WHAT IS A TOXIC CHEMICAL? What is a carcinogen? There really is no way of knowing, according to Lester B. Lave, a Carnegie Mellon University economist, and Dr. Gilbert S. Omenn, a University of Washington physician. That's because among the 70,000 chemicals in widespread use, only 100 or so new chemicals are tested every year.

The reason for such limited testing is economic. The long-term animal bioassays used until now cost more than $1 million each and take two to four years to carry out.But, they write in the current issue of the magazine Nature, there is a quicker and cheaper alternative - new laboratory tests using cultured human cells and one-celled animals. A battery of these tests can classify chemicals about as accurately as the more elaborate studies at about 1 percent of the cost and time.

Mr. Lave and Dr. Omenn come up with what seems like a fairly radical proposal: Classify all chemicals as toxic - consider all guilty - until proven innocent. Rather than continue to tolerate thousands of unregulated carcinogens because there are no animal bioassay test result, subject all chemicals in common use to the new short-term tests.

They acknowledge the increased possibility of error, but say that tests for all is the only way to prevent miscalculation.

The proposal is certain to raise immediate howls of protest from the chemical industry, but it could win long-term support. When the cost of battling liability claims in the future against chemicals today not even suspected of causing injury is considered, the increased cost of additional testing now may be a small price for the industry to pay.

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