(CNN) - A shelter-in-place order for two neighboring Texas cities was lifted Thursday, one day after a massive chemical fire that filled the sky with smoke for days in southeastern parts of the state was finally snuffed out.
The city of Deer Park lifted the order at 11:40 a.m. after earlier concerns about "levels of benzene or other volatile organic compounds within city limits."
At least six area school districts had canceled classes Thursday.
"According to the Texas Commission on Environmental Quality (TCEQ), Harris County HAZMAT teams and a private contractor on-site at Intercontinental Terminals Company (ITC) Deer Park site, air quality readings are improving, and over the last few hours, they have been significantly reduced," said Deer Park Emergency Services Director Robert Hemminger.
The neighboring city of Galena Park also lifted a shelter-in-place order, the city said via Twitter. "We are actively monitoring the air quality and will continue to monitor the situation at ITC in Deer Park," Galena Park Mayor Esmeralda Moya noted in an earlier alert.
The Intercontinental Terminals Company said increased levels of benzene were detected early Thursday. Acute exposure to benzene can cause headaches, dizziness or loss of consciousness, the World Health Organization says. Chronic exposure can lead to cancer.
"These levels are below those that represent an immediate risk," ITC said.
The increased benzene levels, which were detected beginning at 4 a.m., may have been "due to the shifting of the foam in one of our tanks," ITC spokeswoman Alice Richardson told reporters Thursday.
She said company responders were working "as carefully, as safely and as quickly as we can" to solve the issue. The compromised tank contained Pyrolysis gasoline, or Pygas, according to Richardson.
"To the residents that are watching, to my family, to my friends, to the remaining community, ITC cares," Richardson said when asked if she had a message for concerned residents. "We care a lot ... I think they know we will make this right."
Adam Adams, an on-scene coordinator for the US Environmental Protection Agency, said the agency had "not detected any hazardous conditions that are continuous."
Meanwhile, the Texas Commission on Environmental Quality said Thursday it is working with several agencies to assess water quality near the Deer Park facility.
The commission said teams and contractors from Deer Park, Harris County, the Environmental Protection Agency and US Coast Guard are working to evaluate the impact from the fire on Tucker Bayou and the Houston Ship Channel.
There is no threat to the quality of the city's drinking water at this time, according to Deer Park city officials.
Harris County Judge Lina Hidalgo said at a news conference on Thursday that the benzene escaped through the foam that was sprayed to blanket the tanks and put out the flames.
"There's not much wind right now, so it's keeping the product in the area," Hidalgo told reporters. "We are focused on maintaining the foam blanket until we can get to a point where we feel confident that the situation is under control."
Hidalgo said the possibility of vapor escaping from the foam blanket was something they accounted for, and plans were already in place when they received the elevated benzene readings. "We caught benzene vapor immediately early this morning, and our plan was activated and executed as designed," she said.
The Texas National Guard's 6th Civil Support Team, in partnership with local and county hazardous materials teams, have also set up two perimeters around the ITC facility to identify and analyze air quality readings in real time, according to Hidalgo.
Officials don't yet know what caused the release of benzene vapor from underneath the foam blanket, Harris County Fire Marshall Laurie Christensen said at a news conference Thursday morning.
"As they continue to cover and smother that [the tanks], that's going to take time," Christensen said. "We're going to have those layers underneath that have to cool down, and as that foam separates, you're going to see some of those vapors come up."
Responders will continue to apply foam to the tanks throughout the day, according to Christensen. "They're going to continue putting that blanket back together and continue covering in foam."
Christen anticipates that responders will have to continue covering the tanks with foam for approximately 24 hours. "As it breaks down, we're going to re-cover it."
The fire burned over four days at an ITC petrochemical plant in suburban Houston. It was finally extinguished Wednesday.
It began in a single tank Sunday afternoon and quickly spread to a second tank, the company said.
By Tuesday, seven tanks were burning. The tanks contained chemicals that go into making gasoline, including xylene, naptha and pyrolysis gasoline, known as Pygas.
No serious injuries have been reported. The cause of the blaze is under investigation.
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