The Labor Department yesterday ordered a 90 per cent reduction in worker exposure to benzene, a chemical believed to cause leukemia. The department said its action was only the first installment in a new and more agressive effort to rid the workplace of health hazards.

"This is only the beginning," said Eula Bingham, assistant secretary for occupational safety and health, in announcing a temporary emergency standard to cut the permissible level of benzene breathed by more than 150,000 workers in 1,200 refining, rubber, printing and other industrial plants.

It was only the sixth time in the six-year history of the Occupational Safety and Health Administration that an order has been issued putting new standards into effect immediately pending hearings on a permanent regulation. Under normal procedures, new standards cannot take effect until after hearings and assorted other time - consuming steps.

Bingham cited what she called conclusive evidence that benzene, a petroleum derivative that is used in production of rubber, chemicals, detergents and motor fuels, causes leukemia in human beings.

She said a recent survey of exposed workers at two Ohio rubber plants by the National Institute for Occupational Safety and Health showed that the incidence of leukemia among benzene exposed workers was at least five times the normal rate.

No further delay in curbing exposure to benzene could be tolerated, said Bingham, a chemist and environmental health expert who fought for stricter job health standards in private life before President Carter tapped her to head the government's controversial job health and safety program.

The benzene order will take effect May 21.

Rubber workers and others have been pushing for years for protections against benzene as a health hazard and possible carcinogen. An appeal for an emergency order against benzene from the United Rubber Workers and other unions was rejected last year by the Ford administration.

"I believe this signals a new day for an agency which in the past has been criticized for acting too slowly when lives are at stake," said Labor Secretary Ray Marshall, who joined Bingham at the press conference announcing the new benzene standard.

While labor unions generally praised the benzene order as long overdue, the AFL-CIO Industrial Union Department petitioned the U.S. Court of Appeals here for review on the OSHA's new standard, on grounds that its permissible level of benzene exposures is still too high.

Reaction from affected industries was spotty. Goodyear said many of its facilities already meet the new permissible level. Goodrich said it has always controlled exposure but three of its chemical operations may not be in compliance.

The American Petroleum Institute was "very concerned," especially about limitations on benzene's use in unleaded gasoline, said API's refining director, Raymond Wright. Chris Waisenen, labor law attorney for the U.S. Chamber of Commerce, said the need for some benzene curbs is generally recognized.

The emergency order reduces permissible wroker exposure to benzene from its present level of 10 parts per million parts of air to one part per million, based on an eight-hour average, and eliminates the peak level of 50 parts per million for any 15-minute period.

It also requires employers to measure worker exposure to benzene, provide workers with protective equipment and clothing, and institute medical surveillance, educational training and strict record-keeping programs.

OSHA officials estimated the costs of meeting the emergency standards at about $40 million for all the affected industries.