Determinants of outdoor, indoor and personal concentrations of nitrogen dioxide (NO
2) were assessed in a subset of pregnant women of the Spanish INMA (Environment and Childhood) Study. Home indoor and outdoor NO
2 concentrations were measured during 48 h with passive samplers for 50 and 58 women from the INMA cohorts of Valencia and Sabadell, respectively. Women from Sabadell also carried personal NO
2 samplers during the same period. Data on time–activity patterns, socio-economic characteristics, and environmental exposures were obtained through questionnaires. Multiple linear regression models were developed to predict NO
2 levels.In Valencia, median outdoor NO
2 levels (42 µg/m
3) were higher than median indoor levels (36 µg/m
3). In Sabadell, personal NO
2 showed the highest median levels (40 µg/m
3), followed by indoor (32 µg/m
3) and outdoor (29 µg/m
3) levels. Personal exposure to NO
2 correlated best with the indoor NO
2 levels. Temporal and traffic-related variables were significant predictors for outdoor NO
2 levels. Thirty-two percent of the indoor NO
2 variability in the two cohorts was explained by outdoor NO
2 levels and the use of the gas appliances. The model for personal exposure accounted for 59% of the variance in NO
2 levels in Sabadell with four predictor variables (outdoor and indoor NO
2 levels, time spent in outdoor environments and time exposed to a gas cooker). No significant association was found between personal or indoor NO
2 levels and exposure to environmental tobacco smoke (ETS) at home.Personal NO
2 levels were found to be strongly influenced by indoor NO
2 concentrations. The study supports the use of time–activity patterns along with indoor measurements to predict personal exposure to traffic-related air pollution.
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