With Americans tending to make their homes airtight in order to conserve energy, they face another problem--the buildup of radioactive radon gas.

While it is a good idea to seal in heat, the sealing in of everyday poisons that are given off from underground radioactivity, water heaters, gas stoves and smoke, could prove disastrous.

These poisons pose little problem when circulated and transmitted out into the environment The problem develops when they can't escape.

For the most part, Washingtonians can breath a sigh of relief, at least compared to people in other parts of the country.

Dr. John Harley, formerly of the Department of Energy and now associated with the National Council on Radiation Protection and Measurements, has been studying radon gas for some time. He says that other areas of the country, especially the Rocky Mountains, have high levels of radioactivity in the soil, while East Coast states, with the exception of Florida, have low levels.

Richard Guimond, director of the Criterion Standards Division at the Environmental Protection Agency, agrees. "In all likelihood, the Washington area is not that severely affected."

He had some advice, however, for those who want to be sure they are not at risk.

"We do a lot of testing in measuring levels of radon gas in homes," Guimond said. "Many universities are able to do the techniques." Most of the methods, he said, are not suitable for use by amateurs and are not commercially available.

A method known as "track-etch, which uses a material manufactured by Terradex Corp. in Walnut Creek, Calif., is sold to the public, he said.

For a price of around $50, Terradex will send a homeowner a cup that is to be placed in the home for about three months. At the end of the specified time, the cup is to be mailed back to the company, which then will conduct a test to see if the home contains unhealthy levels of radon gas.

Harley explained that radon gas levels can vary greatly from room to room, from day to evening and from winter to summer. So he favors a device such as the track-etch that will measure levels over a period of time.

Harley also suggested some preventive measures. "Radon gas can come out of the soil and into the house," he explained. While he noted that radioactivity can also be emitted by building material, the amount is usually very small.

He suggested that people check the soil for radioactivity levels before they build. For those who have already built, there are ways of limiting the amount that can enter a house. "Homeowners should look around to find leaky spots," said Harley. "A crack in the cellar floor should be sealed," he said. "If a pipe entrance is caulked, this can reduce the level significantly."

Guimond said the EPA is running a study in Butte, Mont., to detect radon levels in homes. EPA also conducted a study in central Florida in the 1970s after "the agency had found elevated radon decay product levels in buildings constructed on land reclaimed from old phosphate mining areas," he said.

An August 1980 task force report by the United States Radiation Policy Council stated, "Interest in indoor radon was first aroused more than 10 years ago when the use of uranium mill tailings in structures in Grand Junction, Colo., came to national attention." Concern has been growing ever since.

The Tennessee Valley Authority has taken a strong interest in it. "For years we operated a phosphorous furnace," said Lewis Wallace, deputy general counsel for TVA. "One of the byproducts was slag, which is melted rock."

He said the slag was sold and used for construction. "In recent years it was found to contain minute traces of radioactivity," Wallace added. So the TVA set out to find if the slag, now converted into cinder blocks and other building material, was emitting radon gas.

"We did find that some houses had high levels of radon gas," said Wallace. But after further studies were conducted, it was discovered that the slag had little to do with the radon gas level in the homes. "Some houses that had high levels of radon had nothing to do with the phosphate," he explained. "It was coming from other sources."

Wallace warned that one of the potentially worst places for a radon gas buildup is an air-tight room in the cellar such as a bomb shelter.

Because Canada has a far worse problem than the United States, the country has developed equipment to help reduce the gas level, such as a heat exchange device, Harley said. "The heat exchange ventilation exchanges inside air with air outside but keeps the heat inside. The only problem is it costs a couple of thousand dollars."

Before running out to buy such a costly device, Harley suggests that people determine first whether they need one--which he said is doubtful in the Washington area.