Orange County authorities on Monday lifted evacuation orders for about 34,000 of the estimated 50,000 residents displaced by a damaged chemical tank at a Garden Grove aerospace plant, saying a crack in the tank and falling temperatures had eliminated the threat of a catastrophic explosion at the GKN Aerospace Transparency Systems facility.
While the worst-case scenario is off the table, a smaller blast or fire remains possible, and a spill risk persists, Orange County Fire Authority division chief Craig Covey said during a Monday news conference. An overnight evaluation of the tank containing 6,000 to 7,000 gallons of methyl methacrylate — a highly flammable and toxic liquid used to make plastics — showed pressure reduction thanks to a crack discovered Sunday.
“It’s not over yet. We still have work to do,” TJ McGovern, interim fire chief of the Orange County Fire Authority, said. “We still have to mitigate a fire and very small explosion concern, and also a spill potential.”
Officials began ordering residents of Garden Grove, near Los Angeles, to evacuate their homes on Thursday after the tank overheated, and by the weekend about 50,000 residents had been told to leave. The tank’s interior temperature fell to 93 degrees F (33.9 C) on Monday from 100 degrees (37.7 C) a day earlier, Covey said.
Orange County Health Director Regina Chinsio-Kwong sought to reassure the roughly two-thirds of evacuees cleared to return. Exposure to methyl methacrylate can cause serious respiratory and neurological problems, and irritation of the skin, eyes and throat, according to the federal Environmental Protection Agency.
“There was no contamination. There were no fumes. There were not vapors that came from this incident,” she said at the news conference. “There was not a leak. So it should be, you should feel comfortable going home even if you’re across the street from that new zone line.”
Purdue University engineering professor Andrew Whelton, who has studied environmental contamination, cautioned that some risk of an explosion remains while the chemical inside stays hot and reactive. Temperatures should fall closer to ambient levels — roughly 60 to 70 degrees F (15.6 to 21.1 C) — before conditions are considered significantly safer, he said.
Whelton said the increasing interior temperature caused the methyl methacrylate to convert from liquid to gas, ramping up pressure. But some of the material may have already hardened into a stable plastic similar to plexiglass, reducing the threat inside the tank.
Orange County Supervisor Janet Nguyen said the South Coast Air Quality Management District would monitor the air for several months, and the EPA would check sewer and storm drains. County health officials have said the chemical is easy to smell and people may detect it over a wide area without being harmed.
Resident Kim Yen, a retiree who lives two blocks from the plant, said she had been closely following the news and was relieved the worst had passed. “I am happy and many of us are happy but, still, we are still on our evacuation,” she said, adding that she was ready to return home but first wanted to be sure it was safe. She called emergency crews “our heroes.”
The parking lot was full Monday at a large park in Fountain Valley, just southwest of Garden Grove, as people sought refuge in an ad hoc shelter or pitched tents outside. Others gathered in the park to celebrate Memorial Day.
GKN Aerospace Transparency Systems, a British company that makes cockpit windows, canopies and windshields for military and commercial aircraft, said its technical specialists worked with the fire authority to remove external insulation from the tank to help cool its contents.
“We apologize for the ongoing disruption this incident is causing and our priority remains its safe resolution, so that residents can return to their homes as quickly as possible,” GKN Aerospace said in a statement. The company employs about 16,000 people across 32 manufacturing sites in 12 countries.
It remained unknown when the plant would reopen. GKN Aerospace agreed in 2025 to pay state regulators more than $900,000 to settle violations involving recordkeeping, permitting and nitrogen oxide emissions, according to a report on the South Coast Air Quality Management District website.
Aircraft manufacturing supply chains are highly concentrated and already strained, leaving little room for disruptions at specialized components facilities, said Richard Aboulafia, managing director of the aerospace consulting firm AeroDynamic Advisory. “There’s just not a lot of margin in the system,” he said.