This work presents the temporal characteristics of Particulate Matter (PM) mass and number concentrations measured inside a naturally ventilated school building, located close to a busy roadway in Chennai city. Two environmental dust monitor instruments (GRIMM Model 107 and Model 108) were used for measuring PM mass and number concentrations. The 1-h mean values of PM10, PM2.5 and PM1 mass concentrations were found to be 262 ± 161, 68 ± 24, 40 ± 15 µg/m3 and 81 ± 26, 56 ± 2, 45 ± 19 µg/m3 during working hours (8am–4pm) and non-working hours (4pm–8am)/holidays, respectively. The PM number concentrations inside the room during working hours were found to be 2.4 × 105, 2.2 × 103 and 8.1 × 102 particles/l in the size range of 0.3–1, 1–3 and 3–10 µm, respectively. The present study reveals that during working hours, indoor PM concentrations of the classroom were influenced by the activities of occupants and during non working hours it was affected by outdoor vehicular emissions. © 2014, The Institution of Engineers (India).