In Valencia, median outdoor NO2 levels (42 µg/m3) were higher than median indoor levels (36 µg/m3). In Sabadell, personal NO2 showed the highest median levels (40 µg/m3), followed by indoor (32 µg/m3) and outdoor (29 µg/m3) levels. Personal exposure to NO2 correlated best with the indoor NO2 levels. Temporal and traffic-related variables were significant predictors for outdoor NO2 levels. Thirty-two percent of the indoor NO2 variability in the two cohorts was explained by outdoor NO2 levels and the use of the gas appliances. The model for personal exposure accounted for 59 % of the variance in NO2 levels in Sabadell with four predictor variables (outdoor and indoor NO2 levels, time spent in outdoor environments and time exposed to a gas cooker). No significant association was found between personal or indoor NO2 levels and exposure to environmental tobacco smoke (ETS) at home.
Personal NO2 levels were found to be strongly influenced by indoor NO2 concentrations. The study supports the use of time–activity patterns along with indoor measurements to predict personal exposure to traffic-related air pollution.