BUTTE, Mont. (AP) — U.S. Minerals, which operated a plant to turn mining waste into roofing materials in Anaconda, has reached a plea agreement in a federal case charging the company with exposing its employees to unsafe levels of arsenic, which can cause cancer.

The company, based in Tinley Park, Illinois, reached an agreement on Aug. 2 to plead guilty to negligent endangerment, a misdemeanor violation of the federal Clean Air Act, The Montana Standard reported Wednesday. The hearing is set for Aug. 23.

The guilty plea acknowledges the company “negligently placed another person in the imminent danger of death or serious bodily injury.”

The agreement calls for a $392,000 fine and for U.S. Minerals' other locations to be under increased oversight by the Environmental Protection Agency and the Occupational Safety and Health Administration during a five-year probationary period. The company would also have to monitor the health of former employees of the Anaconda plant during that time.

Employees who take advantage of the medical monitoring program would not give up the right to pursue civil litigation against U.S. Minerals, under the agreement.

From 2013 to when it closed in June 2021, because of what the company called “significant logistical challenges,” the Anaconda plant converted black slag produced from a century of copper smelting into roofing materials called Black Diamond Abrasive Products.

According to a 2016 report by the National Institute for Occupational Safety and Health, five of six employees tested at the Anaconda plant in July 2015 had elevated levels of arsenic. At the time, respiratory protection was provided but not required, and there was no running water or handwashing stations at the plant

Montana's health department ordered the plant to cease operations in February 2019 after at least two workers had elevated arsenic levels in their urine in 2018. In order to reopen, the plant had to provide employees with showering and handwashing facilities, provide laundering of dirty work clothes, establish a medical surveillance program for arsenic and lead exposure and require respirators in some parts of the plant. The closure order was lifted later that spring.

Long-term exposure to inorganic arsenic can lead to skin cancer and cancer in the bladder and lungs, according to the World Health Organization.

The company was earlier fined nearly $107,000 by OSHA for violations at the Anaconda plant in 2016.

U.S. Minerals Attorney Peter Lacny said he expects the case will be closed by the end of the year. He said he was not aware of any civil lawsuits pending against the company related to the Anaconda plant.