The first step of the exposure modeling was to calculate the weekly means for each station for each pollutant under the condition that more than 2/3 of the half- hourly values were available. The second step was interpolating these means to pollutant surfaces using the inverse distance weighting method (Hwang and
Jaakkola 2008). The weighting function is w(d) ¼ 1/distancep with p ¼ 2, where d
represents distance from the specified monitoring station. Fig. 1 shows w(d) for one week of results for PM10. Mean exposure values for each week during pregnancy were estimated for each birth according to the maternal address. The third step was to calculate the mean values of weeks 3–8 of pregnancy if all weekly means for this period were available.