THE cost of heating a home is expected to be higher than ever this winter, so this is a good time to batten down the hatches by caulking, sealing and weather-stripping every cold air entry point.

But homeowner beware: the quick fix could create a more serious set of problems, because the better you are at sealing icy air out, the more likely you are to keep potentially harmful gases like radon sealed in.

Radon is a naturally occurring radioactive gas that can’t be seen, smelled or tasted. “It is a classic carcinogen,” said Philip Jalbert, the radon team leader for the Environmental Protection Agency in Washington. “We estimate that about 20,000 people die from radon-induced lung cancer every year,” making it the country’s second-highest cause of lung cancer, behind smoking.

The gas can infiltrate a house by seeping up from the soil and through cracks and other openings in a basement floor or crawl space, Mr. Jalbert said. Covering all such cracks lessens the potential for a problem but does not do away with it altogether, as new cracks occur all the time.

While many buyers have their homes tested for radon before purchasing, the agency estimates that one out of four sellers intentionally or unintentionally interferes with the accuracy by moving the test kit or tampering with it in another way. “It happens all the time,” Mr. Jalbert said. Accordingly, the agency suggests that all homeowners test for radon, even if a test was done at purchase, and especially if it wasn’t. (Information on radon is at epa.gov/radon.)

Jeffrey C. May, an indoor-air quality consultant in Tyngsborough, Mass., and the author of “Jeff May’s Healthy Home Tips” (Johns Hopkins University Press, 2008), said that there are two types of kits for homeowners: a long-term test, measuring radon levels over 3 to 12 months; and a short-term one, 2 to 4 days. The agency prefers the long-term test, as it gives a better idea of the year-round average.

The kits measure radon levels in picocuries per liter of air. (A picocurie is one-trillionth of a curie, a standard scientific measure of radiation.) A level of four picocuries or more is an “action level,” at which the agency recommends remedial steps.

“Radon is almost strictly a problem in one- and two-family homes where the basement represents a significant portion of the house,” Mr. May said, making it a concern in both town and country. Since radon typically enters as a result of a drop in air pressure, he said, the level can vary greatly even in a short time: “You can have radon levels go from 2 picocuries per liter of air to 200 over the course of a day.”

Consumer Reports evaluated kits for its September issue. Donald Mays, its senior director of product safety, said that the best buy for a long-term kit was the AccuStar Alpha Track Test Kit AT 100; for short-term, the RTCA 4 Pass Charcoal Canister.

Carolyn Allen, the president of AccuStar, in Medway, Mass., said her company’s kit measures radon levels over 3 to 12 months. “Since levels of radon fluctuate naturally, the more data you collect, the better,” she said. The kit, using plastic film that registers marks as radon decays, costs $28, including postage to send the test to a lab and get the results back.

Andreas George, a consultant at the Radon Testing Corporation of America (RTCA) in Elmsford, N.Y., said his company’s kit tests levels over two days. “We recommend that homeowners buy two kits and place them side by side so we can compare,” he said. The kits, which use charcoal to absorb the radon, cost $45 for two, including postage and results.

So, what do you do if the levels are above 4 picocuries? “You provide an alternative path for it to go from the basement to outside the house,” said Alicia Anderson, a representative for Air Quality Control, a radon-removal company in Lansing, Mich.

Typically, she said, a contractor cuts a hole in the basement floor to create a “radon collection chamber.” Then, PVC vent piping is installed and extended outdoors, and a fan at the end of the vent pipe pulls the gas out of the house. It can cost $660 to $2,500, depending on the house, she said.

That is not inexpensive, but the remedy can not only make the house safer but also provide peace of mind.