The yellow fog arrived five days before Halloween in 1948, swaddling the Pennsylvania city of Donora and the nearby village of Webster in a nearly impenetrable haze. Citizens attending the Donora Halloween parade squinted into the streets at the ghostlike figures rendered nearly invisible by the smoke. The Donora Dragons played their habitual Friday night football game, but, their vision obscured by the fog, ran the ball rather than throwing it. And when terrified residents began calling doctors and hospitals to report difficulty breathing, Dr. William Rongaus carried a lantern and led the ambulance by foot through the unnavigable streets.

On Saturday October 30, around 2 a.m., the first death occurred. Within days, 19 more people from Donora and Webster were dead. The funeral homes ran out of caskets; florists ran out of flowers. Hundreds flooded the hospitals, gasping for air, while hundreds more with respiratory or cardiac conditions were advised to evacuate the city. It wasn’t until the rain arrived at midday on Sunday that the fog finally dissipated. If not for the fog lifting when it did, Rongaus believed, “The casualty list would have been 1,000 instead of 20.”

The 1948 Donora smog was the worst air pollution disaster in U.S. history. It jumpstarted the fields of environmental and public health, drew attention to the need for industrial regulation, and launched a national conversation about the effects of pollution. But in doing so, it pitted industry against the health of humans and their environment. That battle has continued throughout the 20th century and into the 21st, with short-term economic interests often trumping long-term consequences. Donora taught Americans a powerful lesson about the unpredictable price of industrial processes. The question now is whether the lesson stuck.