The risk of developing lung cancer from exposure to various concentrations of radon gas is compared to the risk from cigarette smoking. The concentration of radon gas is related to the radioactivity, or number of radioactive decay events per unit time, per unit volume of air (Curies per liter, or here it is picoCuries per liter, where the "pico-" means one trillionth).

The average indoor level nationwide is about 1 pCi/liter, so this becomes the baseline reference (the "non-smoker risk"). The Environmental Protection Agency (EPA) uses 4 pCi/liter as the threshold for a radon-infested house, which is the level at which there is a discernible health risk. A significantly contaminated house has 10 pCi/liter or more, which corresponds to lung cancer risk for someone smoking at least 1 pack of cigarettes per day.

In some areas of the country that have a lot of uranium in the upper soil or in the building itself (e.g., Leadville, Colorado), household radon values can be as high as 100-200 pCi/liter, which would be comparable to a heavy smoker in terms on lung cancer risk.