## 5 Conclusions  
The present study provides new evidence that prenatal exposure to air pollutants in a cold climate increases the risk of LBW at relatively low levels of air pollution. Our results strengthen the evidence of independent effects of CO and O\({}_{3}\) exposures on the risk of LBW. Our study shows, for the first time, synergistic adverse effects of air pollutants on fetal growth. Our results also indicate the presence of season-specific effects. From the public health perspective, our findings of independent and synergistic adverse effects of low-level air pollution on low birth weight is alarming and underlines the need to reduce air pollution worldwide. **Author Contributions:** Conceptualization, H.A.B., A.K.R., N.S., J.K., M.S.J., and J.J.K.J. ; methodology, H.A.B., A.K.R., H.A., N.S., J.K., M.S.J., and J.J.K.J; software, H.A. ; formal analysis, H.A.B., A.K.R., N.S., and J.J.K.J. ; data curation, H.A. ; writing--original draft preparation, H.A.B. and J.J.K.J. ; writing--review and editing, A.K.R., H.A., N.S., A.K.A., N.R.I.R., J.K., M.S., and M.S.J. ; supervision, J.J.K.J; funding acquisition, J.K., M.S.J., and J.J.K.J. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was supported by the Academy of Finland (grant numbers 266314, 267675, 267995 (APTA Consortium), 310372, and 313073 (GLORIA Consortium)) and by The Research Foundation of the Pulmonary Diseases. N.S. received PhD scholarships from the Health and Biosciences Doctoral Programme (HBS-DP), University of Oulu and the University of Oulu Scholarship Foundation. The funders had no role in study design, data collection or analysis, decision to publish, or preparation of the manuscript. **Conflicts of Interest:** The authors declare no conflict of interest. **Appendix A**  
**Table A1. ** The effect of air pollution exposure during entire pregnancy per 10 unit increase in concentration on low birth weight less than 3000 g (N = 2517), The Espoo Cohort Study, 1984-1990. \begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Single Pollutant**} & \multicolumn{3}{c}{**Low Birth Weight \textless{}3000 g**} & \multicolumn{3}{c}{**Born in Spring–Summer**} & \multicolumn{3}{c}{**Born in Autumn–Winter**} \\ \cline{2-9}  & **Cr. RR** & **Adj. RR** & **Cr. RR** & **Adj. RR** & **Cr. RR** & **Adj. RR** \\  & **(95\% CI)** & **(95\% CI)** & **(95\% CI)** & **(95\% CI)** & **(95\% CI)** & **(95\% CI)** \\ \hline PM\({}_{2.5}\) per 10 ug/m\({}^{3}\) & 0.92 & 0.91 & 0.95 & 0.95 & 0.86 & 0.86 \\ PM\({}_{2.5}\) per 10 ug/m\({}^{3}\) & (0.72–1.16) & (0.71–1.16) & (0.69–1.33) & (0.68–1.32) & (0.60–1.23) & (0.60–1.23) \\ PM\({}_{10}\) per 10 ug/m\({}^{3}\) & 0.93 & 0.92 & 0.96 & 0.95 & 0.88 & 0.88 \\  & (0.75–1.14) & (0.74–1.14) & (0.71–1.28) & (0.71–1.28) & (0.65–1.21) & (0.64–1.21) \\ O\({}_{3}\) per 10 ppb & 0.84 & 0.79 & 0.98 & 0.98 & 0.64 & 0.62 \\  & (0.58–1.24) & (0.51–1.25) & (0.53–1.80) & (0.53–1.80) & (0.34–1.22) & (0.33–1.19) \\ NO\({}_{2}\) per 10 ppb & 0.89 & 0.91 & 0.87 & 0.89 & 0.90 & 0.91 \\  & (0.37–2.12) & (0.37–2.20) & (0.25–2.98) & (0.25–3.13) & (0.26–3.12) & (0.26–3.20) \\ CO per 10 ppb & 1.00 & 1.00 & 1.01 & 1.01 & 0.99 & 0.99 \\  & (0.98–1.02) & (0.98–1.02) & (0.98–1.04) & (0.98–1.04) & (0.96–1.02) & (0.96–1.02) \\ SO\({}_{2}\) per 10 ppb & 0.94 & 0.94 & 0.90 & 0.90 & 0.98 & 0.98 \\  & (0.54–1.63) & (0.54–1.65) & (0.42–1.94) & (0.42–1.94) & (0.43–2.21) & (0.43–2.21) \\ \hline \end{tabular} 
\end{table}
Table 1: The effect of air pollution exposure during entire pregnancy per 10 unit increase in concentration on low birth weight less than 3000 g (N = 2517), The Espoo Cohort Study, 1984–1990.