1948: An inversion layer settles over the rust belt town of Donora, Pennsylvania, trapping industrial pollution in the atmosphere. When it clears six days later, 20 people are dead, another 50 are dying and hundreds will live out their days with permanently damaged lungs.

Inversion occurs when the air near the ground is cooler than the air above it, a reversal of normal atmospheric conditions. When that happens, manmade pollutants are trapped, resulting in smog. The physical conditions around Los Angeles, for example, lead to frequent inversion layers over the basin. That, combined with heavy automobile pollution, consistently gives L.A. the worst air quality in the country.

But L.A. has never seen anything quite like that one week in Pennsylvania, in what became known as the Donora air inversion or, more dramatically, the "Donora Death Fog."

In Donora, an industrial town about 20 miles south of Pittsburgh, pollution from the nearby U.S. Steel smelting plants and Donora Zinc Works was the main culprit. Trapped in a temperature inversion, the pollutants blanketed the town during the night of Oct. 26.

The companies connived with the U.S. Public Health Service to cover up the facts of the incident and succeeded in doing so for half a century. Whistle-blowers were silenced; records disappeared. It wasn't until 1994 that a full accounting of what happened in Donora was finally published.

To Philip Sadtler, an industry consultant sent to evaluate the disaster and who tried without success to expose the corporate cover-up, U.S. Steel was guilty of murder:

"The directors of U.S. Steel should have gone to jail for killing people," Sadtler said shortly before his death in 1996.

In the end, 40 percent of Donora's population of 14,000 became ill as a result of the "death fog," and the town joined a growing list of other places hit hard – and harder – by industrial pollution.

Source: Various

Photo: Donora Zinc Works of the U.S. Steel Corporation is dimly seen through fume-laden smoke and fog.

Bettmann/Corbis

This article first appeared on Wired.com Oct. 26, 2007.