There are no immediate symptoms from exposures to radon. Based on an updated Assessment of Risk for Radon in Homes (see www.epa.gov/radon/risk_assessment.html), radon in indoor air is estimated to cause about 21,000 lung cancer deaths each year in the United States. Smokers are at higher risk of developing Radon-induced lung cancer. Lung cancer is the only health effect which has been definitively linked with radon exposure. Lung cancer would usually occur years (5-25) after exposure. There is no evidence that other respiratory diseases, such as asthma, are caused by radon exposure and there is no evidence that children are at any greater risk of radon induced lung cancer than adults.
See also radon health risks at www.epa.gov/radon/healthrisks.html
Read "A Citizen's Guide to Radon" at www.epa.gov/radon/pubs/citguide.html