January is EPA's National Radon Action Month. Radon exposure is the leading cause of lung cancer among non-smokers and the second-leading cause of lung cancer overall. It's estimated that the radioactive gas causes 21,000 fatal cases of the disease each year. Earlier this year, the President's Cancer Panel singled out radon as one of America's "grossly underestimated" environmentally caused cancer risks.
But January will most likely not be a month of "action" for the EPA as it remains stalled in a decades-long effort to regulate radon in drinking water.
A 1986 amendment to the Safe Drinking Water Act (SDWA) mandated that the EPA create regulations for radon. In 1991, the EPA complied. Its recommendations were much debated and resulted in a 1996 amendment to the SDWA that resulted in a revised EPA proposal in 1999. Yet the issue remains contentious and the EPA has failed to make an enforceable national mandate. As a result, states are left to take the lead. Currently, only nine states have set guidelines for radon limits in drinking water. They are NJ, PA, NH, ME, RI, CT, VT, MA and WI.
While radon-222 is released by naturally occurring uranium in soils throughout the country, its levels fluctuate geographically. Most of the states attempting to set "safe" levels are in the Northeast where radon levels are among the Nation's highest. But radon occurs in equally high levels in the Midwest, Great Plains, and Rockies and, of those states, only Wisconsin has attempted to address the threat in drinking water.
While critics of regulating radon point out that the prime risk of cancer from radon occurs as the gas leaches from soil into a house through cracks in the foundation, the incidence of cancer from water-borne radon is much higher than the risk of cancer from several contaminants that the EPA currently regulates in drinking water.
EPA regulations apply to municipal water systems serving 25 or more people, which accounts for 90% of the water Americans drink. Radon-contaminated water from wells or underground reservoirs releases the gas into a home as it sprays out of a faucet or showerhead.
The two main methods of mitigating radon in drinking water are through aeration, where air is pumped through the water of a well, carrying away the radon gas and granular carbon filtration systems, which filter out radon as it enters the home.
Both options come with hefty price tags. An individual homeowner wishing to treat a contaminated well can expect to spend about $4,000 for installation of a filter or more than $6,000 for an aeration system. And, according to an article by Gayathri Vaidyanathan  in the New York Times of December 7, 2010, cost concerns are a prime reason the EPA has failed to take action. Water utilities claim that the costs of regulating radon in water supplies would be prohibitive and end up being foisted upon customers.
While all water systems serving more than 25 people are required to send customers annual water quality ("consumer confidence") reports, radon is rarely on the list of contaminants. In the rare case that radon is tested for, it is almost always accompanied by a disclaimer that the EPA doesn't currently require a utility to regulate.
To see what your local water district is doing about radon, start your search here. 
Only water providers serving 100,000 people or more are required to supply reports to a publicly available website, however, so you may need to hunt down reports elsewhere. Often, entering the search terms "consumer confidence report" and the name of your city into Google is enough to produce a pdf. To be more precise, use the names of your local water utilities.
For more information on radon:
- Scientific background on radon as a cancer risk. "President's Cancer Panel: Environmentally Caused Cancers Are 'Grossly Underestimated' and 'Needlessly Devastate American Lives,'"  Environmental Health News, May 6, 2010, by Marla Cone.
- "Risk Assessment of Radon in Drinking Water,"  National Academy of Sciences, 1999. NAS Press Office: (202) 334-2138.
- "Reducing Environmental Cancer Risk: What We Can Do Now"  (2008-09 Annual Report), President's Cancer Panel, National Cancer Institute, National Institutes of Health, April 2010. Press Office: 301-496-6641.
- EPA Initial Perspectives of NAS Report: "Risk Assessment of Radon in Drinking Water."  EPA Press Office: 202-564-4355.