In January 2012, one kindergarten and eight elementary school classrooms were monitored. The campaign included simultaneous measurements, indoors and outdoors, of comfort parameters, CO, CO 2 and particles. Automatic monitors using a light scattering technique were employed to measure PM 10 continuously. During occupied periods, low volume samplers were used to daily collect PM 2.5 samples, which were subsequently analysed for carbonates, organic carbon (OC), elemental carbon (EC) and water soluble inorganic ions. With regards to comfort, for many rooms, the schools did not meet the recommended levels. Indoor-outdoor CO 2 ratios between 3 and 12 and indoor levels much higher than 1000 ppm during the occupancy periods denote the highly inadequate ventilations. The results clearly demonstrate that exposure to particulate matter in schools is high. The continuous measurements of PM 10 suggest that the physical activity of pupils, which is assumed to be more marked in younger children, contributes to a constant process of resuspension of sedimented particles. In addition, peak PM 10 concentrations coincident with cleaning activities suggest the necessary of changing practices to improve cleanliness. Around 40% of the PM 2.5 mass is composed of carbonaceous matter, with 4–5 times higher OC mass fractions than EC. It was observed that both OC and EC were significantly influenced by indoor sources. Water-soluble inorganic ions represented around 10–20% of the PM 2.5 mass measured in classrooms. Excepting calcium, in general, the ionic species were present at indoor-outdoor ratios lower than 1, suggesting a major origin in outdoor air.