Firefighters contending with an overheating tank of hazardous chemicals in southern California said they had eliminated the threat of the tank exploding in an overnight operation, Orange county fire officials said on Monday.

The damaged chemical tank in southern California had cracked over the weekend, which authorities were hopeful would relieve pressure and reduce the risk of an explosion.

Officials said crews conducted tank temperature checks at night to reduce risks to firefighters, avoiding daytime operations when heat from the tank made conditions around it most dangerous.

The overnight mission allowed crews to verify the crack and confirm temperatures were falling, the Orange county fire authority division chief, Craig Covey, said on Monday morning. Covey said the results of overnight evaluation of the tank – that the temperature inside had dropped and that pressure had been released – was “incredibly positive news”.

Some 50,000 residents in Garden Grove, a city of roughly 170,000 about 40 miles (60km) south of downtown Los Angeles, and in the surrounding area have been evacuated and were waiting for a resolution.

The tank overheated on Thursday and began venting vapors, leaving local and state officials scrambling to evade the worst-case scenario at the aerospace company site.

No injuries have been yet reported.

Donald Trump approved a federal emergency declaration following a request from the California governor, Gavin Newsom. The declaration makes federal assistance available to emergency responders through the Federal Emergency Management Agency (Fema).

Firefighters have repeatedly sprayed the tank with water in an attempt to cool the chemical inside, methyl methacrylate, which is used to make plastic parts. The tank’s interior reached 100F (37.7C) on Sunday.

The tank at GKN Aerospace Transparency Systems, which makes parts for commercial and military aircraft, holds 6,000 to 7,000 gallons (22,700 to 26,500 liters) of methyl methacrylate.

Monitoring tests found air pollution around the evacuation zone was within normal limits and specialized equipment is being used to ensure gas is not released, state and federal environmental officials said on Saturday.

The first goal of firefighters is to cool off the chemical inside the tank to prevent a leak or explosion.

Drones were monitoring temperatures at 10-minute intervals to watch for any spikes. Containment barriers have been set up to prevent the chemical from getting into storm drains or reaching creeks or the nearby ocean in the event of a spill, Covey, said on social media.

As the interior temperature rises, methyl methacrylate converts from a liquid to a gas and increases the pressure, according to Andrew Whelton, a Purdue University engineering professor, who said the crack could mean product or pressure was being released, reducing the chance of explosion.

“Think of a soda can. If you leave it in a hot car it can explode,” Whelton said. “But if you put a hole in the can, the product is released and the can itself doesn’t explode.”

The tank might eventually cool enough for crews to safely stabilize and drain the remaining material without triggering a spark or ignition, Whelton added. He cautioned that there was still some risk of an explosion while the chemical inside the tank remained hot and reactive.

He said temperatures needed to fall closer to ambient levels – roughly 60 to 70F (15.6 to 21.1C) – before conditions were considered significantly safer.

Aerial photos taken by the Associated Press showed empty streets in the area on Sunday, while several evacuation shelters were open. At a high school in neighboring La Palma, people slept in cars or on mats and sleeping bags on the asphalt.

Garden Grove is next to Anaheim, home to two Disneyland theme parks, which were not under evacuation orders. Park officials said they were monitoring the situation.

Exposure to methyl methacrylate can cause serious respiratory problems, neurological problems, and irritation to the skin, eyes and throat, according to fact sheets about the chemical.

GKN Aerospace Transparency Systems employs about 16,000 people across 32 manufacturing sites in 12 countries, according to its website. The company agreed in 2025 to pay state regulators more than $900,000 to settle violations involving record-keeping, permitting issues and nitrogen oxide emissions, according to a report on the South Coast Air Quality Management District website.

Some Garden Grove residents filed a class action federal lawsuit on Saturday against GKN Aerospace Transparency Systems, which operates the facility where the tank is located. Lawyers for the residents argued that regardless of what happens, property values in the surrounding community are sure to be affected.

GKN Aerospace did not comment on the lawsuit but has apologized to residents and businesses forced to evacuate. It said on Sunday it was “working around the clock to mitigate the risk of a leak”.

The Associated Press and Reuters contributed reporting