The mayors of Superior and Duluth are calling on Husky Energy Inc., to stop using hydrogen fluoride at its oil refinery, citing reports that the highly toxic chemical could injure or kill up to 180,000 people in the Twin Ports and surrounding area if accidentally released.

Their call comes after a panicked evacuation last Thursday of huge swaths of Superior due to an explosion that rocked the Husky refinery. A raging asphalt fire that broke out two hours later was less than 200 feet from the refinery’s hydrogen fluoride tanks, according to firefighters who responded to the blaze.

Superior Mayor Jim Paine said Wednesday that he made the request to Husky CEO Rob Peabody in person on Tuesday. Duluth Mayor Emily Larson issued a news release saying she stands with Paine.

“It is horrifying to consider what could have happened,” Larson wrote.

The morning after the blast and fire, Paine acknowledged at a news conference that the scenario had the potential to be “absolutely catastrophic,” but he said at the time that the evacuation zone was large enough to protect people if the chemical had been released, and that the city had adequate medical transport and supplies to treat exposures.

A few dozen refineries around the nation use hydrogen fluoride, also known as hydrofluoric acid, to refine petroleum, though safer alternatives exist.

Hydrogen fluoride is a fast-acting acid that can cause deep, severe burns or, with sufficient exposure, kill. Exposure can occur through inhalation and skin contact.

The chemical can permanently damage the eyes, skin, nose, throat, respiratory system and bones, according to a 2013 report issued by the United Steelworkers union, which represents many refinery workers. The Center for Public Integrity in a 2011 report also sounded alarms about the use of hydrogen fluoride.

Paine said he has asked Peabody to detail the cost and infrastructure challenges that might prevent Husky from dropping its use of hydrogen fluoride. He also asked the company to disclose all safety measures it has in place to prevent the chemical from harming the public.

A spokeswoman for Husky said Wednesday the hydrogen fluoride unit and its safety systems will be part of the company’s investigation of the explosion and fire. Whether the refinery uses the chemical when it resumes operations hasn’t been decided, she said.

“We will be looking at a number of options for the refinery configuration going forward,” said spokeswoman Kim Guttormson.

The cause of the explosion has yet to be determined.

Kollin Schade, a Husky Energy spokesman, said last week that the blast occurred in the fluidized catalytic cracking area of the refinery. That’s a part of the refining process when crude oil is subjected to heat and pressure to extract gasoline and other light petroleum products.

Even with the support of local residents, Paine and Larson will struggle to persuade Husky to switch to a safer alternative, predicted Steve Goldsmith, a member of a citizens group in Torrance, Calif., that has fought refinery use of hydrogen fluoride for three years.

The group formed after a 2015 explosion at the ExxonMobil refinery in Torrance that, as in Superior, fanned widespread public concern about hydrogen fluoride and the potential risks to densely populated areas nearby.

“You’ve just started on a road that’s going to take awhile,” Goldsmith said of the requests made by the Superior and Duluth mayors.

Thirteen people were injured in the Superior refinery explosion, six seriously enough that they required hospitalization. The evacuation order, which included all neighborhoods within a 3-mile radius of the refinery and 10 miles to the south of it, was lifted Friday morning.

Peabody pledged to rebuild and reopen the refinery at his meeting with Paine, the Superior mayor said.

Husky has also begun taking claims from local residents for losses incurred as a result of the explosion and fire. Some 170 claims have been filed so far, according to Paine.