A crack that formed by chance in an overheated chemical tank in southern California relieved pressure and helped avert a catastrophic explosion, but officials said on Tuesday it still was not safe enough for 16,000 people living closest to the aerospace plant to return home.

The crisis forced 50,000 people to evacuate in and near the Orange county city of Garden Grove last week. Most returned home after the crack formed over the Memorial Day weekend, but the risk of a smaller explosion or potential spill kept evacuation orders in place for about a third of residents.

Exposure to methyl methacrylate – a highly flammable chemical used to make plastics – can cause serious respiratory problems, neurological problems and irritation to the skin, eyes and throat, according to the federal Environmental Protection Agency. The tank at GKN Aerospace Transparency Systems contains 6,000 to 7,000 gallons of the chemical.

Underscoring the dangers of living near hazardous material was an implosion of a chemical tank at a pulp and paper mill in Washington state on Tuesday that injured at least 10 people, while an undisclosed number of others had been killed or remained missing.

Crews at the California plant worked overnight to ensure two nearby tanks were neutralized and would not be affected by the compromised tank, the Orange county fire captain Brian Yau said, adding that material from one of these two tanks was transferred to another that has a neutralizing agent.

The tank overheated because a valve on the cooling system failed, fire officials said.

Crews sprayed water on the tank until the interior temperature stabilized to 92F, down from 100F over the weekend, the fire department said on Tuesday. The hoses put out 1,250 gallons a minute over five days, which meant they used about 9m gallons of water.

The sprinkler system at the facility continues to douse the tank, and the company said its technical specialists and firefighters removed insulation from the tank to help cool it.

The South Coast Air Quality Management District will monitor the air for several months and the EPA will be checking sewer and storm drains for spills, the Orange county supervisor Janet Nguyen said.

As the tank heated up, the chemical converted from liquid to gas, ramping up the pressure and explosion risk, said Andrew Whelton, a Purdue University engineering professor who has studied environmental contamination. Some of the methyl methacrylate may already have hardened into a stable plastic similar to plexiglass, reducing the danger, he said.

“The tank was on track for a catastrophic explosion,” Whelton said. “The formation of a crack seems to have allowed pressure to vent.”

The risk remains of a smaller blast that could send projectiles or even a chemical plume toward nearby homes, he said.

The tank needs to get closer to 60 to 70F before conditions are considered significantly safer, he said.