Pennsylvania Town Suffers Deadly Temperature Inversion

A temperature inversion over Donora, Pennsylvania, trapped atmospheric pollutants for six days, causing more than five thousand people to become ill and the deaths of about twenty residents. The incident led to passage of the 1955 Clean Air Act.

Summary of Event

During the last week of October, 1948, a temperature inversion associated with an anticyclone caused stagnant atmospheric conditions over western Pennsylvania, eastern Ohio, and parts of Maryland and Virginia. In Donora, Pennsylvania, this condition was accompanied by heavy fog, resulting in the formation of a dense smog (smoke and other air pollutants plus fog). On October 27, it was reported that “streamers of carbon appeared to hang motionless in the air and visibility was so poor that even natives of the area became lost.” The smog continued through the next day, with only minor concern voiced by the townspeople. [kw]Temperature Inversion, Pennsylvania Town Suffers Deadly (Oct. 26-31, 1948)
[kw]Deadly Temperature Inversion, Pennsylvania Town Suffers (Oct. 26-31, 1948)
Temperature inversions;Donora, Pennsylvania
Pollution;United States
Temperature inversions;Donora, Pennsylvania
Pollution;United States
[g]North America;Oct. 26-31, 1948: Pennsylvania Town Suffers Deadly Temperature Inversion[02650]
[g]United States;Oct. 26-31, 1948: Pennsylvania Town Suffers Deadly Temperature Inversion[02650]
[c]Disasters;Oct. 26-31, 1948: Pennsylvania Town Suffers Deadly Temperature Inversion[02650]
[c]Environmental issues;Oct. 26-31, 1948: Pennsylvania Town Suffers Deadly Temperature Inversion[02650]
[c]Health and medicine;Oct. 26-31, 1948: Pennsylvania Town Suffers Deadly Temperature Inversion[02650]

The impact on the community, however, was disastrous. On Friday, October 29, illnesses began to be reported. Physicians were flooded with calls for medical assistance, and many people were treated—at hospitals, by the Donora Fire Department, and by the local chapter of the Red Cross—by late that evening. Nearly five hundred people reported respiratory-related illnesses before conditions subsided. It was estimated that five thousand to six thousand additional people may have been affected but did not report their conditions to health officials. Many residents suffered from shortness of breath and coughing. Other symptoms included eye, nose, and throat irritation, headache, and vomiting. In the one-day span between Saturday, October 30, and Sunday, October 31, nineteen deaths were reported in the Donora area. Eighteen of these deaths were believed to have been directly related to the smog conditions; twenty deaths were eventually attributed to the episode.

Donora is a small industrial town located on the Monongahela River about 30 miles south of Pittsburgh. In 1948, the Donora-Webster population was about fourteen thousand, with about one thousand people living in Webster. Donora and Webster are situated on opposite sides of the river.

Much of the economic development in the area centered on the Monongahela River. The river traffic consisted of coal transports from the fields to the mills in Pittsburgh and its surrounding areas. Coal burning was practiced by industrial mills, trains, stores, and homes. The land use along the river bank was largely industrial. Industries in Donora included a steel and wire plant, a slag plant, a sulfuric acid plant, and a zinc-smelting plant. The total area of the plants covered nearly 3 miles along the riverbank, with the steel plant extending about 2 miles south of the Donora-Webster Bridge, and the zinc plant extending about 1 mile north of the bridge. Of the five thousand people employed in Donora, three thousand were employed by these two facilities.

Other industrial activities in the region included two steel plants and a by-product coking plant in nearby Monessen, south of Donora on the opposite side of the river. Another steel and by-product coke plant and a glass company were located in nearby Clairton to the north and Charleroi to the south. A power company and a railroad were situated in neighboring Elrama. All of these companies could have contributed to air pollution.

Industrial air pollution presents a problem when mixing of the pollutants with clean air is not sufficient to dilute the pollutant concentrations to acceptable levels. In order for air pollutants to be diluted, there must be vertical and horizontal mixing of air. Under normal conditions, temperatures decline with altitude, causing vertical mixing. The warmer air near the surface of the earth rises, mixes with the air above it, and is dispersed by winds aloft. This dilution process is important in reducing the concentration of pollutants near the surface. In the case of Donora, the vertical mixing of air was inhibited because the high-pressure system, or anticyclone, caused the temperature profile of the atmosphere to invert—that is, air near the surface was cooler than the air above it, a condition known as a temperature inversion.

Donora was subject to stagnant atmospheric conditions near the surface as well as above the surface. Temperature inversions develop when the earth readily radiates heat energy from its surface on clear nights or when air subsides and warms from compression. On clear nights, the surface readily radiates heat energy to space, causing the surface to cool. This produces a lower-level inversion near the surface. The less frequent but more persistent subsidence-type inversions occur when air subsides in high-pressure systems or in valleys, where air descends along adjacent mountain slopes, producing an upper-level inversion. This can last for days, allowing pollutants to concentrate at excessive levels.

The atmospheric conditions in Donora were such that when the usual morning fog and smoke did not disperse, both inversion conditions were in effect. Under normal conditions, the fog and smoke would break up with the diurnal heating of the valley walls and floor by the sun. On October 26, however, a storm on the East Coast receded, and a high-pressure center moved in from the West and positioned itself over western Pennsylvania. It maintained its position for nearly five days. This high-pressure system brought nearly cloudless skies and very weak low-level winds. The winds drifted from Pittsburgh south toward Donora; very little wind passed through the valley. Clear skies at night caused the surface to cool, producing fog in low-lying areas.

During the day, the fog reflected incoming sun rays, causing surface temperatures to be cooler than usual. These conditions prevented the air near the surface from mixing vertically. The pollutants stagnated over the area until a weak front passed over on October 31. The front changed the wind direction and brought rain and clouds that ended the stagnant pattern the region was experiencing.

The episode focused attention on industrial emissions and the geography and meteorological conditions of the region. A zinc-smelting plant received much of the attention. The plant was situated on forty acres of land adjacent to the river, in a valley surrounded by hills more than eleven hundred feet above sea level. The river valley is about 760 feet above sea level. The smelter’s stacks extended only about 150 feet up, well below the 350-foot hills. When the inversion set in, there was no place for the pollutants to go; they were being discharged below the inversion layer, and the valley location prevented the dispersion of the emissions into the air. The pollutants simply stagnated over the area. Since there was no evidence of any unusual substances in the atmosphere at the time, and there was no accidental occurrence that would have produced unusually high quantities of the usual substances, it was concluded that the meteorological conditions, in conjunction with geography and industrial pollution, were responsible for the episode.

There were twenty fatalities in the Donora area during and shortly after the smog period. Of these, twelve occurred in Donora within seven days of the smog episode. Research suggests that only two deaths would have been expected under normal conditions. The environmental conditions were particularly hazardous to older people. The ages of the people who died ranged from fifty-two to eighty-four years; the majority of them became sick on the second day of the episode.

Many of Donora’s residents believed that the incident could have been avoided. Plant workers accused plant operators of excessive use of reprocessed smoky materials in the outdated furnaces. Plant managers insisted that the plant had operated without incident since 1915. In their view, it was simply an act of God.


This smog episode catalyzed government-supported research and legislation pertaining to air pollution as it relates to public health at the national level. The Donora incident gave impetus to the 1955 Clean Air Act, which provided for research support, training, and technical assistance for air-pollution problems. Before this episode, smog was not widely considered to be an urgent issue in the United States. Also, the event provided a justification for federally sponsored research involving meteorology, atmospheric chemistry, and the physiology of health effects. [kw]Pennsylvania Town Suffers Deadly Temperature Inversion (Oct. 26-31, 1948)

Although it bore a striking resemblance to an incident that occurred in the Meuse Valley of Belgium, in December, 1930, the Donora smog phenomenon was the first such incident in the United States. Donora became a laboratory for studying air quality and public health. Studies were conducted by the Commonwealth of Pennsylvania, the Donora Borough Council, the United Steel Workers, and the U.S. Public Health Service Public Health Service, U.S. (USPHS). The USPHS Division of Industrial Hygiene used Donora and Webster for the first comprehensive survey of health effects of air pollution. This provided the USPHS an opportunity to develop a new program aimed at adopting methods of industrial hygiene to address the threat of air pollution that was to come with postwar industrialization and urban growth.

Many municipalities began to reconsider heavy industrial development. Some cities instituted smoke-control ordinances and initiated more research to understand the threat of air pollution to public health. Air-quality sampling and weather modeling became important components in the search for possible health effects of air pollution. Air-quality samples were used to compare pollutant levels with safety limits in workplaces. Prior to this, emphasis had been on visual observations of emissions and the efficiency of furnaces. Before the Donora smog episode, air pollution was not considered a serious health issue in the United States; in the incident’s wake, the dangers of air pollution were obvious. Temperature inversions;Donora, Pennsylvania
Pollution;United States

Further Reading

  • Bryson, Chris. “The Donora Fluoride Fog: A Secret History of America’s Worst Air Pollution Disaster.” Earth Island Journal 13, no. 4 (Fall, 1998). Available at Bryson outlines the Donora disaster and events leading up to it.
  • Davis, Devra. When Smoke Ran like Water: Tales of Environmental Deception and the Battle Against Pollution. New York: Basic Books, 2002. Davis, an epidemiologist who studies environmental illness, both tells a personal story and discusses air and other types of pollution. She introduces her book with a description of Donora, where several of her family members were affected by the 1948 disaster. A recommended work, written for general readers.
  • Linsky, Benjamin. Air Pollution in Donora: An Analysis of the Extreme Effects of Smog. Elmsford, N.Y.: Maxwell Reprint, 1970. Explains a comprehensive study of the Donora episode by the U.S. Public Health Service. Biological and atmospheric studies are discussed in an effort to understand the cause of the episode. Focuses on epidemiological studies.
  • Shilen, Joseph. The Donora Smog Disaster. Commonwealth of Pennsylvania: Bureau of Industrial Hygiene, 1948. A report on air quality immediately following the Donora episode. Looks at constituent concentrations beginning the morning of October 31, 1948, and atmospheric conditions that prevailed during the days of the episode.
  • Snyder, Lynne P. “The Death-Dealing Smog over Donora, Pennsylvania.” Environmental History Review 18 (Spring, 1994): 117-135. Recounts the activities that surrounded the Donora episode. Discusses the role of the local zinc works in contributing to the incident. Presents local media and personal accounts.
  • Stradling, David. Smokestacks and Progressives: Environmentalists, Engineers and Air Quality in America, 1881-1951. 1999. New ed. Baltimore: Johns Hopkins University Press, 2003. Considered a soon-to-be classic in environmental history, this book looks at the early years of one of the first environmental movements in the United States—the crusade against coal and other types of smoke that began around 1900.
  • Townsend, James G. “Investigation of the Smog Incident in Donora, Pa. and Vicinity.” American Journal of Public Health 40 (February, 1950): 183-187. A vivid description of conditions in the Donora community during the 1948 incident. Gives insight into the character of the town and its people.
  • Turk, Jonathan. Introduction to Environmental Studies. 3d ed. New York: Saunders College Publishing, 1989. A brief summary of the incident. Touches on the factors pertaining to the meteorological conditions and epidemiological findings.

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