Photo: The Canadian Press

Harris County Judge Lina Hidalgo talks about the air quality in east Harris County during a press conference at TranStar Thursday, March 21, 2019, in Houston.

Authorities on Thursday warned people to stay indoors after high levels of benzene were detected in the air near a scorched petrochemicals storage facility outside of Houston.

Firefighters on Wednesday extinguished the blaze at the Intercontinental Terminals Company in Deer Park, which started Sunday and destroyed several large tanks that contained gasoline and chemicals used in nail polish remover, glues and paint thinner. They continued to spray foam on the site Thursday to try to prevent flare-ups.

Officials said Wednesday that benzene levels near the facility didn't pose a health concern. But authorities issued a shelter-in-place order Thursday due to "reports of benzene or other volatile organic compounds" in Deer Park, which is about 15 miles (24 kilometres) southeast of Houston. Several school districts also cancelled classes for the day due to the air quality concerns.

Harris County Judge Lina Hidalgo, who is the county's top administrator, said at a news conference Thursday that light winds were helping to keep the vapours from spreading more broadly.

"Outside of that immediate area we're not seeing elevated levels right now," said Hidalgo.

Dr. Umair Shah, who heads the county health agency, said there's still only a minimal public health risk, but he cautioned that the elderly, pregnant women and other vulnerable groups should try to limit their exposure.

"The most important thing is that the levels that have been detected are still not high enough for the level of concern that people may be having," he said.

The county fire marshal, Laurie Christensen, said the benzene vapours may be escaping from gaps in the foam that firefighters have been spraying to try to prevent flare-ups at the site.

The Texas National Guard's 6th civil support team was assisting at the scene Thursday. The team of about a dozen was helping to contain hazardous materials and provide other assistance to local emergency responders.

Environmental groups said residents who live near the facility have experienced various symptoms, including headaches, nausea and nose bleeds. According to the Centers for Disease Control and Prevention, long-term exposure to the highly flammable chemical causes harmful effects on the blood, including bone marrow.

The Environmental Protection Agency conducted air quality tests throughout the Houston area, both on the ground and from a small airplane, and "measured no levels of hazardous concentrations," EPA official Adam Adams said Wednesday.

Some residents who live near the facility, though, said they didn't have confidence in the air quality test results.

"Everything has been wrapped up in this nice perfect bow in saying that there were no problems. Every air quality was perfect. Every wind was perfect, blowing it away. And if everything was so perfect, why did it happen?" longtime Deer Park resident Terri Garcia said.