No injuries were reported after the pressurised tank overheated on Thursday and began venting vapours in Garden Grove, according to the Orange County Fire Authority.
Evacuation orders were issued for 40,000 people, and several shelters remained open Saturday, including at three high schools.
Fire Authority Division Chief Craig Covey said Friday evening that efforts to cool the tank appeared to be working.
But on Saturday, he backtracked, saying a reading conducted by drones on Friday showed the temperature on the outside of the tank, not the inside.
Authorities haven't said why the tank began overheating.
But cooling the tank is important because the chemical's flashpoint is 50 degrees Fahrenheit (10 degrees Celsius), according to the National Institute for Occupational Safety and Health.
If the chemical heats up, it can release a vapour that is harmful to people's health.
It can cause respiratory issues, itching and burning eyes, nausea and headaches, said Dr Regina Chinsio-Kwong, the Orange County health officer.
Covey said emergency crews were able to see the troubled tank's temperature gauge in person late Friday.
"Unfortunately, I do have to report that the temperature was 90 degrees (Fahrenheit)," Covey said Saturday.
"Yesterday morning, it was 77 degrees (Fahrenheit) when we backed down. It's been averaging about a degree an hour increasing."
The concern is that the tank could fail and crack, releasing the liquid chemical onto the ground, or it could explode, Covey said.
If the tank spills its contents, containment barriers have been set up to prevent the toxic chemical from getting into storm drains or reaching creeks or the nearby ocean, he said.
The damaged tank is located at GKN Aerospace, which makes parts for commercial and military aircraft.
The tank holds between 22,700 and 26,500 litres of methyl methacrylate, used to make plastic parts.
"Letting this thing just fail and blow up is simply unacceptable to us," Covey said.
In a statement, GKN Aerospace said it was "fully focused on working with emergency services, specialised hazardous material teams and the relevant authorities to ensure the safety of the local community, our employees and everyone else involved".
Initially, residents in Garden Grove were ordered to leave.
Evacuation orders were expanded on Friday to some residents of five other Orange County cities — Cypress, Stanton, Anaheim, Buena Park and Westminster.
Some residents with pets said they planned to sleep in their cars.
Marco Solano, a 32-year-old resident, spent Friday night at his parents' home.
He said he was frustrated by the situation and was monitoring the news to see if he could go back to his home.
"I don't think that they should have dangerous chemicals in a neighbourhood area, especially that dangerous that they have to evacuate people," Solano said.
Solano, who has multiple jobs, said he wasn't feeling well Saturday, believing the stress of the chemical leak was exacerbating his anemia and ulcerative colitis.
"Right now, I'm extremely tired. I have no energy. I'm super weak," he said.
He said he went to his apartment after work on Friday to grab some belongings and saw other residents who had not evacuated, and he was worried for them.
Garden Grove is about 61 kilometres south of downtown Los Angeles and next to Anaheim, home to Disneyland's two theme parks, which were not under evacuation orders as the Memorial Day weekend got underway.
Park officials said they are monitoring the chemical incident and supporting employees impacted by the evacuations.
