The Occupational Safety and Health Administration has moved to tighten its rules governing worker exposure to the pesticide ethylene dibromide (EDB), a chemical that has been shown to cause cancer in animals and is widely used to kill agricultural pests in harvested fruit and grain crops.

However, OSHA Administrator Thorne G. Auchter refused labor organizations' requests for a more stringent emergency standard, saying that emergency standards have been vulnerable when challenged in court.

OSHA is asking for comments by March 1 on a proposed revision of the exposure limit, which stands at 20 parts per million parts of air for workers exposed over an eight-hour period.

The request for comments does not indicate what new standards might be under consideration, but informed observers said OSHA is clearly moving to tighten, not loosen, the exposure limit.

Workers are exposed to EDB either by working near grain and fruit fumigation chambers or by transporting or handling fruit after it has been treated.

California, where EDB has been widely used for fruit sent out of state in the wake of the Mediterranean fruit fly infestation, has adopted an exposure limit of 130 parts per billion. That is 150 times stricter than the federal standard. Last month OSHA formally solicited comments on whether the state standard interferes with interstate commerce.

Earlier this month, a federal circuit court judge in California upheld the state standard, which had been challenged by Florida citrus growers who used to send their fruit to California and through that state to Pacific markets.

The Carter administration's Environmental Protection Agency last December proposed to ban EDB in July, 1983. The EPA is expected to extend the deadline to 1985, or whenever an alternative procedure for treating fruit and grain crops will be available. Irradiation with gamma rays is the preferred alternative.

OSHA has been criticized sharply by the AFL-CIO for not issuing an emergency standard.

OSHA's news release cites "new evidence" for re-examining the 10-year-old standard. An OSHA official said that included a review of relevant data by scientists at the National Institute of Occupational Safety and Health.