About one in every three persons will get cancer in their lifetimes. The extra risk from radon depends on its concentration in air and water, and the number of years of living with that radon. Cancer can develop only after decades of exposure, typically at age 60 or older. The risks are simply proportional to the radon levels in air or water, and the duration of exposure to that radon. There are not believed to be threshold concentrations, below which there are no cancer risks.
Smokers are at much greater risk from radon in air than nonsmokers.
The ingestion risk can be eliminated by using water from a low-radon source for the small amounts of water drank from the tap without boiling or cooking, which drives out the radon. The inhalation risk is harder to reduce. There is currently no U.S. EPA standard for radon in drinking water. In most of the state radon in water is not a concern. Most of the risk is from radon escaping the water into the air you breathe, and is the same as for radon entering the air directly from the soil.