The relationship between indoor and outdoor particulate air pollution was investigated at an urban background site on the Payambar Azam Campus of Mazandaran University of Medical Sciences in Sari, Northern Iran. The concentration of particulate matter sized with a diameter less than 1 mu m (PM1.0), 2.5 mu m (PM2.5), and 10 mu m (PM10) was evaluated at 5 outdoor and 12 indoor locations. Indoor sites included classrooms, corridors, and office sites in four university buildings. Outdoor PM concentrations were characterized at five locations around the university campus. Indoor and outdoor PM measurements (1-min resolution) were conducted in parallel during weekday mornings and afternoons. No difference found between indoor PM10 (50.1 +/- 32.1 mu g/m(3)) and outdoor PM10 concentrations (46.5 +/- 26.0 mu g/m(3)), indoor PM2.5 (22.6 +/- 17.4 mu g/m(3)) and outdoor PM2.5 concentration (22.2 +/- 15.4 mu g/m(3)), or indoor PM1.0 (14.5 +/- 13.4 mu g/m(3)) and outdoor mean PM1.0 concentrations (14.2 +/- 12.3 mu g/m(3)). Despite these similar concentrations, no correlations were found between outdoor and indoor PM levels. The present findings are not only of importance for the potential health effects of particulate air pollution on people who spend their daytime over a period of several hours in closed and confined spaces located at a university campus but also can inform regulatory about the improvement of indoor air quality, especially in developing countries.