A fire at a huge refinery in the San Francisco Bay area has been contained, but the Chevron facility is continuing to spew thick, black smoke, which authorities fear may be toxic.
More than 200 residents complaining of breathing difficulties have flooded nearby hospitals.
"We heard a big explosion," San Pablo resident Rafael Bastos said. "We thought it was the neighbors, and then we went outside and saw the big smoke."
Chevron officials can't confirm reports of an explosion, but a spokesperson says a diesel leak is responsible for the fire.
The general manager of the refinery said they started investigating after detecting the leak.
"Part of our response process is to actually get out," Chevron Refinery General Manager Nigel Hearne said. "We call out to the job site, and when the leak increased, we evacuated the area. The increasing leak is what actually caused the fire."
Diesel fuel continued to feed the fire, sending more smoke billowing into the air. Residents say they began to feel light-headed and nauseous from the smell.
Shortly after it began, county health officials issued a "shelter in place" warning in an automated phone message. That means residents should stay indoors, turn off all heaters, air conditioners and fans, and cover cracks around doors with tape or damp towels.
"Given that the plume is high up there -- but there may be some concerns with winds and when it starts to cool down -- that the plume may start coming down to the ground," Contra Costa County Health Department hazardous-materials specialist Trisha Asuncion said.
Air measurements have not picked up any harmful readings, but health officials say they will continue to monitor air quality.
One plant employee was treated for superficial burns on his wrist.