The Labor Department yesterday announced new regulations to substantially reduce worker exposure to benzene in an effort to prevent hundreds of leukemia deaths caused by the potent chemical.

The regulations are aimed at reducing by 90 percent the maximum amount of benzene to which 240,000 workers are exposed in the petroleum, chemical, printing, paint, rubber fabricating and other industries in which the clear, colorless liquid is used as a solvent.

Under the new rules, employers will be required to provide workers with properly fitted respirators, protective clothing and regular medical tests and to meet the new standard of maximum exposure by February.

The stringent limits are expected to prevent at least 326 deaths from leukemia and other blood disorders over a working lifetime of 45 years, said Assistant Labor Secretary John A. Pendergrass, who heads the department's Occupational Safety and Health Administration.

Labor representatives hailed the new standard but criticized OSHA for its long delay in formulating it.

Industry spokesmen challenged OSHA's assessment of the health risks of benzene and said the stricter exposure limits are unncessary.

Yesterday's announcement caps a long, complicated regulatory history involving benzene, a derivative of petroleum and a basic building block of many chemicals that was first determined to be a cause of leukemia in 1976. A study by the National Institute of Occupational Safety and Health in 1977 showed that workers exposed to the chemical face a fivefold increase in the incidence of leukemia.

The institute recommended as far back as 1976 that the 90 percent cutback in exposure limits be immediately imposed in an emergency order.

OSHA complied with the recommendation, but it was overturned twice by the U.S. Court of Appeals for the 5th Circuit, which sided with industry contentions that OSHA failed to demonstrate health benefits from the stricter standard.

In July, 1980, the U.S. Supreme Court affirmed the rulings, and called on the agency to demonstrate that benzene poses a significant health threat, that tighter workplace standards could reduce the risk and that implementation of such regulations is technically feasible.

OSHA took until December, 1985 to complete its assessment and repropose the benzene exposure limits that were unveiled as final yesterday -- one part per million parts of air over an eight-hour workday. The old standard was 10 parts per million.ing leukemia if OSHA put out the standards when it should have."

The American Petroleum Institute disagreed in a statement citing "substantial uncertainty about any health risk from benzene" under the old standard and criticizing the OSHA's assessments as "flawed because they overstate the risk."