Homeowners can buy reliable radon test kits at local hardware stores and from the National Radon Hotline at 800-SOS-RADON (800-767-7236). The only way to know if radon exists at elevated levels is to test.

To find a qualified radon service professional to test or mitigate, or to purchase a measurement device, EPA recommends consumers contact their State Radon Contact to see if there are any state-mandated requirements for such contractors.

* Virginia requires that contractors who test or mitigate be listed with groups known to EPA or the Virginia Department of Health. The Virginia House this session unanimously passed a bill to require that mitigation work meet certain standards. The bill is pending in the Senate.

The state clearinghouse is the Virginia Department of Health Indoor Radon Program at www.vdh.state.va.us/rad/rn_home.htm (800-468-0138). The state maintains a list of companies that EPA says are proficient in screening or testing for radon and another list of contractors for mitigation.

* Maryland has no radon program. It refers residents to the regional EPA office in Philadelphia at 800-438-2474, x2086.

The state Geological Survey, however, provides a 1988 report on radon (www.mgs.md.gov/esic/brochures/radon.html) that describes why houses built in central Montgomery, western Howard, eastern Frederick and central Carroll counties can have high radon rates and why even houses built along the coast can be sitting on uranium-rich phosphate beds.

Montgomery County is the only Maryland county with an active radon program. It offers free kits and analysis to county residents because an estimated 35 percent of county homes exceed EPA's recommended action level. Call 240-777-7770, e-mail radon@askdep.com or write to the Montgomery County Department of Environmental Protection, 255 Rockville Pike, Ste. 120, Rockville, Md. 20850.

Montgomery was the first county in the country to require that "passive" mitigation systems be installed in new housing. Passive systems are activated later if radon is found. Howard County's building code requires that all new housing have active mitigation systems.

* The District's program is run by the city Environmental Health Administration. It offers educational materials and testing assistance for the elderly and handicapped. A free radon test kit is available at 202-535-2302.

* The EPA also suggests contacting one or both of the two privately run National Radon Proficiency Programs that list contractors.

The programs are:

The National Environmental Health Association at www.radongas.org. P.O. Box 2109, 24 Cardinal Ridge Rd., Fletcher, N.C. (800-269-4174).

The National Radon Safety Board at www.nrsb.org. P.O. Box 703, Athens, Tex. 75751 (866-329-3474, toll free).