The United Steelworkers union said the fire at an Exxon Mobil refinery in California is a reminder of the lopsided policies in the downstream energy industry.

A blast at the 750-acre refinery in Torrance, Calif., shook nearby buildings and left at least four people with minor injuries. Most of the damage was contained to the site of what was described as a small fire.

Dave Campbell, secretary of the USW Local 675, which represents workers at the facility, said the blast occurred near a fluid catalytic unit, part of the refinery's infrastructure used to process gasoline.

"We thank God that no worker at ExxonMobil - hourly, management, or technical - was killed or seriously injured," he said in a Wednesday statement.

USW workers at 11 refineries are striking against what they see as unfair labor practices, adding workplace safety is a major concern in U.S. refineries.

"Our members work in dangerous and too often deadly conditions," said USW International President Leo W. Gerard said in a statement. "While employers have reaped billions of dollars in profits over the past several years, they have done little to improve conditions for workers and surrounding communities."

USW said five workers died in refinery incidents in the United States every year for five years running.

Torrance Mayor Patrick Furey issued a brief shelter-in-place following Wednesday's incident.