Toxic chemicals in the environment may cause widespread behavioral and mental as well as physical problems among those exposed, but under existing law there are rarely tests for these subtler effects, the American Association for the Advancement of Science was told yesterday.

A panel of scientists outlined recent findings that many chemicals besides lead and mercury affect the brain and the nervous system, often indirectly.

This is "a major new frontier in toxicology research," which previously has focused mainly on the cancer-causing effects of chemicals, said Bambi Batts Young of the Center for Science in the Public Interest.

A recent "truly alarming" study by the National Center for Health Statistics found lead levels high enough to require medical treatment in 2 percent of all white children and 12 percent of black children, with 18.5 percent of inner-city black children so affected, Young said.

"We've known from antiquity that lead can cause incurable mental damage. Unfortunately, we're still letting it happen," she told a news conference.

She said even small doses of lead cause distraction, vagueness, difficulty in following directions and a decline in intelligence test scores.

But lead is so widespread in the environment that nobody is lead-free, making it impossible to set up a controlled experiment on the effects of small doses, said Dr. Bernard Weiss, professor of toxicology at the University of Rochester, New York.

He said it comes from auto exhaust, the solder in food cans, paint and other sources.

If the drug thalidomide had caused a 10 percent decline in intelligence instead of grossly deformed infants, we might never have noticed it, Weiss said. Recent research supports earlier theories that some chemical food additives tend to make some children overactive, he said.

Dr. Charles V. Voorhees of Childrens Hospital in Cincinnati said one in every 750 U.S. and European babies is born alcoholic, a level equal to the number born with Down's syndrome (mongolism).

Fetal alcoholism, he said, is "probably the most common known cause of mental retardation," yet there are no systematic tests for the problem. Research is now conclusive that more than one ounce of pure alcohol per day, or two stiff drinks, causes some mental deterioration in adults, he added.

Dr. Kent Anger, chief of behavioral research at the National Institute for Occupational Safety and Health, said the agency plans a September conference on ways to test new chemicals for behavioral and nerve effects.

Workers in a fabric-coating plant in Columbus, Ohio, suffered loss of control of their leg muscles after exposure to a petroleum-based solvent called MBK (methyl-N-butyl ketone), a component of dyes, lacquers and thinners, he said.

The plant easily substituted another solvent when the problem was spotted. "Neurotoxicity needs to be a basis for the regulation of chemicals," he said.

The brain has been found to have receptors for chemicals that were previously thought to affect only other parts of the body, reported Dr. Ellen Silbergeld of the National Institutes of Health.

The female hormone estrogen appears to alter the behavior of male rodents as well as their sex characteristics. Other chemicals do not affect the brain directly, but disrupt the formation of some essential blood compounds whose loss does affect the brain, she said.