Radon gas is the second leading cause of lung cancer globally, trailing only tobacco smoke.

Radon-222 is a radioactive noble gas that originates from the natural decay chain of Uranium-238, which is found in varying concentrations in the Earth's crust. As uranium decays into radium and then radon, the gas migrates through soil pores and enters the atmosphere or accumulates inside buildings. The Environmental Protection Agency (EPA) has established an action level of 4.0 picocuries per liter (pCi/L) for indoor air quality.When radon is inhaled, it continues to decay into short-lived isotopes called radon daughters, such as Polonium-218 and Polonium-214. These alpha-emitting particles lodge in the respiratory tract and release bursts of energy that directly damage the DNA of bronchial epithelial cells. This genetic damage can trigger mutations that eventually lead to the development of malignant tumors.According to the World Health Organization (WHO), radon causes between 3% and 14% of all lung cancers in a given country, depending on the average local radon levels. In the United States, the Surgeon General issued a national health advisory in 2005 warning that radon is the leading cause of lung cancer among non-smokers. Modern building codes now often include radon-resistant construction techniques to mitigate this risk in high-radon areas.