Statistical analysis
The analyses were carried out using the statistical software R (version 2.14.1) (R Core Team, 2013). Pearson's chi-square, the Student's t-test and the Wilcoxon rank sum test were used to assess differences between children included in this analysis and those from the original birth cohorts for categorical, normally distributed continuous variables, and continuous variables with a skewed distribution, respectively. Unadjusted correlations between ambient air pollutants and bone turnover markers were assessed with the Spearman correlation coefficient.