The blast occurred in an overhead duct system shortly before 10:00 in an industrial area of La Habra, a Los Angeles suburb. It took more than 50 firefighters about 40 minutes to douse the resulting fire.
TV news reports showed workers with blackened faces and hands outside the plant, which was identified on a company website as Gorilla's Polishing. It is part of Gorilla's Chrome, which plates and polishes metal, particularly car wheels.
Mike Bremer, an operations manager at the neighboring VIP Rubber, told the Orange County Register that he felt the explosion in his office.
"It made my heart jump up into my throat," Bremer said. "This felt like a bomb going off. It was one big, loud explosion and a steam sound after that."
Bremer ran outside and saw about 20 to 30 employees from Gorilla's Polishing standing outside the burning building.
"It was chaotic. A couple of people I saw were burned, badly burned," he said. "They had their shirts off."
The blast and fire injured 11 people, including two men who were taken to hospitals in critical condition with third-degree burns over 90% of their bodies, Los Angeles County Fire Department Inspector Scott Miller said.
Most of the others suffered smoke inhalation, he said.
A preliminary investigation suggested lint and dust in a duct system caused the explosion, Miller said.
Employee David Lopez, aged 32, told the Register that the employees were polishing tire rims at the time of the explosion.