While there's no longer a risk of a major explosion at the GKN Aerospace Transparency Systems plant in Garden Grove, there's still a chance for a smaller blast or a fire, Orange County Fire Authority division chief Craig Covey said during a news conference on Monday.
An overnight evaluation of the tank containing 22,700 to 26,500 litres of methyl methacrylate, which is highly flammable, showed a reduction of pressure inside the tank thanks to a crack that was discovered on Sunday.
About two-thirds - roughly 34,000 - of the evacuated residents can go home as a result, Covey said.
"It's not over yet. We still have work to do," Covey said.
"We still have to mitigate a fire and very small explosion concern, and also a spill potential."
Officials began ordering residents of Garden Grove, near Los Angeles, to evacuate their homes on Thursday after the tank overheated, and by the weekend about 50,000 residents had been told to leave.
Officials said they needed to cool the tank to prevent a toxic leak or explosion. The tank's interior had cooled to 33.9C, Covey said on Monday, down from 37.7C) a day earlier.
Orange County Health Director Regina Chinsio-Kwong said she wanted to reassure everyone who is returning home that they can feel safe. Exposure to methyl methacrylate can cause serious respiratory problems, neurological problems and irritation to the skin, eyes and throat, according to the federal Environmental Protection Agency.
"There was no contamination. There were no fumes. There were not vapours that came from this incident," she said at the news conference.
"There was not a leak. So it should be, you should feel comfortable going home even if you're across the street from that new zone line."
