U.S. Rep. Betty McCollum is asking Marathon Petroleum to stop using hydrogen fluoride at its St. Paul Park refinery, citing EPA records that say an accidental leak of the highly toxic chemical could, in the worst-case scenario, put 1.7 million people in the Twin Cities at risk.

The request, made in a letter sent Wednesday to Marathon Petroleum’s CEO Gary Heminger, notes that Marathon is in the midst of purchasing the refinery from San Antonio-based Andeavor.

“The presence and use of this dangerous chemical within a major population center is simply unacceptable,” wrote McCollum, the ranking Democrat on the Interior-Environment Appropriations Committee, which is the committee that funds the EPA.

The refinery’s worst-case scenario is spelled out in documents on file with the EPA’s regional office in Chicago, the Star Tribune has reported.

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, which is used in the production of gasoline, 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.

An explosion and fire April 26 at the Husky Energy Inc. refinery in Superior, Wis., took place within 200 feet of a hydrogen fluoride system there. According to worst-case scenarios spelled out in EPA documents, that could have put 180,000 people in the Twin Ports area at risk if the chemical had leaked.

The far larger St. Paul Park refinery has the largest amount of hydrogen fluoride of any facility in Minnesota.