American families who have constructed themselves snug homes out of concrete blocks may be exposed to more radioactive material than persons who live next door to nuclear plants, experts said today.

The concrete blocks in question were made from "slightly radioactive" phosphate slag supplied to block manufacturers by the Tennessee Valley Authority's fertilizer facilities at Muscle Shoals, Ala., from 1934 until last December.

The TVA, which was warned of the potential hazard of the construction materials in 1975 by Alabama officials, has annually distributed from 5,000 to 70,000 tons of the fertilizer byproduct, according to Dr. Harold Parker, spokesman for the agency's Muscle Shoals chemical operations. He said most of it had been sold to block manufacturers in Alabama, Mississippi, Georgia and Tennessee, and that the remainder had gone into fertillizers distributed worldwide.

While emphasizing that no current studies implicate the blocks in raising the radiation exposure to dangerous levels, Parker acknowledged that TVA had "briefly" stopped the sale of the slag to block manufacturers in 1975 after a warning by Aubrey Godwin, director of the Alabama division of radiological health.

"We thought they had stopped selling it for building material in 1975," Godwin said today in an interview. Alabama officials became concerned about the radioactive slag at that time, he said, because Environmental Protection Agency studies in Florida were raising questions about high radiation exposure in homes built on reclaimed phosphate mines.

A still-uncompleted EPA study of Florida homes built on phosphate mine residue shows that 15 percent of the homes surveyed have above.03 "working levels" of exposure to radon gas, according to EPA expert Allan Richardson. A person spending a lifetime in that exposure would double his chances of developing lung cancer, he said.

EPA, TVA, and the state of Alabama met Tuesday to consider a year-long study of homes in Alabama constructed from the blocks to determine if radiation exposure levels are higher in similar fashion to those in Florida.

Godwin said his financially strapped division was having "a problem in deciding whether we ethically should accept money from TVA."

In halting sales of the slag in December, TVA said it did not believe "there is a health hazard, but we thought the safe, responsible thing to do was to stop selling the byproduct until the [Florida] guidelines are developed."

The experts acknowledged they have little data on exposure level risks in concrete block homes or those having block foundations. TVA says its sampling indicates the radiation level may be no higher than in buildings constructed of other natural radioactive materials, such as granite and clay tile.

The chief of TVA's radiological hygiene branch, Ernest Belvin, said today, however, that measurements in the unoccupied basement of his office building constructed from the blocks in question showed exposure levels of.05. That level of exposure was cited in a 1970 surgeon general's report as requiring remedial action in homes built on uranium mine tailings in Grand Junction, Colo.

EPA has proposed a Florida standard of.02, or about ten times lower than the amount of radiation naturally occurring in the environment.

In its statement aimed at reassuring homeowners, TVA said measurements showed use of the blocks increased the naturally occurring radiation level by 35 millirems. Nuclear plants are prohibited by federal regulations from increasing the nearby natural exposure level more than 25 millirems.

TVA's Belvin said the exposure level in block homes using the slag would depend on several factors, but chiefly the amount of ventilation. "If you have five kids opening doors, the level is lower; if you're a retired couple staying inside, it's higher," he said.

"It's a risk-benefit analysis," Belvin continued. "If you want to smoke, you take a certain risk. If you want to live in a house built with phosphate slag, you take a certain risk."