The effect of outdoor air and indoor human activity on mass concentrations of PM10, PM2.5, and PM1 in a classroom

The 12-h mass concentration of PM10, PM2.5, and PM1 was measured in a lecturing room by means of three co-located Harvard impactors. The filters were changed at 8 AM and at 8 PM to cover the periods of presence and absence of students. Concentrations were assessed by gravimetry. Ambient PM10 data were available for corresponding 12-h intervals from the nearest state air-quality-monitoring network station. The data were pooled into four periods according to the presence and absence of students-Monday-Thursday day (workday daytime), Monday-Thursday night (workday night), Friday-Sunday day (weekend daytime), and Friday-Sunday night (weekend night). Average indoor workday daytime concentrations were 42.3, 21.9 and 13.7 μg m−3, workday night were 20.9, 19.1 and 15.2 μg m−3, weekend daytime were 21.9, 18.1 and 11.4 μg m−3, and weekend night were 24.5, 21.3, and 15.6 μg m−3 for PM10, PM2.5, and PM1, respectively. The highest 12-h mean, median, and maximum (42.3, 43.0, and 76.2 μg m−3, respectively) indoor concentrations were recorded on workdays during the daytime for PM10. The statistically significant (r=0.68,P<0.0009) correlation between the number of students per hour per day and the indoor coarse fraction calculated as PM10−2.5 during daytime on workdays indicates that the presence of people is an important source of coarse particles indoor. On workdays, the daytime PM10 indoor/outdoor ratio was positively associated (r=0.93) with an increasing indoor coarse fraction (PM10-2.5), also indicating that an important portion of indoor PM10 had its source inside the classroom. With the exception of the calculated coarse fraction (PM10-2.5), all of the measured indoor particulate matter fractions were significantly highly correlated with outdoor PM10 and negatively correlated with wind velocity, showing that outdoor levels of particles influence their indoor concentrations.