FARGO, N.D. -- The back yard of Earl and Betty Lou Alberts sweeps gently to the banks of the Red River, opening to a wooded glen on the other side. The house sits on the rustic fringes of Fargo, a refuge, seemingly far from the intrusions of modern day life, where the couple hand-feed raccoons and watch deer prance past tall oaks.

But the pristine image of the estate was shattered last week. According to the Environmental Protection Agency, the Albertses may be exposed to a health risk at home far greater than they would face living downwind of an industrial polluter or next door to a toxic-waste dump.

The couple's home was identified as one of the nation's most polluted by radon -- an invisible, odorless gas produced by decaying uranium in rock and soil. North Dakota itself topped a seven-state survey that found more pervasive contamination of homes than previously was realized. The survey prompted the EPA and the surgeon general to urge property owners nationwide to have their homes tested for the naturally occurring radioactive substance.

The EPA says radon is second only to cigarette smoking as a lead-ing cause of lung cancer, claiming 20,000 lives a year. The EPA estimates that lifetime exposure to radon at four picocuries per liter of air equals the health danger of smoking 10 cigarettes daily.

Although high radon levels were found in large parts of Indiana, Massachusetts and Pennsylvania, the hottest spots fell in the path of glaciers that lumbered through the north-central states of Minnesota and North Dakota thousands of years ago, depositing Canadian clay speckled with uranium.

Nearly two-thirds of the houses surveyed in North Dakota had more than four picocuries of radon per liter of air, the level at which the EPA recommends corrective action. Four percent of the houses contain higher levels of the gas than the levels uranium miners are permitted to work under.

Five of the top 10 radon readings in the seven states were in North Dakota, including the three highest -- 184, 134 and 127 picocuries. The Albertses home ranked sixth, with a reading of 86 picocuries.

Richard J. Guimond, head of the EPA's Office of Radiation, said radon levels reported for North Dakota represent a "serious" health threat.

"They have a tremendous amount of {radon}, and it's pervasive," Guimond said. "If you add up the projected deaths caused by every other kind of pollution, they just don't come close to the deaths caused by radon. We think they ought to take it very, very seriously."

News of the radon findings had trickled out since owners of the 1,600 surveyed homes received their test results this summer. A people rooted in the flat, expansive stretches of land here, North Dakotans apparently were too preoccupied with the drought to register much concern at the time.

But the national health advisory last week was trumpeted in front-page headlines and prime-time coverage. It created the urgency that normally accompanies a prairie fire.

"When I heard it on the TV, I went back and looked at the report that was sent to us," Earl Alberts said. "Then I began thinking seriously about it. If you live in this house, it's like smoking three or four packs of cigarettes per day. It causes a little apprehension."

Alberts was not the only one belatedly upset. The state health department received 200 inquiries Tuesday, the day after the announcement in Washington, said Jim Killingbeck, the department's environmental scientist. The only time more people have called, he said, was in August 1985, when a truck carrying radioactive materials collided with a train.

The North Dakota chapter of the American Lung Association also fielded many frantic calls. "We've tried to keep people from going off the deep end" by reminding them that smoking kills many more people than radon and that radon has existed for a long time, spokeswoman Carol Guler said.

North Dakotans quickly bought out radon test kits available in stores, emptying shelves at the K mart in Bismarck in the first two hours of business last Tuesday, store manager Jim Till said.

At Jerry's Trustworthy Hardware, manager Jeanette Cook said only this summer's demand for plant root feeders matched last week's request for the $9 to $20 kits recommended by the EPA and the state as a homeowner's first step to measure the problem.

Mark Pescion, a radon consultant from Devil's Lake, used typical Dakotan understatement to size up the potential market in inspecting for and ridding homes of radon. "It seems like something that could keep a guy busy," he said.

There is a perverse irony in North Dakota's ranking in household air pollution. Only it and Hawaii are in compliance with every federal outdoor air-quality standard. There is only one Superfund toxic-waste cleanup project in the rural state of 650,000 residents.

The radon problem apparently dates to the era of glaciers, which overrode North Dakota and left rocks, silt and clay containing bits of uranium that remain under buildings today, said Sid Anderson, associate state geologist. When the clays dry, they fracture, releasing uranium ions, or radon, he said.

In winter, as heating systems dry clays under homes, radon flows through cracks in the basement floors and walls and accumulates in the energy-tight structures.

Earl Alberts, 58, who teaches business administration at nearby Moorhead State University in Moorhead, Minn., said he had a vague knowledge of radon but "never dreamed" of the problem so far from uranium mines.

"This is beautiful out here," said Alberts, pointing to his grounds. "It's the last place you'd expect" pollution.

The couple bought the 1 1/2-acre spread three years ago after selling real estate, consulting and investment companies. As they head into their 60s and plan for retirement, they are now trying to sell the ranch-style home five miles south of Fargo.

Like many North Dakota homeowners, the Albertses look on the EPA disclosures like they might a summer of no rain. They are willing to spend whatever it takes to rid the house of radon, starting with the hairline cracks in the linoleum-tiled floor of the basement. But reducing levels 20 times higher than acceptable appears a daunting task.

The choices are bleak if they cannot. Who would pay their price of $100,000 for a contaminated house? Alberts asks. He would be obligated to disclose the pollution.

The alternative is remaining in a house known to be dangerous to their health. "What are people going to do? Live there and slowly die?" asked Alberts, who smokes a pack of cigarettes daily. "Even if I quit {smoking}, it's like not quitting."

"I imagine people would have to walk away from it and lose years of hard work and savings," he said. "I could. But I'd have to wait till I'm 80 to retire."

According to Killingbeck, radon can be reduced by dilution with fresh air or sucking it out of the ground with a pipe inserted below the floor slab and hooked to an exhaust fan. Those measures should work for all but the worst cases and cost about $1,500, he said.

In a seven-state survey, the Environmental Protection Agency found more homes contaminated by radon than expected. This chart shows the percentage of homes in each state with radon levels greater than the standard of four picocuries per liter.

State...........Homes

Arizona.........8%

Missouri........18%

Indiana.........26%

North Dakota....63%

Massachusetts...24%

Pennsylvania....37%

Minnesota.......46%