What we know about the chemical tank incident in Southern California — and what questions still linger

An issue with a valve caused the contents of a chemical storage tank in Garden Grove, California, to overheat, prompting fears of a possible disaster that threatened to expose residents to toxic plumes and destroy buildings.

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Since Thursday, Southern California officials — with help from state and federal agencies — have tried to avert what they feared could have become one of the worst chemical incidents in California history. The emergency also meant thousands of concerned residents in the city southeast of Los Angeles were ordered to evacuate their homes with no scheduled return date.

By Monday evening, the threat of a “boiling liquid expanding vapor explosion,” or BLEVE, had been averted , Orange County fire officials said.

Here’s what to know about what happened.

What is methyl methacrylate?

At issue is a storage tank at a GKN Aerospace facility containing about 7,000 gallons of methyl methacrylate.

Liquid methyl methacrylate is a toxic chemical used to manufacture resins and plastics, such as Plexiglas.

Exposure to methyl methacrylate can cause eye or skin irritation, and inhaling it can lead to coughing, wheezing, dizziness, headache or shortness of breath.

What was the risk?

The Orange County Fire Authority reported late last week that the tank containing the chemical had begun to heat up and bulge.

That meant one of two horrifying scenarios : Either the pressure in the tank would get so high that it would crack and the chemical would spill out, or the tank would rupture and the chemical would vaporize and explode. Experts feared the blast would cause two nearby tanks of methyl methacrylate to explode, as well.

“That was what we were handed — a leaking tank or a tank that blows up,” Fire Division Chief Craig Covey, the incident commander, said Friday.

By Monday morning, the fire department said, it had eliminated the threat of a BLEVE — a scenario in which flammable liquid inside the tank becomes hotter than its boiling point, creating so much pressure that the tank explodes, accompanied by a massive fireball.

Andrew Whelton, a professor of civil, environmental and ecological engineering at Purdue University in Indiana, compared the process to leaving a soda can in your car in the middle of summer.

“If you leave it in there, it’s going to blow up because the pressure in the soda can get too great,” he said. However, “if you put a hole in it, it will let some of the gases out.”

Southern California officials confirmed Monday that a crack in the tank was releasing pressure — but not leaking the chemical — and that the internal temperature was coming down. Firefighters have also been using sprinklers and hose lines to cool the tank.

Around 60,000 people were evacuated from the area over the concerns. By Monday evening, the orders had been reduced to around 16,000 residents.

Why did the tank overheat?

An issue with a valve in the tank’s recirculating refrigeration system set off a dangerous chain of events, officials said.

“The tank overheated because it’s held by a recirculating refrigerating system — that’s what kept it at 50 degrees,” said Covey, the incident commander. “One of the valves in that system froze up, so it was no longer being circulated.”

The tank then went into a heating-up process, he said.

There will be a transparent investigation into what happened, Garden Grove Mayor Stephanie Klopfenstein assured residents Monday evening.

Has the risk been eliminated?

At its peak, the tank reached at least 100 degrees Fahrenheit — the highest reading on the temperature gauge, suggesting the actual temperature may have been even higher. The boiling point for methyl methacrylate is around 212 degrees, according to BASF, a large chemical manufacturer. By Monday, the temperature had dropped to 93 degrees.

“The fact that it is decreasing is a positive sign, because it means that there’s no heat being generated inside that chemical tank,” Whelton said.

However, he said the risk of a chemical spill or a smaller explosion wasn’t off the table, given the lingering uncertainties about what’s happening inside the tank. Some chemical engineers think it’s possible that the methyl methacrylate in the tank reacted with itself, creating a chain of molecules that bind like Legos to become solid.

“Once it’s in that condition, it doesn’t pose a threat anymore, so that’s one of the theories about why the temperature is decreasing,” Whelton said.

By Monday, temperatures had cooled and some of the material had “cured,” or turned into a solid state, in good news for firefighters and response crews, officials said.

Interim Orange County Fire Chief TJ McGovern said Monday evening that while the threat of a catastrophic explosion was believed to have passed, there was still more work to be done.

McGovern said firefighters “still have to mitigate a fire and very small explosion concern, and also a spill potential.”

Are there environmental concerns?

Air monitoring with 20,000 instruments in the region has detected “no exceedances” in the region throughout the incident, Chris Myers, the federal on-scene coordinator for the Environmental Protection Agency, said Monday evening.

The crack is at the top of the tank, which, Whelton said, could in theory allow vapor and gases to escape.

On Monday, President Donald Trump approved California’s request for a presidential emergency declaration to free up more resources, such as equipment and personnel, from the Federal Emergency Management Agency. California Gov. Gavin Newsom also declared a state of emergency in Orange County on Saturday.

A White House official said FEMA has sent a team to the State Emergency Operation Center and activated its Interagency Modeling and Atmospheric Assessment Center, which models airborne hazards. The Environmental Protection Agency has enabled air monitoring at 20 locations around the area, the official said.

The Orange County District Attorney’s Office has launched an investigation , and two Garden Grove residents have filed a class-action lawsuit against GKN Aerospace, the company that owns the facility.

In response to a request for comment, a spokesperson for GKN Aerospace directed NBC News to an online statement saying its technical specialists worked with the Orange County Fire Authority to stabilize the tank.

“The team safely and successfully removed external insulation material from the tank in order to help advance efforts to cool its contents,” the statement reads. It continues: “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.”

Covey, the incident commander, said Friday that authorities were able to neutralize a nearby tank by adding a compound to it but couldn’t do the same for the one on the brink of exploding because the valves were “broken” and “gummed up.”

Judith Enck, a former EPA regional administrator who is now president of Beyond Plastics, a nonprofit group trying to end plastic pollution, said: “This is why you need environmental regulators to do inspections. A fair question for EPA and the South Coast Air Quality Management District is ‘When is the last time this particular tank was inspected, and what was found?’”

Whelton said investigators should consider how often the tank was maintained or cleaned out or whether the chemical itself had solidified and clogged the valves.

“Hopefully it’s going to turn out to be just a giant emergency for many people involved, the chemical is not released, and there is no physical destruction of buildings,” he said. “That’s the ideal situation, and that’s still in the cards.”