Emergency crews in Southern California have been working through the night to stabilise a massive storage tank containing around 7,000 gallons of methyl methacrylate, a highly flammable chemical used in plastics and resin manufacturing.

Authorities said the chemical inside the tank overheated on Thursday, raising concerns that pressure could build up and cause a catastrophic explosion.

Thousands forced to leave homes across Orange County

Officials warned that a potential blast could affect an area spanning nearly nine square miles in Orange County, prompting evacuation orders across multiple cities, including Westminster, Stanton, Buena Park, Cypress, Garden Grove and Anaheim.

Residents living near the site have reported symptoms including dizziness, nausea and irritation in the throat and nose.

“I forgot to close the window during Thursday night, and I regret it. My throat and the inside of my nose feel sore, and my wife felt dizzy Friday night,” Stanton resident Van Ly said after evacuating.

California State Senator Tom Umberg said the temperature inside the tank had exceeded monitoring limits.

“The tank was at 100 degrees, or at least that's as high as the thermometer would go,” Umberg said.

Firefighters race to prevent 'worst-case scenario'

Firefighters and hazardous materials teams are continuing efforts to cool the tank and release pressure safely.

Officials said a crack spotted in the tank over the weekend may actually be reducing the immediate risk by slowly releasing pressure.

“In a weird world that's the best case scenario, believe it or not,” said incident commander Craig Covey. “Because once it comes out it is no longer an explosive hazard.”

However, authorities warned that the situation remains extremely dangerous if pressure is still trapped inside the structure.

“If the crack does not extend all the way, pressure could still be trapped inside, leaving the possibility of a catastrophic worst-case scenario,” Orange County Fire Chief Brian Fennessy McGovern said.

EPA monitoring air quality as residents await updates

Emergency officials said teams from the Environmental Protection Agency and local fire departments have so far not detected dangerous levels of toxic fumes in surrounding neighbourhoods.

Even so, authorities are urging residents to stay away from evacuation zones until operations are complete.

“We know that you're out of your homes, we want to get you back, but we cannot do that until it's deemed safe,” McGovern said.

Fire crews are expected to continue monitoring the tank overnight, with officials promising further updates once the operation progresses.