## 3 Results
### Exposure Distributions  
Table 2 presents the mean values and distributions of air pollutants and ambient temperature for the entire pregnancy. The mean concentrations of PM\({}_{2.5}\), PM\({}_{10}\), CO, NO\({}_{2}\), O\({}_{3}\), and SO\({}_{2}\) during the entire pregnancy were 19.6 ug/m\({}^{3}\), 21.4 ug/m\({}^{3}\), 295.1 ppb, 4.3 ppb, 3.9 ppb, and 23.8 ppb, respectively. The mean temperature was 4.8 \({}^{\circ}\)C. Table 2 presents also skewness and kurtosis of the distributions of air pollutants in the footnote. Based on these measures, only the distribution of SO\({}_{2}\) deviated substantially from normal distribution (kurtosis 5.266). Table 3 represents the correlations of the average concentration of air pollutants. In most cases, the correlation was positive and the lowest Pearson correlation coefficient was found between CO and SO\({}_{2}\). For O\({}_{3}\), however, we detected negative correlations between O\({}_{3}\) and CO and between SO\({}_{2}\) and O\({}_{3}\), with the coefficient r being \(-\)0.26151 and \(-\)0.36899, respectively. The Spearman correlations between SO\({}_{2}\) and the other pollutant did not differ substantially from the corresponding Pearson correlation coefficients (PM\({}_{2.5}\) = 0.90292, PM\({}_{10}\) = 0.92383, CO = 0.92383, and NO\({}_{2}\) = 0.95573). \begin{table}
\begin{tabular}{c c c c c} \hline
**Characteristic** & \multicolumn{2}{c}{**Low Birth Weight \(<\)3000 g**} & \multicolumn{2}{c}{**Non-Low Birth Weight**} & \multicolumn{1}{c}{**Total**} \\  & \multicolumn{2}{c}{\(n\) (\%)} & \multicolumn{2}{c}{\(n\) (\%)} & \multicolumn{2}{c}{\(n\) (\%)} \\ \hline Total & 344 (13.40) & 2216 (86.56) & 2568 (100.00) \\ Mean birth weight (Mean \(\pm\) SD) & 2742.02 \(\pm\) 228.59 & 3675.24 \(\pm\) 406.62 & 3523.41 \(\pm\) 559.77 \\ Gender & & & & \\ boy & 157 (45.64) & 1149 (51.85) & 1311 (51.05) \\ girl & 187 (54.36) & 1067 (48.15) & 1257 (48.95) \\ Maternal age at delivery (years) 1 & & & \\ \(<\)25 & 59 (17.25) & 308 (14.03) & 370 (14.53) \\
25–30 & 115 (33.63) & 845 (38.48) & 960 (37.71) \\ \(>\)30 & 168 (49.12) & 1043 (47.50) & 1216 (47.76) \\ Family socioeconomic status 2,3 & & & \\ high & 76 (22.22) & 520 (23.57) & 597 (23.36) \\ low or medium & 266 (77.78) & 1686 (76.43) & 1959 (76.64) \\ Single parent or guardian & & & & \\ yes & 35 (10.17) & 147 (6.63) & 183 (7.13) \\ no & 309 (89.83) & 2069 (93.37) & 2385 (92.87) \\ Maternal smoking in pregnancy & & & & \\ yes & 77 (22.38) & 287 (12.97) & 364 (14.17) \\ no & 267 (77.62) & 1925 (87.03) & 2204 (85.83) \\ Environmental tobacco smoke exposure of the pregnant mother 4 & & & \\ yes & 17 (6.91) & 84 (4.88) & 101 (5.11) \\ no & 229 (93.09) & 1639 (95.12) & 1874 (94.89) \\ \hline \end{tabular}  
* The ages of 22 persons were missing. 2 High vs. low or medium parental education and occupation. 3 There were 12 persons with missing information in this category. 4 There were 593 persons with missing information on prenatal environmental tobacco smoke exposure. Birth weight was missing among 8 babies, and exposure information was missing among 43 mothers.  
\end{table} 
Table 1: Characteristics of the study population, The Espoo Cohort Study, 1984–1990.