In this study, indoor air quality parameters were investigated in a naturally ventilated school building located near an urban roadway in Chennai. Monitoring was done during the winter (34 days) and summer (26 days) of 2011. The mean value of indoor PM 10, PM 2.5, PM 1 and CO concentrations were found to be 149 ± 69, 61 ± 29, 43 ± 24, 0.10 ± 0.18 and 95 ± 61, 32 ± 16, 18 ± 9 μg/m 3, 0.11 ± 0.14 ppm, respectively for winter and summer seasons. PM data analysis indicated that the indoor PM 10 (60% of time) and PM 2.5 (27% of time) levels were often exceeding the National Ambient Air Quality Standards (NAAQS). The indoor -outdoor (I/O) ratios of PM 10, PM 2.5 and PM 1 and CO were found to be 2.52 ± 2.71, 1.44 ± 0.67, 0.97 ± 0.18 and 0.51 ± 0.38, respectively. The high I/O value of PM 10 concentration indicates significant contribution from the activities of occupants inside classroom. The low I/O values for finer PM fractions and CO concentrations confirm the vehicular emission contribution from the adjacent road. The influence of classroom occupancy, comfort parameters, outdoor traffic and meteorological parameters on indoor pollutant concentrations were also investigated. Results showed good correlation between PM with comfort and outdoor meteorological parameters. © 2012 Elsevier Ltd.