HARRISBURG, Pa. (AP) — U.S. Steel has agreed to settle a lawsuit that accused the Pittsburgh-based company of violating federal clean air laws by operating plants without its desulfurization controls for more than three months, emitting clouds of sulfurous gas into surrounding towns.

The settlement with environmental groups Clean Air Council and PennEnvironment and the Allegheny County Health Department was filed in federal court Monday for a judge to review, the groups said.

PennEnvironment and the other plaintiffs accused the steel producer of more than 1,200 violations of its air pollution permits.

They put the value of the settlement at $42 million, including $37 million worth of improvements to U.S. Steel’s pollution control and plant reliability systems at its Mon Valley Works plants.

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The rest is a $5 million penalty that U.S. Steel agreed to pay to fund clean air efforts. It is one of the largest-ever fines nationally in a citizen-enforced lawsuit under federal clean air laws, Clean Air Council and PennEnvironment said.

“This historic announcement should send a message to all illegal polluters who put the health and environment of Pittsburghers at risk,” David Masur, executive director of PennEnvironment, said at a news conference Monday. “We will not sit by while illegal air pollution rains down on nearby communities and the Pennsylvanians who live in them.”

U.S. Steel said it regretted the “accidental” emissions and that it strives to comply with environmental regulations.

“When we miss that mark, we will make changes so we can do better,” said Kurt Barshick, the company’s Mon Valley Works vice president, said in a statement.

The environmental groups sued in 2019, after a Christmas Eve fire at the Clairton coke works plant caused $40 million in damage.

The fire damaged pollution control equipment and led to repeated releases of sulfur dioxide, the lawsuit said. Sulfur dioxide is a colorless, pungent byproduct of fossil fuel combustion that can make it hard to breathe.

In the wake of the fire, Allegheny County warned residents to limit outdoor activities, with residents saying for weeks afterward that the air felt acidic, smelled like rotten eggs and was hard to breathe.

The fire knocked out pollution controls at its Mon Valley plants, but U.S. Steel continued to run them anyway, the groups said.

The lawsuit also cited repeated breakdowns at the Clairton plant, including one in 2019 in which the company reported a release of 525,000 pounds of coke oven gas from a pressure release valve. Allegheny County, which is home to Pittsburgh and the Mon Valley Works plants, said U.S. Steel has already spent about half of the $37 million on improvements.

U.S. Steel also must permanently close approximately 60 of the worst polluting coke ovens, the groups said. The ovens turn coal into coke, a raw ingredient in the steelmaking process.