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.

Firefighters responded, 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.