The same qualities that made PCBs a blessing to industry are what make them so troublesome to the environment and public health. The liquid chemicals are so stable and nonflammable that they make an excellent insulator in electrical equipment. But PCBs also are virtually indestructible in the environment and in the tissues of living things, including humans.

PCBs, or polychlorinated biphenyls, consistently have produced cancer, birth defects, liver damage and reproductive problems in laboratory animals. Although similar effects in humans have not been proven conclusively, scientists regard the results of animal studies as a warning to people, according to Environmental Protection Agency officials.

PCBs are known to cause chloracne, a severe, disfiguring and painful form of acne, in humans.

An EPA ban on the production of PCBs took effect in 1979. But at least 750 million pounds of the chemical are still in use, according to Suzanne Rudzinski, chief of the chemical regulation branch of the agency's Office of Toxic Substances.

Once PCBs leak into soil and water, Rudzinski said, they are passed on to aquatic life, animals and, ultimately, to humans.

"What we're aiming at is trying to eliminate PCBs in the environment and reduce risks of exposure to humans," Rudzinski said yesterday. "No new PCBs are being produced so, through attrition, they're going to be eliminated."

A majority of people in the United States have measurable amounts of PCBs in their fat tissues, where the toxic substance accumulates, according to federal officials who have run tests on segments of the U.S. population. Since the ban went into effect, such levels have declined dramatically, according to a 1982 EPA study, Rudzinski said.