, D.; Jaakkola, J.J.K. A review of the literature on the effects of ambient air pollution on fetal growth. Environ. Res. 2004, 95, 106-115. Srám, R.J.; Binková, B.; Dejmek, J.; Bobak, M. Ambient air pollution and pregnancy outcomes: A review of the literature. Environ. Health Perspect. 2005, 113, 375-382. Ritz, B.; Wilhelm, M.; Hoggatt, K.J.; Ghosh, J.K.C. Ambient air pollution and preterm birth in the environment and pregnancy outcomes study at the University of California, Los Angeles. Am. J. Epidemiol. 2007, 166, 1045-1052.