The 1948 Donora smog was a historic air inversion resulting in a wall of smog that killed 20 people and sickened 7,000 more in Donora, Pennsylvania, a mill town on the Monongahela River, 24 miles southeast of Pittsburgh. The event is the basis of the Donora Smog Museum.The fog started building up in Donora on October 27, 1948. By the following day it was causing coughing and other signs of respiratory distress for many residents of the community in the Monongahela River valley. Many of the illnesses and deaths were initially attributed to asthma. The smog continued until it rained on October 31, by which time 20 residents of Donora had died and approximately a third to one half of the town's population of 14,000 residents had been sickened. 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 ten 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 the rain ended the weather pattern.