The measuring equipment was placed on a table in the back of the room opposite the front wall, which accommodated the white- and blackboards. Three Harvard impactors (Air Diagnostic and Engineering Inc., Naples, ME, USA) with quiet pumps operating at 10 L min−1 (for PM10 and PM2.5) and at 23 L min−1 (for PM1) were used for sampling (Marple et al., 1987; Turner et al., 2000). The inlets of the impactors were situated in the breathing zone of the sitting students—130 cm above the floor.