Many of the injured suffered burns to their faces and forearms, said Greg Siggins of the California Division of Occupational Safety and Health, which is investigating the blast. The number of injured could grow as the investigation continues, Siggins said.
Occupants of a neighboring building reported the explosion shortly before 10 a.m. in La Habra, Los Angeles County Fire Inspector Keith Mora said.
Arriving firefighters saw flames in the building, he said. The fire was contained in about a half-hour. The eight people who were hurt were taken to hospitals.
A preliminary investigation suggested lint and dust in a duct system caused the explosion, fire Inspector Scott Miller told the Orange County Register.
The building in a commercial district houses a metal plating and polishing firm. A phone message left at the business was not immediately returned.
About 30 workers from the plant waited about a half-block from the scene as an acrid smell of smoke hung in the air, the Register reported.
Employee David Lopez, 32, said the employees were polishing tire rims at the time of the explosion, and several of his co-workers were burned.
La Habra is a suburb on the border of Orange and Los Angeles counties about 20 miles southeast of downtown LA.