The Donora Death Fog...

https://www.aiche.org/chenected/2011/10/donora-death-fog-crisis-led-modern-air-pollution-laws

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You may not have heard of the Donora Death Fog, but if you are interested in climate change, you should have. This notorious weather event is arguably one of the most pivotal moments responsible for the adoption of air quality regulations in the United States. Though more of a "Smog" than a "Fog," this aptly named phenomenon left 20 dead and half a town hospitalized in its wake during the fall of 1948. It showed all of the characteristics of an atmospheric inversion, an event in which air stops circulating and is trapped close to the ground. The combination of trapped toxic gasses and early morning mists yielded disastrous effects.

In the early morning hours of October 26, 1948, a fog settled over the town of Donora, Pennsylvania, the home of U.S. Steel Corporation's Donora Zinc Works and American Steel and Wire. As the day wore on, the fog became progressively thicker and witnesses even claimed that it was so thick and potent they could taste it. By October 29, the inversion had trapped so much pollution and fog that attendees of a local high school football game noted that they couldn't even see the players on the field. Doctors ordered the elderly and those having trouble breathing to leave town, which became impossible as visibility was reduced to nothing - firefighters had to abandon attempts to deliver oxygen to suffering citizens as they were unable to navigate the town--in the middle of the day. On the morning of October 30, the two U.S. Steel plants ceased operation. The following morning the fog had begun to dissipate leaving many surviving residents with permanent respiratory damage.

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Located 37 miles south of Pittsburgh, Donora is nestled in the Monongahela River Valley. In the valley, the cool ground contributed to a strong temperature inversion. Under normal conditions, the air close to the planet's surface is warmest and decreases in temperature with increasing altitude. The warm air at the surface begins to rise replaced by cooler air that is then warmed by the sun hitting the Earth's surface. That warmed air rises and causes constant air flows. During a temperature inversion, however, the cool air gets stuck at the surface. The early morning fog in the valley was created when the cool front and high pressure system interacted with moisture in the air, which reflected and blocked the sun's ability to warm the cool air at the surface of the Earth. With little air movement, the pollution emitted by the steel-belt town had nowhere to go but into the valley, increasing in concentration. Particulates like zinc, cadmium and lead emitted by the plants contributed to the sun-blocking, compounding the problem and ensuring a stagnant layer of air.

The now poisonous air contained more than just particulates--it also contained hydrofluoric acid, sulfur dioxide, and carbon monoxide. In a news clip published in Chemical and Engineering News on December 13, 1948, it was revealed that the victims of the smog experienced acute fluorine poisoning determined by the extreme fluorine levels in the blood of the deceased (12 to 25 times the normal levels) and the near-asthmatic breathing of the survivors. Humans were not the only victims--all of the crops in the area perished. Corn, which is highly sensitive to fluorine exposure, was especially devastated.

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