Indoor and Outdoor Concentrations of PM2.5, Soot and NO2 at Homes, Pre-Schools and Schools in Sweden

Abstract

P-623 Introduction: In developed nations people typically spend about 90% or more of their time indoors. Investigating the relationship between indoor and outdoor air pollution levels is important to enhance the understanding of the health effects of outdoor air pollution. Studies describing both the outdoor and indoor atmospheric environment are limited. Methods: PM2.5, soot and NO2 were measured indoors and outdoors at 20 homes, 10 classrooms and 10 pre-schools during 1 December 2003 – 1 July 2004 over 9 sessions. Each session covered 8–12 sampling sites from mostly all types of sites and areas (city center and 3 suburban areas). No environmental tobacco smoke exposure occurred in any of the indoor environments and no gas stoves were used. At each site, two 14-day average air samples were collected with Harvard Impactors and diffusive samplers, along with source-receptor ventilation measurements. Each site was sampled twice. PM2.5 filters were analyzed gravimetrically and for reflectance (soot). Results: The overall mean indoor and outdoor PM2.5 levels measured indoors during all three seasons were 8.5 and 9.0 μg/m3, respectively. The outdoor levels differed significantly from indoor levels during spring only. The overall indoor levels for soot and NO2 were 0.82 m−110−5 and 12.1 μg/m3, respectively. The respective outdoor levels were 0.98 m−110−5 and 14.3 μg/m3. For both NO2 and soot the outdoor levels were significantly higher compared to the indoor levels during winter only. The overall mean I/O ratios were 1.02 (R2=0.29), 0.90 (R2=0.50) and 0.99 (R2=0.71) for PM2.5, soot and NO2 during all seasons, respectively. PM2.5 and soot were moderately correlated (outdoor: 0.69; indoor: 0.58). Soot and NO2 were better correlated outdoors (0.73) than indoors (0.30). Ventilation rates displayed no seasonal variation at all the sites and seemingly did not influence NO2 and soot I/O ratios. There was a tendency of higher PM2.5 I/O ratios at very low ventilation rates. Discussion and Conclusions: Our results suggest that, for some periods of the year only, outdoor levels were higher than indoor levels: for PM2.5 during spring, and for soot and NO2 during winter. Local outdoor NO2 levels were good proxies for indoor NO2 levels, indicating that gaseous pollutants from traffic easily enter Swedish houses. Local outdoor PM2.5 levels were less predictive of the indoor levels, indicating less infiltration and the presence of contributing indoor sources. Soot was related to NO2 in the outdoor environment, but seemed to infiltrate less predictably into the indoor environment.