In recent years, technology development and using various tools have led to produce environmental contaminations. Radon (Rn-222), a natural radioactive gas, is one of the dangerous and fatal indoor air pollutants. Since radon is a major cause of lung cancer after smoking, in the present research, radon measurements have been done in 30 dwellings of Tehran using penetrative chambers. These measurements were done by passive method in a 3-month period. According to research achievements, the average radon concentration in Tehran is measured 104 Bq/m3. Maximum and minimum concentrations are 460.2 (Shahid Baqeri in west) and 31 Bq/m3 (Shahid Araqi in north), respectively. Risk assessment was done using Environmental Protection Agency method. Results showed that independent of age, the lung cancer death rates were about 14 and 12 times greater for ever smokers than never smokers among males and females, respectively. Moreover, it was concluded that females are more at risk than males. Generally, people who were older than 70 and also ever smokers are more at risk and must be followed up.