Tehran, the capital city of Iran, is one of the largest cities in the world. It extends over an area of approximately 2,300 square kilometers and has a population of more than 10 million people. The city is suffering from atmospheric pollution arising from the rapid urbanization during the last 3 to 4 decades. The city will develop towards the west to Karaj, and large volumes of traffic will be going east-west by the year 2015. With more than 2 million vehicles in Tehran, most of which are more than 20 years old, traffic is the major source of air pollution in Tehran. The city of Tehran has a significant air pollution problem and suspended particulate matter (SPM) appears to rank high on the list pollutants. The monthly average of PM-10 in Tehran is high in autumn and low in spring. Maximum average value observed in September is over 370 microg/m(3); the minimum in March (102 microg/m(3)) and early April (65 microg/m(3)). The diurnal variation PM-10 concentration have been studied.