Exposure to radon is the second leading cause of lung cancer in the United States. Radon can be inhaled into the lungs, where it undergoes radioactive decay. As it decays, radon releases tiny bursts of energy called alpha particles, which can harm sensitive lung tissue by damaging the DNA. This damaged DNA can lead to lung cancer.
The main type of cancer caused by radon in water is also lung cancer. This is because a large fraction of radon in water is released to the air when water is used in common household tasks such as showering, washing clothes, and other uses where water can become vaporized.