An industrial tank containing about 7,000 gallons of a highly flammable toxic chemical remains on the brink of explosion or rupture in Southern California for the fourth consecutive day, keeping tens of thousands out of their homes.

The tank is at a plant in Garden Grove that belongs to GKN Aerospace, a company based in the United Kingdom that manufactures aircraft components.

The tank became increasingly pressurized on Thursday, releasing gas that could trigger an explosion. However, fire officials said on Sunday afternoon that the tank may have formed a crack, possibly helping to relieve the pressure. They said they were evaluating the development.

Firefighters have been dousing the tank with copious amounts of water in an attempt to cool it, but GKN Aerospace’s team was unable to inject a neutralizing agent to reduce the chemical’s instability because of a failed valve.

“No one has ever had this situation before because the chemical is so volatile,” said TJ McGovern, the interim fire chief of the Orange County Fire Authority, who called the situation “unprecedented.”

The chemical inside the tank, methyl methacrylate, is used in the manufacture of resins and acrylic plastics, most notably plexiglass.

According to the Environmental Protection Agency, exposure to methyl methacrylate can irritate the eyes and skin and make it difficult to breathe, among other symptoms. Birth defects have appeared in animals exposed to the chemical.

On Saturday, local fire officials said the temperature inside the tank had risen more than 20 degrees, and the temperature continues to fluctuate at around 90 degrees, according to Chief McGovern.

There is fear of a “thermal runaway,” which could further generate heat, build pressure and cause a blast, said Elias Picazo, an assistant professor of chemistry at the University of California.

Alternatively, he said, a tank failure — in which the tank ruptures but does not necessarily explode — could lead to a controlled leak that could then be neutralized.

“I think the temperature within the tank has been steadily increasing and that’s indicative that the reaction is moving forward,” he said.

It is possible, officials said on Saturday, that the increase in temperature is occurring because the liquid inside the tank is solidifying. If so, and if the tank holds, that could make a rupture less likely.

A specialized team composed of officials from the fire departments in Los Angeles, San Bernardino County, Orange County and Long Beach were working on alternative solutions to prevent the tank from breaching, Chief McGovern said on Sunday. He did not provide details.

In a video posted to social media on Sunday, Mr. McGovern said the team had found a potential crack in the tank, which could potentially relieve some of the internal pressure.

“With this new information, it could change our trajectory and our strategy to this event,” he said.

Senator Thomas J. Umberg, a state legislator who represents the area, said that “several courageous firefighters” discovered a small crack last night at about 8:30 p.m., after approaching the tank to adjust the water being sprayed on it.

The fire authority said in another post that areas outside of the evacuation zone were “currently considered completely safe and day-to-day activities can continue as normal.”

Read more here: New York Times