GARDEN GROVE, Calif. (AP) — 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 Tuesday it still wasn't 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 around 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.

Isabel Mendez was among those still waiting to return to her mobile home. She said she broke out in a rash on her face and developed tingling lips and a sore throat while evacuating last week. After spending several expensive nights in a hotel, she is now staying with her mother north of Los Angeles.

She remains uncertain about returning home because she does not trust official assurances that the area is safe.

"Of course it is still dangerous," she said.

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 (22,700 to 26,500 liters) 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, Orange County Fire Capt. 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.

"That's what kept it at 50 degrees," or 10 degrees Celsius, said TJ McGovern, interim chief of the Orange County Fire Authority.

"Due to that failure, the tank went into the heating-up process because it wasn't continuing to be chilled," he said at a press conference Monday evening.

Crews sprayed water on the tank until the interior temperature stabilized to 92 F (33.3 C), down from 100 F (37.7 C) over the weekend, the fire department said Tuesday. The hoses put out 1,250 gallons a minute over five days, which meant they used about 9 million 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.

Fire officials also tested storm drain water and found it was clean, McGovern said.

"There was no contamination," Orange County Health Director Regina Chinsio-Kwong said at a Monday news conference. "You should feel comfortable going home even if you're across the street from that new zone line."

Concern among residents

The crisis occurred in central Orange County, which is a densely populated area made up of a cluster of cities including Garden Grove. The city of 170,000 people, along with neighboring Westminster, is home to Little Saigon, the largest Vietnamese community outside Vietnam. It is also next to Anaheim, home to Disneyland's two theme parks, which were not under evacuation orders.

Garden Grove Unified School District said three schools remained closed and it was unclear if they would reopen before the school year ends this week.

Chinh Nguyen, 62, was among those unable to go back to his home in Stanton. He said the first night of the evacuation, he and his wife and two adult children slept at his wife's hair salon.

"We slept like dogs on the ground," he said. "We had no choice. We had nowhere to go."

They are now at a shelter in a high school in nearby Huntington Beach. He's grateful for the help but Nguyen said he hopes to return home before food that he left for his 10 parakeets runs out.

Environmental risks remain

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, 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 70 degrees Fahrenheit (15.6 to 21.1 degrees C) before conditions are considered significantly safer, he said.

GKN is a British company that supplies aircraft manufacturers

GKN Aerospace Transparency Systems makes cockpit windows, canopies and windshields for military and commercial aircraft. It employs about 16,000 people across 32 manufacturing sites in 12 countries, according to the company website.

"We apologize for the ongoing disruption this incident is causing and our priority remains its safe resolution," the company said, "so that residents can return to their homes as quickly as possible."

Weber reported from Los Angeles and Bellisle from Seattle. Associated Press journalists Leah Willingham in Boston; Jamie Stengle in Dallas; and Ethan Swope in Garden Grove, California, contributed to this report.