Nearly all Michigan residents--97 percent--have been contaminated with the toxic chemical PBB following a 1973 agricultural accident in the state, according to estimates in this week's Journal of the American Medical Association.

The authors of the article said they feared that the widespread contamination could lead to increased rates of liver cancer and diseases of the nervous system in Michigan during the next 20 or 30 years.

There is an additional danger that PBB may combine with other toxic chemicals such as DDT and PCB, which already contaminate the population, to cause cancer and other diseases.

In an editorial accompanying the study, the AMA journal described the finding as "dramatic" and said it "is another reminder of our vulnerability to the increasing number and volume of potentially toxic chemicals used in manufacturing, agriculture, pest control, and food processing."

PBB, short for polybrominated biphenyl, is a chemical that was used as a fire retardant. In 1973, between 1,000 and 2,000 pounds of PBB was accidentally mixed with livestock feed and sent to hundreds of farms in west-central Michigan.

The error is believed to be the worst case of accidental contamination in agricultural history; about 1.5 million chickens, 30,000 cattle, 5,900 swine, and 1,470 sheep had to be destroyed.

Farmers first noticed something was wrong when dairy farms had a sudden decrease in milk production, when abnormal growths were spotted on animal hooves, and more cows aborted their calves.

But in the months before the contamination was discovered, meat and dairy products from the Michigan region were sold and consumed in the state.

PBB is absorbed into the body and remains there for a long time, as do DDT and PCB. Therefore, an intake of even small amounts can accumulate in body tissues.

The study that led to the estimates began in 1978. Blood and tissue samples from 1,738 people in the state showed that 97 percent had traces of PBB in their fat tissues. The amounts ranged from less than one part PBB per billion parts tissue up to 36,700 parts per billion.

Not surprisingly, the highest levels of contamination were found in farmers in the area where the accident occurred, according to the study, which was done by Mary S. Wolff, Henry A. Andersen, and Irving J. Selikoff, of the Mount Sinai School of Medicine at the City University of New York.

The group also tested for the level of PBB in the blood, and found 70 percent of those tested had a detectable amount.

"It's important now to follow up" on the findings, said study leader Wolff, since it sometimes takes two decades for increased cancer to show up in the population.