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Heavy haze surrounding the Donora zinc works of the American Steel and Wire Company, Donora, Pennsylvania.
The 1948 Donora smog was a historic air inversion that resulted in a wall of smog that killed 20 people and caused respiratory problems for 6,000 people of the 14,000 population of Donora, Pennsylvania, a mill town on the Monongahela River 24 miles (39 km) southeast of Pittsburgh. The event is commemorated by the Donora Smog Museum. Sixty years later, the incident was described by The New York Times as "one of the worst air pollution disasters in the nation's history". Even 10 years after the incident, mortality rates in Donora were significantly higher than those in other communities nearby.
Hydrogen fluoride and sulfur dioxide emissions from U.S. Steel's Donora Zinc Works and its American Steel & Wire plant were frequent occurrences in Donora. What made the 1948 event more severe was a temperature inversion, a situation in which warmer air aloft traps pollution in a layer of colder air near the surface. The pollutants in the air mixed with fog to form a thick, yellowish, acrid smog that hung over Donora for five days. The sulfuric acid, nitrogen dioxide, fluorine, and other poisonous gases that usually dispersed into the atmosphere were caught in the inversion and accumulated until rain ended the weather pattern.