Although at least 80 workers have died in the last decade in fireworks factories -- including 21 killed Tuesday in a devastating explosion in Oklahoma -- such plants would be exempt from random safety inspections next year under an Occupational Safety and Health Administration policy adopted by the Reagan administration in 1981.

OSHA officials said yesterday they have never inspected the Aerlex Corp. plant near Hallett, Okla., because of past exemptions and because OSHA, for unexplained reasons, was unaware that the plant existed.

"Even though it may sound like a high-hazard industry, if they have evidence of a reasonable safety rate, it does not go on the high-hazard list" of plants that will receive unannounced visits by OSHA inspectors, said David F. Demarest, deputy undersecretary of labor.

"What change we may make in that policy is an unknown quantity," Demarest said. "Right now we are looking at how to make sure this kind of thing doesn't happen again."

Margaret Seminario, AFL-CIO associate director for health and safety, said the federation "has been pointing out for five years that we don't think anybody should be exempt. We have been trying to impress upon people that this is a crazy policy . . . and this illustrates it."

The policy, adopted four years ago by then-OSHA chief Thorne G. Auchter, is intended to "target" inspectors to those industries where the dangers are greatest, said OSHA spokesman Jack McDavitt. He said the agency believes the targeting has been effective, but added, "I suppose reevaluating the targeting will be a possibility, but it is too early" to tell.

Under the policy, companies aren't subjected to random inspections when their annual record of "lost workday" accidents is below the most recent national average for all industries.

Fireworks manufacturing is lumped into a category of miscellaneous "chemicals and chemical preparations." That industry is currently considered above the national average because it had an injury rate of 3.8 accidents involving lost workdays per 100 workers in 1982, higher than the national rate of 3.4. But fireworks companies would become exempt when the new fiscal year starts in October because their 1983 rate was 3.3, below the national average of 3.4, according to John Miles, OSHA director of field operations.

Fatalities are not included in this computation, according to OSHA.

OSHA, which has reduced its inspection staff by roughly 25 percent over the past five years, now inspects exempted facilities only after a fatality has occurred or employes have complained. The AFL-CIO said that roughly three-quarters of all manufacturers are exempt because their industries are below the national injury average.

"Under OSHA, this fireworks plant is a safe plant," said Seminario. The plant, she noted, is grouped with firms that manufacture chemicals such as the one responsible for the Bhopal, India disaster.

Investigators from at least six state and federal agencies yesterday were sifting through the rubble of the Oklahoma factory and planned to interview five survivors of the blast to try to determine the cause. The Treasury Department's Bureau of Alcohol, Tobacco and Firearms and the Transportation Department regulate and inspect the storage and transportation of explosive materials.

The BATF inspected the explosives storage at the Aerlex plant on April 24 and found "record-keeping violations," but no safety problems, said BATF spokesman Dot Koester. Aerlex has a Class B federal license to use "black powder," which is classified as a "low explosive" that is often used for July 4th fireworks displays, Koester said.

Aerlex usually had a payroll of about 15 employes, but had hired extra help to increase production for the busy Independence Day season, according to William White Jr., area director of OSHA's Oklahoma City field office. He said state labor officials are investigating reports that the firm illegally employed underage workers.

White said his office was unaware that the firm existed because it was not included on a master list of employers prepared in Washington. He said his office supplements the list using state data, but said Aerlex was dropped from listings after its plant burned down in 1979 in an incident that involved no injuries. When Aerlex rebuilt its factory, he said, it was inadvertently never included on OSHA employer lists. But, he said, because of the agency policy, "we wouldn't have inspected them anyway," unless a complaint was received.

According to BATF data, 80 deaths and 150 injuries have resulted from more than 60 explosions at fireworks factories since 1976. Of those, 42 deaths and 86 injuries occurred at BATF-licensed facilities, while 38 deaths and 64 injuries occured at unlicensed plants making illegal fireworks. The most common illegal fireworks are known by such names as cherry bombs, hammerheads, silver salutes, M-80s and M-100s.

Aside from workplace injuries, the Consumer Product Safety Commission estimates that more than 9,800 people, most of them children, were injured last year using fireworks. Fire officials estimate that more than half the injuries result from using illegal explosives.