A fire burning at a petrochemical storage facility in suburban Houston could burn for two more days as firefighters take a defensive posture and let the blaze burn through fuel stored in tanks at the site, officials said Monday.

Ray Russell, spokesman for Channel Industries Mutual Aid, which is helping in the response, said firefighters have had “pretty good success controlling the fire” and stopping it from spreading to other tanks. The tanks that are burning contain gas, oil and chemicals, according to Intercontinental Terminals Company, which owns the facility.

In one tank, Russell said, crews are working to pump out a flammable liquid to deprive the fire of fuel. Even with that effort, the fire could burn until Wednesday, he said.

A column of black smoke rose from the plant, but the city of Deer Park and ITC said tests indicated the air was not dangerous as of late Monday morning. Schools in Deer Park and La Porte were shut down as testing continues.

The cause of the fire remains under investigation.

Asked whether the result of air-quality tests could be released to the media, ITC spokeswoman Alice Richardson said they had already been provided to city officials and she would check on whether she could share them with reporters.

A private air monitoring contractor declared the readings “favorable,” Deer Park’s Office of Emergency Management said just before noon (1 p.m. ET). The latest results indicate “no detections during the latest reporting period exceeded recommended action levels,” the office said.

Low levels of “particulate matter” were detected early Monday, the company said, and “a single, volatile organic compound detection has been found 6 miles southwest of the facility. These readings are currently well below hazardous levels.”

ITC reported the fire began in a single tank on Sunday afternoon and spread to a second tank. Richardson told reporters that firefighters were using foam in their efforts to douse the blaze and they were hoping that once the fire was contained, they could close the tank valves and the fire would put itself out.

By Monday morning, seven of the Deer Park facility’s 242 tanks were involved in the fire, and the blaze spread to an eighth tank before 5:30 a.m., the company said. Later, however, David Wascome, ITC’s vice president of terminal operations, said only seven tanks were affected and that one of the tanks originally cited was empty. The fire is confined to an area containing 15 tanks, he said.

“Although the risk of explosion is minimal, we continue to take precautions to further reduce this possibility,” the company said.

One tank stores naphtha, another contains xylene, the latest to catch fire contains toluene and the others hold “gas blend stocks used in the production of finished gasoline, and base oil commonly used as machine lubricants,” ITC said.

The tank containing the naphta, which is highly flammable, was the one being pumped, the company said.

Xylene is a solvent that occurs naturally in petroleum, and swallowing or breathing the substance can cause death, while nonlethal exposure can cause eye, nose, throat and skin irritation, among other maladies, according to the National Library of Medicine.

Naphtha is a petroleum product resulting from the distillation of natural gas or crude oil, the library says. It can be an eye and nasal irritant. Toluene occurs naturally in crude oil and is used as a gasoline additive, “and damage to the central nervous system is the main concern following exposure to toluene in the air,” the library says.

ITC describes naphtha and xylene as “components in gasoline.” Toluene, it said, is used in the production of nail polish remover, glue and paint thinner.

All ITC employees are accounted for, and no injuries have been reported, the company said. Authorities reopened Highway 225 early Monday and lifted an order that residents of Deer Park stay inside with with their windows closed and central cooling or heating units turned off, the city’s Office of Emergency Management said.

Gov. Greg Abbott said state officials are monitoring the situation, and his office had “ordered that all state resources be made available to local and industry officials and urge residents to continue heeding the warnings of local officials.”

According to ITC, the Deer Park terminal opened in 1972 and has capacity for 2.2 million cubic meters — more than a half billion gallons — of storage for “all kinds of petrochemical liquids and gases, as well as fuel oil, bunker oil and distillates.” The facility has ship and barge docks, rail and truck access and pipeline connections.