The federal government, alarmed by the small number of homeowners who have tested their properties for the health-threatening presence of radon, is considering a proposal to impose testing on homes up for sale.

Fewer than 5 percent of houses nationwide, according to Environmental Protection Agency estimates, have undergone radon screening in the five years since dangerous levels of the cancer-causing agent were discovered in homes.

Testing a home is considered the critical first step in dealing with the radioactive gas that occurs naturally in rock and soil because it is undetectable by sight, smell or taste.

Tests results of four or more picocuries of radon per liter of air indicate unacceptably high radon levels, according to the EPA. At those levels, homeowners should respond, the agency recommends, by reducing the penetration of the gas, which typically entails sealing of basements and venting techniques.

The American Lung Association attributes 5,000 to 20,000 lung cancer deaths annually to radon.

Although the EPA has been looking at a point-of-sale testing requirement, it has not formally sought permission from Congress to do so, said Dennis Wagner, chief of the agency's radon public information section.

Yet, the very consideration of such a proposal worries the country's real estate sales community, said Pat Campbell-White, a Rehoboth Beach, Del., real estate broker and the National Association of Realtors' spokeswoman on environmental issues.

With about 6 percent of U.S. homes changing hands annually, it would take years before a mandatory point-of-sale test would make a substantial dent in the problem, Campbell-White said. That means, she added, that "some families in a neighborhood would be subjected to the risk of elevated levels of radon for an extended period of time."

A better solution, she said, is to require current owners to check their homes' radon levels.

The public's apathy toward radon testing has confounded the EPA, Wagner said. In 1985, when the dangers of radon were first publicized, "the agency thinking was that people would panic because there is this invisible killer, this deadly radioactive gas creeping up out of their basements. The exact opposite has happened."

The fact that 5 percent of the nation's housing stock has undergone testing is a positive sign given that the problem was recognized five years ago, said Peter M. Sandman, director of the environmental communications research program at Rutgers University. "I am not suggesting the problem is solving itself, but {the prevalence of testing} is about where we would expect it to be."

People are most receptive to getting a test done when they are about to purchase a home, Sandman added.

"It is much easier to persuade a buyer that they ought not to buy without knowing whether they are buying a problem," Sandman said. "It is much harder to persuade to test in a house already occupied because if a problem is found then owners are going to have to cope with it and they would rather not."

While American Lung Association official Ronald H. White said he believes that the sales process is not the ideal point at which to test a home, he has concluded it is the only time many people are willing to do so.

"We encourage people not to wait because it is awfully late if you are selling to find out that there is a problem," White said. "We are talking about avoidance of a very dangerous and mostly fatal disease and we are talking about living in a radon environment for many years at a time."

The time pressures inherent in many real estate transactions also push buyers to use short-term tests that take two to three days, White said, when a longer-term test of up to 12 months would yield more reliable results. Both testing methods, he said, cost $10 to $25.

The lung association encourages buyers to avoid the time trap, White said, by getting sellers to agree to escrow the cost of potential mitigation repairs in the event long-term testing results show elevated levels.

Homeowner apathy toward radon testing is partly because of a lack of public outrage, said Sandman, who has studied the issue.

"There is no coercion," he said. "There is no radon industry putting radon in our basements." He contrasted the indifference to testing to the public's intense reaction to nuclear and hazardous waste health threats.

A feeling that the problem is not imminent also contributes to the testing malaise, Sandman said.

A misconception that mitigation repairs are costly also accounts for some of the resistance, White said. Actually, he said, the cost of reducing radon levels typically ranges from $200 to $1,500.

Another unfounded fear is that a high radon reading will hurt the value of a home, White said. Most buyers, though, treat elevated radon levels like they would termite damage and ask the seller to make the necessary repairs, he added.

Mark T. Simpson, director of mortgage underwriting standards for the Federal National Mortgage Association, which buys mortgages and packages them for sale to investors, said that since there is little fear that a borrower will default on a loan if radon is found in the home, it has not made its lenders insist on radon testing of the homes they finance.

"Radon is one of those hazards that is relatively inexpensive to mitigate. Typically people are not going to walk away from their homes for the few hundred to few thousand dollars that it takes to solve the problem," Simpson said.

The EPA also is weighing other alternatives to shake homeowners out of their lethargy, although they also apply primarily to buyers, Wagner said. They include requiring disclosures in sales contracts that would advise that a radon test be performed, reveal any known test results, or simply state the health hazards associated with radon.

Some local realty boards have developed standard contract forms for use by buyers who want to make a radon test a condition of the sale in much the same way that home inspections are handled. The Northern Virginia Association of Realtors began offering a radon inspection addendum a year ago, while the Montgomery County Association of Realtors made its radon contract form available a year earlier.

For the most part though, real estate agents do not help the radon testing cause, White charged. Sales agents "generally wish these issues would go away because it complicates their lives and their deals," he said.

Realtor Campbell-White said that agents have an "affirmative obligation" to disclose all material facts about a house and radon is a material fact." She also said the National Association of Realtors encourages its members to tell homeowners about radon when listing their properties for sale, "but we tell them not to panic. {A radon problem} can be corrected simply and economically."