By Brendan O’Brien

HOUSTON (Reuters) – Residents of two Houston-area cities were told to stay indoors and schools in six communities were closed on Thursday due to air pollution after a petrochemical plant fire.

The three-day blaze at Mitsui unit Intercontinental Terminals Co (ITC) in Deer Park, Texas, was extinguished on Wednesday after it destroyed 11 giant fuel tanks. No injuries were reported, but authorities testing the air detected high levels of benzene, a toxic chemical which has been linked to cancer.

The City of Deer Park, 20 miles (32 km) east of Houston, issued a shelter-in-place advisory to its 34,000 residents after reports of high levels of benzene or other volatile organic compounds (VOCs) within city limits, the municipality said on its website.

Residents were advised to remain indoors, turn off air conditioning and heating systems, and close doors and windows, making sure to plug any gaps, holes or cracks with wet towels or sheets.

A state highway was closed in the city and the Deer Park Independent School District and five other nearby school systems canceled classes.

The city of Galena Park, a community of about 11,000 people east of Deer Park, also issued a shelter-in-place advisory.

“Given our very conservative air quality standards, we are at a level where out of an abundance of caution there should be a shelter in place,” Lina Hidalgo, the chief executive of Harris County which encompasses Houston and its suburbs, during a news conference.

“This is a dynamic situation,” Hidalgo said. “That is why we have so many monitors in place.” Officials said they did not know when the shelter-in-place advisories will be lifted.

Inhaling benzene, a chemical with a pungent odor can cause minor irritation to skin, eyes and the respiratory system, while severe exposure can harm the nervous system or lead to unconsciousness, according to experts. The U.S. Environmental Protection Agency (EPA) classifies benzene, a component of gasoline stored in some of the tanks that burned, as a known carcinogen.

ITC said workers monitoring the scene of the fire had detected increased levels of benzene but said these levels did not represent an immediate risk.

The state’s environmental regulator said monitors detected up to 190.68 parts per billion of benzene in Deer Park early Thursday, a level that can cause headaches and nausea.

Workers were removing liquids from some tanks, the Texas Commission on Environmental Quality said, adding that vapors from exposed fuels still at the site can escape.

The fire, which began on Sunday morning, destroyed 11 tanks holding up to 80,000 barrels of gasoline and other fuels. The cause of the blaze has yet to be determined.

Harris County Fire Marshal Laurie Christensen said firefighters have placed and continue to reapply a foam blanket on the burn area to stop the possible escape of dangerous fumes. It is unclear what caused the release of benzene, she said.

The Texas Commission on Environmental Quality, which is investigating the incident, estimated that on the first day of the fire, 6.2 million pounds of carbon monoxide and thousands of pounds of nitrogen oxides, sulfur dioxide, and toluene were released. The regulator has cited ITC for violations of state air-emissions rules 39 times over the past 16 years.

The EPA is to test local waterways for possible contamination from the millions of gallons of water and foam dropped on the fire since Sunday.

(Reporting by Brendan O’Brien in Milwaukee; additional reporting by Gary McWilliams in Houston; Editing by Bernadette Baum and David Gregorio)