In the wake of two recent fatal explosions at fireworks factories, the Labor Department has announced that by next July 4 it plans to inspect all U.S. fireworks plants with 10 or more employes.

"The recent tragic death of 30 workers engaged in the manufacture of fireworks demands that this industry be subjected to close scrutiny by the Occupational Safety and Health Administration," Secretary William E. Brock said in making the announcement last week. "We are determined to assure that all fireworks manufacturers know and follow the safety and other worker protection standards that apply to them."

Twenty-one workers died in a June 25 explosion at a plant in Oklahoma and nine were killed in a similar accident May 20 in Ohio.

After the explosion at Aerlex Corp. near Hallett, Okla., OSHA acknowledged that it had never inspected the plant because it was unaware that it had been rebuilt after burning down in 1979. Officials said that, starting Oct. 1, it would have been exempted from random inspections under a Reagan administration policy that exempts industries that have injury rates below the national average for the previous year. Fireworks makers are included in the category of miscellaneous "chemicals and chemical preparations."

Brock noted, however, that the classification covers more than 100 types of manufacturers and that there is no reliable injury rate data for fireworks firms. He also pointed out that the injury rate does not include fatalities.

"Given these facts," he said, "we can't permit statistics of questionable relevance to determine our regulatory approach to a manufacturing process that common sense as well as experience tells us involves considerable potential risk to the workers engaged in it."

OSHA will coordinate inspections with the Treasury Department's Bureau of Alcohol, Tobacco and Firearms, which regulates and inspects storage and transportation of explosive materials.