An estimated 40,000 people in Orange County, California, have been ordered to evacuate over fears that a tank containing a toxic chemical may explode, officials said Friday.

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Authorities in Orange County said they’re unable to control the valves on a tank at an aerospace manufacturing facility in Garden Grove.

“It fails or it blows up,” said Craig Coby, Orange County Fire Authority incident commander at the scene.

Initial evacuation orders were issued Thursday after a “vapor release” at the manufacturing site in Garden Grove. The situation seemed to be stable, but on Friday, the valve on a tank became inoperable, the Orange County Fire Authority said.

The tank contains methyl methacrylate, a toxic chemical used to manufacture resins and plastics.

In a video announcement, Coby urged caution for people in the area of the leak, which includes parts of the cities of Garden Grove, Cypress, Stanton, Anaheim, Buena Park, and Westminster.

Garden Grove is about 35 miles southeast of downtown Los Angeles.

“It is a highly toxic substance,” OCFA Division Chief Nick Freeman said at an afternoon news briefing. “It’s extremely flammable, and in its current state, very reactive, and can cause that explosion.”

The tank was not emitting the chemical on Friday, authorities said, and no injuries have been reported.

Dr. Regina Chinsio-Kwong said the emergency is unprecedented and it is difficult to predict an outcome.

“This is a unique situation,” she told reporters. “We don’t have information of a similar situation where this happened, right. So we’re going into unique times, and we have limited information.”

According to the U.S. Environmental Protection Agency, methyl methacrylate can cause lung and skin irritation, headaches, coughing and wheezing, and has been suggested but not proven as a possible cause of colon and rectal cancers.

Garden Grove Police Chief Amir El-Farra told reporters that the evacuation zone includes about 40,000 people.

The EPA is supporting local authorities with air monitoring. A spokesperson said OCFA is the lead agency handling the emergency.

Authorities first responded to reports of a “vapor release” from multiple chemical storage tanks at the facility in the 12100 block of Western Avenue in Garden Grove about 3:22 p.m. Thursday, the OFCA said in a statement.

Initial evacuation orders were rolled back after authorities were able to cool the tanks, the OFCA said. But lack of valve control prompted the renewed evacuations on Friday, it said. A tank holding about 6,000 to 7,000 gallons is being cooled by a sprinkler system and an unmanned hose line, the agency said.

A spokesperson for the manufacturer on site, GKN Aerospace, said by email, “The situation at our Garden Grove site remains ongoing, and we are fully focused on working with emergency services and the relevant authorities to ensure the safety of our employees and the local community.”