In response to a chemical industry petition, the Environmental Protection Agency yesterday proposed exempting nearly half of all newly developed chemicals from a detailed agency review.

The proposal, based on suggestions from the Chemical Manufacturing Association, would exempt from pre-manufacturing review about 400 of the roughly 900 chemicals developed each year.

John Todhunter, assistant administrator of EPA for pesticides and toxic substances, said at a news conference that the types of chemicals that would be exempted from comprehensive health and environmental review are not the "reactive, bad things" but are merely "slight new variations on chemicals that have been around for years."

The decision on whether a chemical could be exempted basically would be left to a "qualified expert" paid by the manufacturer, a proposal consistent with EPA's attempts to shift more responsibility for testing and evaluation to the private sector.

The firm would have to give EPA a brief description of the new chemical two weeks before manufacturing begins, and EPA could veto the decision of the company's expert. But critics question whether that would give the agency enough time or information to spot a potentially hazardous product.

An EPA draft of the proposal warned that the plan would probably generate strong protests from labor, public interest groups and environmentalists.

Rep. James J. Florio (D-N.J.), chairman of the House subcommittee that oversees toxic substance regulation, said in a recent speech that the proposal, if adopted, would make the "U.S. population the test rats for new chemical development" from Europe, where testing requirements would be more rigorous. But Todhunter denied that charge.

Todhunter said the change would allow EPA to use its resources in a "more focused" manner, to assure that potentially hazardous chemicals are evaluated completely. It could encourage industry efforts to develop more advanced chemical technology, he added.

The proposal also would exempt certain chemicals when a firm produces less than 22,000 pounds annually, and certain "chemical intermediates," or chemicals that are combined to produce others and are used only at their manufacturing site, if a company expert finds that the new chemical is not thought to cause cancer, birth defects or other adverse health and environmental effects.

When a company produces less than 2,200 pounds per year of a chemical, it would be exempted without any expert review of the health and environmental effects.