Introduction: Exposure to toxic components in indoor PM is associated with a wide spectrum of adverse respiratory and cardiovascular health effects. The fine PM pollution in ambient air is currently a major health concern in Iran and is driving increasing research interest. Due to air pollution in Tehran metropolitan, it is necessary to study the concentration and size distribution of particles inside and outside the building. Materials and methods: Hence, for this study, concentration and size distribution of particles matter was calculated with diameters of PM≤0.4, PM0.4-0.7, PM0.7-1.1, PM1.1-2.1, PM2.1-3.3, PM3.3-4.7, PM4.7-7, PM7-11, PM≥11 and TSP during two seasons in the lab building in the Tehran. Measurements on the aerodynamic size of atmospheric aerosols carried with Anderson type 1-ACFM Cascade Impactor with six-stage. The length of each collection period was about 24 h. Results: The results show that the effect of outdoor air pollution on the concentration of particles in the indoor environment is significant. According to these results, the lowest value is for particles with a diameter greater than 11 μm. the highest value of this ratio is dedicated to PM≤0.4 and with the increase of the aerodynamic diameter of the particles, the I/O decreases as well. A similar trend was recorded for concentration of TSP. The highest difference in the concentration of TSP in indoor and outdoor was 60.25 and 188.36 μg/m3, respectively. Conclusion: This effect is due to factors such as the lack of standard ventilation, old doors and windows and the life of the building.
