The blast Thursday at the 97-year-old refinery on the edge of San Francisco Bay prompted Richmond officials to sound the city's sirens, warning residents to stay indoors.
An estimated 600 people went to three area hospitals. Some complained of nausea and a metallic taste in their mouths or burning sensation in their throats. Three firefighters were injured.
"This is something that obviously is of concern to Chevron. We're taking this very seriously," said company spokeswoman Dawn Soper. "We'll work hard to ensure anything like this is prevented in the future."
Adrian Stanton of the Richmond Fire Department said the smoke was considered a health hazard.
Chevron spokesman Terry Swartz said there were toxins in the smoke, but he downplayed the risk.
"If the question is, is there a health hazard to our employees or the community, I don't believe there is," he said.
Some residents complained the explosion was just the latest health risk from Chevron and other nearby refineries.
The blast occurred just a month after a fireball engulfed five workers at the Tosco Avon refinery in Martinez on Feb. 23, killing four and leaving one critically injured.
"Basically, they need to shut it down or something, because the majority of these babies are being born with asthma," said Angela Brawley, who lives near the Chevron refinery.
The thick, black smoke was visible across the bay in San Francisco and Marin County. Firefighters had contained the blaze about 5 hours after it began, though they expected to continue fighting hot spots, said Soper.
The unit where the explosion occurred - a processing and manufacturing site for gasoline and jet fuel - was shut down indefinitely for investigation.