A Thermal Inversion Example in Donora | AP Environmental Science | Khan Academy

• [Instructor] Hey there, friends. Today we're gonna learn about air pollution, and to start off, we're going back in time to the small town of Donora, Pennsylvania, in October of 1948. (Light music) Walking into this small industrial town, you can immediately sense that something is wrong.

It's the middle of the day, but there's a thick yellowish smog everywhere, enveloping everything and even blocking out the sun. It's so dark that streetlights are on during the daytime. It stings your eyes, and it's hard, even painful, to breathe. What we're experiencing is the Donora death fog, one of the worst air pollution disasters in the United States.

Donora was an industrial town full of steel plants and mills, which released toxic emissions, such as hydrogen fluoride and sulfur dioxide, when processing steel and other metals. Normally, these poisonous gases would disperse into the atmosphere, but this time, there was a temperature inversion, which caused a blanket of warm air to cover a layer of colder air near the surface and right over Donora.

Consequently, the toxic emissions were essentially trapped under the warm air. Over the course of several days from October 26th to October 31st, these toxic emissions had accumulated so much that half of the 14,000 people living in Donora suffered from respiratory problems, and 20 people died.

Relief only came when the steel mills were shut down and a rainstorm alleviated the smog. But, following the deadly Donora smog, the public began to realize just how dangerous and life-threatening air pollution could be, and citizens demanded change. Donora became a turning point in US history and was the start of the Clean Air Movement.