In the fall of 1948, the small industrial town of Donora, Pennsylvania, faced a serious environmental crisis resulting from a severe temperature inversion that trapped air pollutants close to the ground. The town's economy relied heavily on two major zinc smelting plants, which emitted large quantities of sulfur dioxide, lead, and other hazardous substances. On October 27, a combination of weather conditions, including low temperatures and high humidity, created a stagnant atmosphere that allowed pollutants to build up over the urban area.

By the morning of October 28, the air quality had deteriorated significantly, leading to a dense fog that enveloped the town. Residents reported difficulty breathing, coughing, and other respiratory ailments. Public health officials rushed to address the situation as hospitals filled with patients suffering from symptoms related to the toxic air these emissions created. This health crisis lasted for several days, ultimately leading to the death of at least 20 individuals and causing thousands to experience severe illness.

The Donora incident became a pivotal moment in the history of environmental health and regulation in the United States. It highlighted the dangers of unchecked industrial emissions and prompted local and federal officials to take action to improve air quality standards. The event contributed to the establishment of more rigorous regulations regarding air pollution that would eventually culminate in the creation of the Clean Air Act of 1970, a significant piece of legislation aimed at protecting air quality in the U.S.