Emergency officials lifted an evacuation order Monday for some of the people who live near a damaged tank containing hazardous chemicals in Southern California after temperatures inside the tank fell enough to eliminate the risk of a catastrophic explosion.

There is no longer a risk of catastrophic explosion of the tank containing methyl methacrylate at the GKN Aerospace Transparency Systems plant in Garden Grove, near Los Angeles, Orange County Fire Authority division chief Craig Covey said during a news conference.

Officials ordered about 50,000 Garden Grove residents to evacuate their homes on Thursday after the tank overheated and pressure built up inside. A crack that was discovered Sunday has relieved some of that pressure and the temperature of the highly flammable gas inside is continuing to fall, authorities said.

An overnight evaluation of the tank containing 6,000 to 7,000 gallons of methyl methacrylate produced “incredibly positive news,” Covey said earlier Monday, although the risk to public safety is “ongoing.”

Authorities have detected no hazardous chemical levels as of Monday.

Officials said they needed to cool the tank to prevent a toxic leak or explosion. The tank’s interior had cooled to 93 degrees F, Covey said Monday, down from 100 degrees a day earlier.

RISKS REMAIN

The tank might eventually cool enough for crews to safely stabilize and drain the remaining material without triggering a spark or ignition, said Andrew Whelton, a Purdue University engineering professor who has studied environmental contamination.

Whelton cautioned there is still some risk of an explosion while the chemical inside the tank remains hot and reactive. He said temperatures need to fall closer to ambient levels — roughly 60 to 70 degrees Fahrenheit — before conditions are considered significantly safer.

After the tank overheated on Thursday and pressure built, officials said it began releasing vapors. As the interior temperature rises, methyl methacrylate — which is used to make plastics — converts from liquid to gas, increasing the pressure and risk of explosion, Whelton said.

Some of the methyl methacrylate may already have hardened into a stable plastic similar to plexiglass, reducing the risk inside the tank, he said.

The release of pressure likely released at least some chemical into the atmosphere, and what responders are able to detect depends partly on the type and placement of monitoring equipment.

“If the device isn’t right next to the tank, then it’s not necessarily surprising that they’re not detecting high magnitudes of chemicals,” Whelton said.

Exposure to methyl methacrylate can cause serious respiratory problems, neurological problems and irritation to the skin, eyes and throat, according to an EPA fact sheet.

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

Relief among residents after hearing the latest update

Authorities have not defined what a catastrophic explosion might mean, but said Monday the worst-case scenario is off the table.

Kim Yen, a retiree who was forced to evacuate her Garden Grove home, said she has been closely following the news and was relieved to learn that the worst has passed.

“I am happy and many of us are happy but, still, we are still on our evacuation,” she said.

Yen, who lives two blocks from the plant, said she’s ready to return home but first wants to be sure it’s safe. And, she said, she’s been worried about the emergency crews.

“They are really our heroes,” Yen said.

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 there or pitched tents outside. Other people gathered in the park to enjoy Memorial Day.

GKN is a British company that supplies aircraft manufacturers

GKN Aerospace Transparency Systems, which owns the plant, is a British company that makes cockpit windows, canopies and windshields for military and commercial aircraft.