PM2.5, soot and NO2 indoor–outdoor relationships at homes, pre-schools and schools in Stockholm, Sweden

In developed nations people spend about 90% of their time indoors. The relationship between indoor and outdoor air pollution levels is important for the understanding of the health effects of outdoor air pollution. Although other studies describe both the outdoor and indoor atmospheric environment, few excluded a priori major indoor sources, measured the air exchange rate, included more than one micro-environment and included the presence of human activity. PM2.5, soot, NO2 and the air exchange rate were measured during winter and summer indoors and outdoors at 18 homes (mostly apartments) of 18 children (6–11-years-old) and also at the six schools and 10 pre-schools that the children attended. The three types of indoor environments were free of environmental tobacco smoke and gas appliances, as the aim was to asses to what extent PM2.5, soot and NO2 infiltrate from outdoors to indoors. The median indoor and outdoor PM2.5 levels were 8.4 μg m−3 and 9.3 μg m−3, respectively. The median indoor levels for soot and NO2 were 0.66 m−1 × 10−5 and 10.0 μg m−3, respectively. The respective outdoor levels were 0.96 m−1 × 10−5 and 12.4 μg m−3. The median indoor/outdoor (I/O) ratios were 0.93, 0.76 and 0.92 for PM2.5, soot and NO2, respectively. Their infiltration factors were influenced by the micro-environment, ventilation type and air exchange rate, with aggregated values of 0.25, 0.55 and 0.64, respectively. Indoor and outdoor NO2 levels were strongly associated (R2 = 0.71), followed by soot (R2 = 0.50) and PM2.5 (R2 = 0.16). In Stockholm, the three major indoor environments occupied by children offer little protection against combustion-related particles and gases in the outdoor air. Outdoor PM2.5 seems to infiltrate less, but indoor sources compensate.