Benzene is mother's milk to the chemical industry -- a gasoline additive, a building block for paints, solvents and thousands of other chemical products, a byproduct of steelmaking.

But there is another, sinister side to benzene. When inhaled it is rapidly absorbed in the blood. The sysmptoms of overexposure begin with headaches and breathlessness and grow worse. In some chemical plants, heavier exposures over long periods of time -- more than motorists or other members of the public would encounter -- have caused leukemia, a fatal cancer, among some of the workers.

No one knows where that fatal line is drawn. There is no scientific study that can pinpoint the amount of benzene exposure that triggers leukemia.

Into that frontier of ignorance, the Supreme Court plunged yesterday.

In a decision of vital importance to the chemical and steel industries and their employes, the court struck down a regulation by the Labor Department's Occupational Health and Safety Administration that would limit industrial exposures to benzene to the lowest "feasible" level.

The OSHA rule would have required substantial engineering changes in industrial plants at an estimated cost of $266 million in the first year, and recurring costs of $34 million annually, according to the OSHA. The industry said costs of compliance could be far higher. The OSHA issued the ruling in 1977, but it has not gone into effect because of court challenges.

The case has become a test of how much proof government regulators must have in setting limits on toxic and cancer-causing chemicals in the work place.

The OSHA benzene regulation was based on studies showing that relatively high exposures had caused high rates of cancer among several groups of workers -- shoemakers in Turkey and chemical workers in two Ohio plants.

In the United States, exposures to benzene have been reduced gradually in the past three decades as the connection between the chemical and leukemia received more attention.

But the OSHA concluded that the limit of 10 parts of benzene in one million parts of air in a plant environment presented too great a risk given the ignorance about an individual's vulnerability to cancer-causing chemicals.

"There is no known level that can be considered safe," said attorney George Cohen, representing the AFL-CIO which joined the OSHA in defending the agency's standard.

A solid front of business organizations, from the American Petroleum Institute to the Chamber of Commerce, had attacked the OSHA ruling as excessive and lacking any scientific foundation.

"OSHA's suggestion that it is impossible to quantify the value of human life is correct, but totally irrevelant," the chamber said in a court brief. Industry is not trying to put a price tag on a worker's life, but the OSHA should be required to consider the relative hazards of dangerous chemicals before imposing exposure limits to make certain that the money spent on industrial safety goes where it is most needed, the chamber said.

The industry's witnesses contended that although benzene is deadly in relatively high concentrations, it is a weak cancer agent compared with other industrial carcinogens. Richard Wilson of the Harvard Energy and Environmental Policy Center said the risk of contracting cancer from an exposure to 10 parts per million of benzene is no more than the risk of death by drowning.Two weeks exposure to benzene at the level is a hazard equal to one day of breathing the air of Washington or Boston, he said or drinking a gallon of chlorinated water, he said.

The OSHA challenged these arguments, saying there is too little experience with the effects of low levels of benzene exposure on workers to permit conclusions about ultimate health risks.

The Supreme Court's ruling yesterday gives little guidance to the agency in making such determinations.