Highest concentrations tend to occur indoors particularly in homes on sites that have a high radon releasing potential. The EPA guideline is actually based on an average one- year exposure. Such exposures have the potential to cause lung cancer. There is no safe level of exposure to radon and its radioactive decay products. The risk of developing lung cancer from radon exposures is a lifetime one.
Risk increases with exposure concentration and duration.
If you are a smoker or are exposed to environmental tobacco smoke your risk is significantly increased . Most radon risk assessments factor in present or past smoking. Of those estimated 16,000 lung cancer deaths per year, less than 1000 are expected to occur among never smokers. That's a relatively low risk. If you are a smoker, radon is not the health risk you should be focusing on.