Determinants of indoor and outdoor levels of no2 in a cohort of 4-years-old children

Background and Aims: Home outdoor NO2 has been used as a marker of personal exposure to traffic-related air pollution. However, recent studies suggest that exposure to indoor NO2 may be a risk factor in itself for respiratory health and neurodevelopment. We aimed at assessing the relationship between indoor and outdoor NO2 levels and their determinants in a subset of the INMA birth cohort of Sabadell, Spain. Methods: Indoor and outdoor NO2 levels were measured with passive samplers during two measurement campaigns of one week (1st campaign in fall/winter, 2nd campaign in summer) at home addresses of 96 children. Characteristics of the home surroundings (street configuration, building density, and traffic-related variables) were obtained by the field workers and by Geographic Information Systems (GIS). Information on sources of NO2 indoor levels (gas appliances, tobacco smoke) and additional variables (e.g.use of extractor fan, air conditioning) were obtained through questionnaires in each campaign. Results: Mean levels of outdoor NO2 were 36.7 µg/m3 in fall/winter and 34 µg/m3 in summer, while mean levels of indoor NO2 were 22.8 µg/m3 in fall/winter and 26 µg/m3 in summer. The correlation between indoor and outdoor levels was lower in fall/winter (r=0.35) than in summer (r=0.51). The main determinants of outdoor NO2 were the street configuration (continuous buildings on both sides vs. other configurations) and traffic-related GIS variables. Indoor NO2 levels were influenced by tobacco smoke, time using gas cooking, time opening windows to ventilate, house age, and an interaction between outdoor NO2 levels and use of air conditioning. Conclusions: The influence of traffic-related NO2 pollution on indoor levels is higher in summer than in fall/winter, probably due to differences in ventilation patterns. Gas cooking and tobacco smoke are the main indoor sources of NO2, whereas ventilation, use of air conditioning, and house age are additional determinants.