Distance to major road
The closest distance between each child's home address and a major road, which was defined as a road with more than 5000 vehicles per day, was also included as an indicator of traffic-related air pollution exposure. Furthermore, as we found a sharp decrease of concentrations of air pollutants when the distance to a major road is bigger than 350 m, we also dichotomized this variable (= < 350 m versus >350 m to major road).