On Aug. 11, 2025, an explosion at U.S. Steel’s Clairton Coke Works outside Pittsburgh — a 392-acre riverside industrial facility and the largest of its kind in the Western Hemisphere — killed two U.S. Steel workers and injured 11 others, according to the Chemical Safety Board, a federal agency investigating the incident.
Six months later, workers remain rattled and community concerns about air pollution from the plant are heightened. The blast comes on top of a string of other accidents at the Clairton plant and a history of legal battles between U.S. Steel and Allegheny County regulators. Some current and former workers at Clairton Coke Works say poor management and underinvestment have exacerbated air pollution and undermined workplace safety at the plant. And the August blast came after Nippon Steel's $15 billion acquisition of U.S. Steel in June 2025. Nippon Steel did not provide a response to written questions.
Pittsburgh's Public Source and The Associated Press interviewed current and former workers at Clairton Coke Works, experts, former and current government officials, reviewed court documents and analyzed federal data to produce this report.
Here are key takeaways from the report:
What we know about the explosion
Industrial ovens at Clairton Coke Works, which opened more than a century ago, heat coal at high temperatures for hours to make coke, a key component in steelmaking. The heat removes impurities, producing a flammable byproduct called coke oven gas.
According to the Chemical Safety Board, the August explosion happened while workers were closing and opening a gas isolation valve in a basement after pumping water into the valve. U.S. Steel’s written procedure did not mention the use of water and a U.S. Steel supervisor directed workers to pump the water, the agency said. Kurt Barshick, U.S. Steel’s vice president of the Mon Valley Works, said during an October presentation to residents that workers trapped “3,000 PSI water inside of a valve that’s rated for 50 PSI.” The valve cracked and gas filled the area, Barshick added.
Drew Sahli, the Chemical Safety Board’s investigator in charge, said there was a “release of coke oven gas” and that the gas “contacted an ignition source” and exploded. The agency is still investigating how the gas was released, Sahli said.
In a written statement, U.S. Steel stressed its commitment to safety and said that it has “strengthened several safety protocols” based on its own ongoing investigation, including prohibiting the use of high-pressure water for valve cleaning.
Before the August blast, the plant already had a history of accidents and explosions, including a 2009 blast that killed a maintenance worker and another explosion in 2010 that injured 14 employees and six contractors.
Workers question management
Some current and former workers at the Clairton plant fault U.S. Steel’s management of the aging Clairton Coke Works, saying that it has caused a range of operational problems. For instance, they describe difficulties getting coke oven doors, which can leak emissions, replaced.
Current and former workers also point to a fire at the Clairton plant that occurred on Christmas Eve in 2018. It shut down pollution control equipment and led to repeated releases of air pollution, according to a lawsuit filed by environmental groups after the incident. An engineer hired by the plaintiffs wrote in a report filed in the case that he found "no indication" that U.S. Steel "has an effective, comprehensive maintenance program for the Clairton plant." Sahu also wrote that the 2018 accident "preventable by a robust inspection and preventive maintenance program and by better plant design."
In a 2024 consent decree settling the lawsuit, U.S. Steel agreed to measures including investing close to $20 million in facility upgrades. As part of the consent decree, U.S. Steel admitted no liability.
In its statement, U.S. Steel said that the company's "overall transformation efforts have improved our company’s performance, created a robust maintenance program, and improved employee safety over time.”
Plant regularly under fire for pollution
The Clairton plant has been the largest local source of air pollution in recent years, according to the Allegheny County Health Department. County Health Department regulators routinely clash with U.S. Steel over alleged violations of the plant’s operating permit, such as excessive emissions or failing to use pollution control equipment. In 2023, the Allegheny County Health Department fined U.S. Steel more than $2 million for violations at Clairton Coke Works.
In response to questions from Public Source and AP, the Allegheny County Health Department that air monitoring stations near the Clairton plant “have measured a 15-25% reduction in annual average particle pollution concentrations compared to ten years ago.”
Nationally, Clairton Coke Works’ environmental compliance track record is an outlier, according to a Public Source and AP analysis of federal Clean Air Act data from about 14,000 facilities. The analysis found that Clairton Coke Works is classified by the EPA as a “high-priority violator” – only about 11% of major emitters fall into that category. It’s even rarer for facilities to garner financial penalties on the magnitude that Clairton Coke Works has faced in the last five years, the analysis shows. Just 11 facilities, including Clairton Coke Works, have faced $10 million in penalties or more in the last five years.
U.S. Steel said in its statement that the company spends “$100 million annually on environmental compliance at Clairton alone and has consistently achieved an environmental compliance rate exceeding 99% for regulated activities per year at our Clairton Plant.”
New ownership arrives
In the nearly $15 billion deal to buy U.S. Steel, Nippon Steel pledged to invest $14 billion in domestic steelmaking operations. Much of that money remains publicly uncommitted, and U.S. Steel has been firm that it wants to keep the Clairton plant operating.
“The Clairton Coke Plant is an important part of our North American Flat-Rolled integrated operations,” the company said in November. The company added that a “steady coke supply remains critical” and that the “Clairton Coke Plant will be maintained for the next generation of steelmaking.”
However, U.S. Steel has not publicly committed to spending money at the Clairton plant to expand production, extend its life, improve efficiency, upgrade safety or reduce its polluting air emissions. Of the $14 billion, U.S. Steel has said $2.4 billion will go toward its Pittsburgh-area plants. A portion of that money will be spent on building a new hot strip mill to replace the one at its Irvin plant, just down the Monongahela River from Clairton, that processes steel into massive sheet rolls.
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