Here’s what you’ll learn when you read this story:

• At the tail-end of October 1948, the Pennsylvania town of Donora experienced a deadly smog event that eventually led to the deaths of 20 people and the sickening of thousands more.
• The smog—generated by the smokestacks at the American Steel and Wire plant and the Donora Zinc Works, and exacerbated by local weather and geography—contained traces of hydrogen fluoride, carbon monoxide, nitrogen dioxide, and heavy metals.
• The tragedy was one of the instigating events that led to the eventual passage of the Clean Air Act in 1970.

Before the arrival of the American Steel and Wire plant and the Donora Zinc Works, the town of Donora, Pennsylvania—located roughly 25 miles southwest of Pittsburgh—was a quaint farm town nestled between two 400-foot hills. Lying along a bend in the Monongahela River known as “horseshoe bottom,” the small village was known as West Columbia before officially incorporating in 1901 as Donora (an amalgamation of the names of the two owners of a steel company that purchased land along the river). Today, that name is now best remembered for a deadly smog that killed nearly two dozen people, sickened thousands, and kickstarted an environmental revolution.

During the dying days of October 1948, an eerie fog settled on Donora as families walked in Halloween parades and watched the local high school football team (the Dragons) square off against a rival. Donora and its approximately 14,000 or so residents were used to the poor visibility. But as this choking pollution continued unabated, what was once seen as a fixture of factory life soon turned into a living nightmare.

Within a few days, residents of Donora—along with the nearby town of Webster—began making frantic calls to the town’s eight resident doctors complaining of respiratory distress. Due to the poor atmospheric conditions, emergency vehicles needed to be escorted with flashlights, and emergency responders eventually resorted to providing oxygen door to door. While doctors urged those with chronic health conditions to evacuate, poor visibility made travel impossible. By two a.m, on Saturday, October 30, the first death from smog was reported in Donora, and within 12 hours, there’d be 16 more. When the fog eventually lifted on Halloween night, a total of 20 people had died, an estimated 1,440 contracted a serious illness, and nearly 5,000 reported mild or moderate symptoms, according to an article in the American Journal of Public Health.

Three major factors contributed to this sadly avoidable disaster. The first—and biggest—contributor was the collection of toxic emissions that streamed from the American Steel and Wire plant and the Donora Zinc Works. A subsequent analysis of the fog found that these emissions included hydrogen fluoride, carbon monoxide, nitrogen dioxide, heavy metals, and multiple sulfur compounds. The second was pure bad luck: a weather event known as a “temperature inversion” effectively trapped cold air underneath warm air, allowing the pollution to stagnate. The third was the town’s location between two hills, which further concentrated the pollution.

Sadly, an eerily similar tragedy had occurred 18 years earlier in the Meuse River Valley in Belgium—one that also included industrial pollution, temperature inversion, and unfortunate geography—that killed at least 60 individuals in just 24 hours. While experts at the time warned that this series of events, though unlikely, could happen again, the prediction was mostly ignored, much to the detriment of Donora and its residents.

While the 1948 Donora smog would be far from the last smog event of the century—including major NYC smogs of the 50s and 60s, and the infamous Great Smog of 1952 in London—it’s remembered today as one of the original catalysts behind environmental revolution that led to the passage of the Clean Air Act in 1970. But though strides have been made since 1948, pollution continues to wreak havoc on the environment, and subsequent protections intended to mitigate the negative health effects of harmful emissions are actively under threat.

Humanity has paid a deadly cost to learn a valuable lesson. It remains to be seen if that lesson will truly stick.