The incident occurred at an aerospace facility in Garden Grove, southeast of Los Angeles, where a storage tank holding about 7,000 gallons (26,000 liters) of methyl methacrylate developed a leak. Emergency officials warned that the situation posed significant risks to nearby communities due to the chemical’s flammable nature and the possibility of a thermal runaway reaction.

Incident Commander Craig Covey said emergency planners were preparing for two primary scenarios: a complete tank failure resulting in a large chemical spill or an explosion that could affect neighboring tanks containing fuel and other hazardous materials.

“We are setting up these evacuations in preparation for these two options,” Covey said, describing the threat as serious and requiring immediate precautionary measures.

Garden Grove Police Chief Amir El-Farra said approximately 40,000 people were covered by the evacuation order, although several thousand residents chose to remain in their homes despite warnings from authorities.

Emergency crews continuously sprayed water on the tank in an effort to reduce its temperature and stabilize conditions. Later on Friday, Covey reported that cooling operations had succeeded in lowering the tank’s temperature to about 61 degrees, closer to what officials described as a safe operating range.

Authorities continued searching for alternatives that could further reduce the risk of either a major spill or an explosion.

Orange County Health Officer Regina Chinsio Kwong said the evacuation zone was established to protect residents from potential exposure to hazardous vapors in the event of a catastrophic failure.

She urged the public to report any detection of a strong fruity odor associated with methyl methacrylate, while noting that the smell alone did not necessarily indicate harmful exposure levels.

No injuries had been reported as of Friday evening, and officials had not determined the cause of the leak, which was first identified on Thursday.

Emergency responders were also preparing containment measures to prevent any released chemical from entering storm drains, waterways or channels that flow into the Pacific Ocean.

According to the U.S. Environmental Protection Agency, methyl methacrylate can irritate the skin, eyes and respiratory system. Acute or prolonged exposure may also result in respiratory and neurological effects.