The risk of a catastrophic explosion at a damaged chemical tank in Southern California has been eliminated following a close overnight inspection that confirmed a crack in the tank relieved pressure and cooled the chemical, authorities said Monday.

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, Orange County Fire Authority division chief Craig Covey said 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."

However, evacuation orders remained in place for about 50,000 people in Garden Grove, California, located south of Los Angeles.

Covey said falling temperatures and the release of pressure from the tank were allowing officials to "turn the corner on this incident" after days of concern about a possible explosion.

There has been no chemical leak as of early Monday, but the Orange County Fire Authority said the risk to public safety is "ongoing."

READ MORE: 40,000 under evacuation orders after chemical tank leak in Southern California

Vapors began venting from the site on Thursday

After the tank overheated Thursday and began venting vapors, 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 100 degrees (37.7 Celsius) Sunday, an increase of 10 degrees Fahrenheit (5.5 Celsius) since Saturday, according to Democratic state Sen. Tom Umberg. On Monday, Covey said the temperature fell to 93 degrees F (33.9 degrees C).

Officials working to prevent a disaster

Gov. Gavin Newsom declared a state of emergency Saturday and said he asked President Donald Trump to issue an emergency declaration to bolster federal support for local and state officials.

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 used to make plastic parts.

Efforts to cool the tank were nonstop

The first goal of firefighters was 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 were 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 earlier.

As the interior temperature rises, methyl methacrylate converts from a liquid to a gas and increases the pressure, according to Purdue University engineering professor Andrew Whelton, who had said earlier that the crack could mean product or pressure is 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."

An explosion that could spread the chemical over a broad area and send shrapnel flying would be the worst-case scenario, he said.

Dealing with displacement and health concerns

Aerial photos taken by The Associated Press showed streets in the area were empty 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 Disneyland's two 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.

Whelton said if an explosion occurs, it will be crucial to conduct detailed air monitoring specifically for methyl methacrylate and not just generic tests for volatile organic compounds as officials did after a 2023 train derailment in East Palestine, Ohio, which released more than 115,000 gallons (435,000 liters) of vinyl chloride after officials blew open five tank cars and burned the chemical.

Orange County health officials said the chemical is easy to smell and people may notice it over a large area without being harmed.

Residents taking legal action

Some Garden Grove residents filed a class-action federal lawsuit 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 impacted.

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

GKN Aerospace agreed in 2025 to pay state regulators more than $900,000 to settle violations involving recordkeeping, permitting issues and nitrogen oxide emissions, according to a report on the South Coast Air Quality Management District website.

Associated Press journalist Ethan Swope in Garden Grove, California, contributed to this report.