40,000 evacuate over LA chemical tank explosion risk

US firefighters warned a tank of toxic chemicals in California is heating up, adding to fears of a catastrophic explosion that has already forced tens of thousands of people to evacuate.

About 40,000 residents were ordered to leave their homes in the Garden Grove area of Orange County, southeast of Los Angeles, yesterday after the tank began to leak, sending fumes over a heavily populated area.

The tank contains 26,000 litres of methyl methacrylate, a volatile and flammable liquid used to make plastics, with firefighters warning the situation was serious.

Orange County Fire Authority Incident Commander Craig Covey said an emergency team had ventured into the area overnight, seeking to neutralise the "explosive potential" posed by an additional 15,000 gallon tank nearby should the 7,000-gallon tank blow up, and were then able to view the temperature gauge on the 7,000-gallon tank.

In a short video posted on social media, he said: "Unfortunately, I do have to report that the temperature was 90 degrees.

"Yesterday morning, it was 77 degrees when we backed out.

"It's been averaging about a degree an hour increasing, so that's the bad news."

He said firefighters are seeking ways to cool the tank.

Aerial footage filmed by local TV stations showed jets of water being sprayed at the tank, which has a capacity of 34,000 gallons.

Authorities described the tank exploding or rupturing as the only possible outcomes, but Mr Covey said: "Letting this thing just fail and blow up is unacceptable."

"Our goal is to find something and not allow that to happen, not to let it damage our community, not let it damage our environment," he added.

He said residents had followed directions to evacuate and the only people at risk now were the emergency responders.

Orange County Health Officer Regina Chinsio Kwong said yesterday the large exclusion zone around the tank was a necessary precaution.

"If it does explode and there is a vapor, you are all safe as long as you are out of the zone that was determined to be an evacuation zone," she said.

No injuries have been reported and there is no immediate indication as to what caused the leak, which was initially reported on Thursday.

Responders were working to put containment barriers in place to prevent any spilled material from reaching storm drains or river channels that funnel into the ocean.

The US Environmental Protection Agency said methyl methacrylate is irritating to the skin, eyes and mucous membranes in humans.

"Respiratory effects have been reported in humans following acute (short-term) and chronic (long-term) inhalation exposures," a fact sheet on the agency's website says.

"Neurological symptoms have also been reported in humans following acute exposure," it added.