## 3 Results
### Exposure Distributions
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Pollutants** & **Mean \(\pm\) SD** & **Minimum** & **25 th Percentile** & **Median** & **75 th Percentile** & **IQR** & **Maximum** \\ \hline PM\({}_{2.5}\) (ug/m\({}^{3}\)) & 19.62 \(\pm\) 4.50 & 3.01 & 16.93 & 19.53 & 22.10 & 5.17 & 38.48 \\ PM\({}_{10}\) (ug/m\({}^{3}\)) & 21.35 \(\pm\) 5.12 & 3.17 & 18.47 & 21.15 & 24.06 & 5.59 & 43.78 \\ CO (ppb) & 295.09 \(\pm\) 51.50 & 119.89 & 264.02 & 295.97 & 329.74 & 65.72 & 482.45 \\ NO\({}_{2}\) (ppb) & 4.31 \(\pm\) 1.23 & 0.21 & 3.63 & 4.41 & 5.07 & 1.44 & 8.73 \\ SO\({}_{2}\) (ppb) & 3.94 \(\pm\) 1.95 & 0.19 & 2.76 & 3.75 & 4.71 & 1.95 & 15.88 \\ O\({}_{3}\) (ppb) & 23.79 \(\pm\) 2.80 & 11.20 & 21.64 & 23.93 & 26.00 & 4.36 & 36.93 \\ Temperature (\({}^{\circ}\)C) & 4.82 \(\pm\) 2.84 & \(-\)6.58 & 2.59 & 4.94 & 7.28 & 4.69 & 15.50 \\ \hline \end{tabular}  
* Note: Normality tests: PM\({}_{2.5}\) skewness = 0.104, kurtosis = 1.724; PM\({}_{10}\) skewness = 0.276, kurtosis = 2.086; CO skewness = \(-\)0.228, kurtosis = 0.448; O\({}_{3}\) skewness = \(-\)0.093, kurtosis = \(-\)0.217; NO\({}_{2}\) skewness = \(-\)0.374, kurtosis = 1.029; SO\({}_{2}\) skewness = 1.667, and kurtosis = 5.266.  
\end{table} 
Table 2: Distributions of air pollution and temperature parameters during entire pregnancy, Espoo 1 January 1983–31 March 1990. Table 4 shows the effects of average air pollution exposures during the entire pregnancy on the risk of low birth weight. The risk of LBW\({}_{3000}\) increased in relation to the highest quartile of CO in the two-pollutant model that adjusted for PM\({}_{10}\) (adjusted RR 1.44; 95% CI 1.04-2.00). The effect estimates for CO showed a similar pattern among babies born during spring and summer (adjusted risk ratio [aR] 1.61; 1.01-2.55) and during autumn and winter (aR 1.26; 0.78-2.03) when adjusting for PM\({}_{10}\). In addition, among babies born during spring and summer, the two-pollutant model that adjusted for O\({}_{3}\) showed increased risk of LBW\({}_{3000}\) (crude RR 1.29, 1.17-3.13). Exposure to O\({}_{3}\) was related to an increased risk of LBW\({}_{3000}\) among babies born during spring and summer in the single-pollutant model (aR 1.80, 1.10-2.94) and in the two-pollutant model adjusting for the other pollutants (e.g., adjusting for CO: aRR 1.88, 1.15-3.09). The risk of LBW\({}_{3000}\) was also increased in relation to NO\({}_{2}\) in the full model and among babies born during autumn or winter, although the lower 95% confidence interval (95% CI) was \(<\)1.00 (Table 4). The risk of LBW\({}_{3000}\) was not related to PM\({}_{2.5}\), PM\({}_{10}\), or SO\({}_{2}\) exposure in any of the models. However, when SO\({}_{2}\) was adjusted for CO in the two-pollutant model, we found an increased risk with an aRR of 1.72 (95% CI 1.06-2.81). We neither found any linear association when fitting air pollutant exposures as continuous variables in the models (Table A1) nor any increased risk when using exposure cutoff values at Q\({}_{3}\) (median value) in our analyses (Table A2). Table A3 presents the results of the sensitivity analyses using the traditional definition of low birth weight less than 2500 g (LBW\({}_{2500}\)). The risk of LBW\({}_{2500}\) increased in relation to PM\({}_{2.5}\). The adjusted RRs among children born in warm seasons varied from 2.31 to 2.45, being statistically significant in the single-pollutant model and when adjusting for O\({}_{3}\). The adjusted RRs of CO were systematically elevated and mainly statistically significant with higher effect estimates among children born during the warm season. Also, the adjusted RRs for SO\({}_{2}\) were elevated. The effect estimates for O\({}_{3}\) were elevated and statistically significant in all two-pollutant models, and the effect was stronger among children born in the warm season.