Anexplosion at a U.S. Steel plantsouth of Pittsburgh killed two people and injured 10 others. The CEO has vowed to find out what caused Monday's blast while union officials say representatives will work to ensure a thorough investigation. The plant, Clairton Coke Works, is one of four major plants in Pennsylvania owned byU.S. Steel,an icon of the domestic steel industry and American industrialization for over a century. The plant is in the Mon Valley, a region of the state synonymous with steel. Here's what to know about the explosion: Blast felt nearby Allegheny County Emergency Services said a fire at the plant started around 10:50 a.m. The explosion, which was followed by several smaller blasts, sent black smoke spiraling into air. The blast could be felt in the nearby community and prompted county officials to warn residents to stay away from the scene so emergency workers could respond. One worker was rescued from the smoldering rubble hours later. Allegheny Health Network said it treated seven patients from the plant and discharged five within a few hours. The University of Pittsburgh Medical Center said it was treating three patients at UPMC Mercy, the region's only level one trauma and burn center. Past explosions The plant converts coal to coke, a key component in the steel-making process. It's the largest coking operation in North America. Coke is made by baking coal in special ovens for hours at high temperatures, which removes impurities that could otherwise weaken steel. The process creates what's known as coke gas — made up of a lethal mix of methane, carbon dioxide and carbon monoxide. Clairton Coke Works, which has 1,400 workers, has had explosions and fatal accidents before: Plant pollution concerns After Monday's blast, the county health department initially told residents within 1 mile (1.6 kilometers) of the plant to remain indoors and close all windows and doors. The department lifted the advisory after its monitors didn't detect levels of soot or sulfur dioxide above federal standards. In 2018, a Christmas Eve fire damaged pollution control equipment and led to repeated releases of sulfur dioxide, according to a subsequent lawsuit. The fire prompted Allegheny County to warn residents to limit outdoor activities. Residents said it was hard to breathe for weeks afterward and that the air felt acidic and smelled like rotten eggs. U.S. Steelsettled the lawsuitlast year with an agreement to spend $19.5 million in equipment upgrades and $5 million on local clean air efforts and programs. Dr. Deborah Gentile, the medical director of Community Partners in Asthma Care, studied asthma levels after the fire and found twice as many patients sought medical treatment. One of her colleagues found patients living near the plant had increased symptoms of asthma, including coughing, wheezing and shortness of breath. In 2019, U.S. Steel agreed tosettle a lawsuitregarding air pollution from Clairton for $8.5 million. A storied history and new ownership U.S. Steel has been a symbol of industrialization since it was founded in 1901 by J.P. Morgan, Andrew Carnegie and others. It's beenthe icon of the American steel industrythat once dominated the world market until Japan, then China, became preeminent steelmakers over the past 40 years. Its steel has helped build everything from the United Nations building in New York City to the New Orleans Superdome. Japan's largest steelmaker,Nippon Steel, in June finalizeda deal to acquire U.S. Steelfor nearly $15 billion. The agreement took more than a year to come together after U.S. Steel shareholders approved it as the parties surmounted obstacles created by national security concerns andpresidential politicsin a premier battleground state. The deal includes what is known as a"golden share" provisionthat gives the U.S. government the power to appoint a board member and a say in company decisions that affect domestic steel production and competition with overseas producers. The combined company is the world's fourth-largest steelmaker in an industry now dominated by China.