Reduction of PM2.5 mass is not sufficient for reducing toxicity, as PM2.5 induced biological effects are also observed in less polluted sites. In addition, water-soluble and organic fractions of PM2.5, as well as metals, are capable of generating ROS, which can subsequently damage DNA . To better understand the dependence of the toxic potential of such particles on their composition and origin, additional toxicological studies are needed. The most important environmental issue in Tehran is air pollution, which has health, economic, and social effects on its population of over 11 million. The large population, along with large-scale fossil fuel-burning activities, has led to dramatic changes in the atmosphere of Tehran. Currently, there are almost 3.5 million vehicles on the streets of Tehran, which have a capacity for only 2million vehicles, and the non-standard fuel used by these vehicles is the major source of air pollution in Tehran. An investigation of air quality in Tehran in 2014 showed that the Air Quality Index (AQI) during that year included 16 clean days, 233 healthy days, 112 unhealthy days for sensitive groups, and 4 unhealthy days for the general population.