Air quality assessment

We used portable devices to measure ultrafine particles (UFPs) with a diameter 10–300 nm (Aerasense NanoTracer; Phillips, Eindhoven, The Netherlands), and particulate matter (PM with diameter < 2.5 μm) (AEROCET 531; MetOne Instruments Inc., Grants Pass, Oregon, US) in the school and at the playground on the examination days as part of the field work. The measurements were performed in the morning (9–12 a.m.). For each child, the measured outdoor pollution levels of the 10 min recess when children were at the school playground, before the study visit were used. Thus, outdoor exposure levels reflect ambient air pollution during the last time that the child was outside, which was approximately one hour before saliva donation. The measurements are significantly correlated with modeled concentrations at the school site (Additional file 1: Table S1).

We used a spatial temporal interpolation method to model the daily residential exposure levels (μg/m³) of PM_2.5, at each child’s home address. This method takes into account land cover data obtained from satellite images (CORINE land cover data set) [48] and pollution data of fixed monitoring stations in combination with a dispersion model [49]. The model calculates the daily interpolated exposure concentrations in a high resolution receptor grid based on information from the Belgian telemetric air quality networks, point sources, and line sources. Overall model performance was evaluated by leave-one-out cross-validation. Validation statistics of the interpolation tool gave a spatial temporal explained variance of more than 0.80 for PM_2.5 [50]. We used this model to estimate the residential exposure on the day (lag 1) of the examination, the day (lag 1) before the examination and two days (lag 2) before the examination as well as the average exposure of the 48 h before the examinations.

Since the parameters for fine and ultrafine PM were measured or interpolated in a exposure window ranging from hours to two days before sampling, they reflect recent exposure.