Chemical and morphological properties of particulate matter (PM10, PM2.5) in school classrooms and outdoor air

Studies have shown high concentrations of particulate matter (PM) in schools. Further insights into the sources and the composition of these particles are needed.During school hours for a period of 6 weeks, outdoor air and the air in two classrooms were sampled. PM was measured gravimetrically, and PM filters were used for the determination of the elemental and organic carbon, light absorbance, and 10 water-soluble ions. Some filters were further analyzed by scanning electron microscopy (SEM) and energy dispersive microanalysis (EDX).The median PM10 concentrations were 118.2 μg m−3 indoors and 24.2 μg m−3 outdoors; corresponding results for PM2.5 were 37.4 μg m−3 indoors and 17.0 μg m−3 outdoors. Using PM10 and PM2.5 data, we calculated the following indoor/outdoor ratios: 0.3 and 0.4 (sulfate), 0.1 and 0.2 (nitrate), 0.1 and 0.3 (ammonium), and 1.4 and 1.6 (calcium).Using the measured sulfate content on PM filters as an indicator for ambient PM sources, we estimated that 43% of PM2.5 and 24% of PM10, respectively, were of ambient origin. The composition of the classrooms' PM (e.g., high calcium concentrations) and the findings from SEM/EDX suggest that the indoor PM consists mainly of earth crustal materials, detrition of the building materials and chalk.Physical activity of the pupils leads to resuspension of mainly indoor coarse particles and greatly contributes to increased PM10 in classrooms. The concentration of fine particles caused by combustion processes indoors and outdoors is comparable.We conclude that PM measured in classrooms has major sources other than outdoor particles. Assuming that combustion-related particles and crustal materials vary in toxicity, our results support the hypothesis that indoor-generated PM may be less toxic compared to PM in ambient air.