Levels and indoor-outdoor relationships of size-specific particulate matter in naturally ventilated Portuguese schools.

Indoor exposure to particulate matter (PM) has received great interest due to the epidemiological evidence of its health impact, particularly in susceptible populations such as children. The present study investigated indoor concentrations of three PM size fractions in 11 naturally ventilated schools with static heating systems, and the relationship between indoor and outdoor PM concentrations. The study was performed in Porto, Portugal, during winter and included school buildings and individual classrooms with walk-through surveys, as well as indoor and outdoor air monitoring. Mean 12-h indoor daytime concentrations PM₁₀, PM(2.5), and PM₁ were 140, 95, and 91 μg/m³, respectively. During the day, PM(2.5) and PM₁ concentrations were lower indoors than outdoors (indoor/outdoor ratios of 0.83 and 0.8, respectively), whereas PM₁₀ showed the opposite trend. Concentrations decreased significantly during the night, 49% for PM₁₀ and 27% for PM(2.5) and PM₁. These findings reflect the significant contribution from the activities of occupants inside classrooms to higher indoor levels of PM₁₀ levels, whereas the fine fraction of PM(2.5) and PM₁ is primarily influenced by outdoor concentrations. This study provides a link between size-specific PM in Portuguese schools with contribution of outdoor versus indoor air. Our results suggest that exposure to PM is high and highlights the need for strategies that provide healthier school environments.