At the turn of the 20th century, Donora, PA was just a sleepy farm community located in a narrow valley near the Monongahela River, with cliff walls rising over 400 feet on either side, about 30 miles away from Pittsburgh, the booming metropolis. Just across the river lay the town of Webster, Donora’s “sister” city.

But, as the century progressed, change began to come to these two small communities. By 1902, Carnegie Steel had installed a facility in the immediate region, complete with more than a dozen furnaces. In a few short years, by 1908, Donora had the largest volume of railroad freight traffic in the region. And just a few years after that, by 1915, the U.S. Steel’s Donora Zinc Works and its American Steel & Wire plant started production as well.

This quiet little community in western Pennsylvania was soon a hive of activity, its economy thriving and population growing, up to about 14,000 by the mid-20th century.

But industry changed the valley in ways that weren’t so wonderful, too.

By 1918 the American Steel & Wire Company paid off its first fine for air pollution damage to health.

The steel mills and a zinc smelting plant released excessive amounts of sulphuric acid, carbon monoxide and other pollutants into the atmosphere constantly. It wasn’t long before residents landowners, tenants, and farmers began to complain–and eventually sue–for damages attributed to smelter effluent—the loss of crops, fruit orchards, livestock, and topsoil, the destruction of fences and houses, as well as adverse health side effects to those living near the plants. During the 1920s and 1930s, the owner of the plants paid off local residents several times for damages caused by the contamination. Still, there remained little to no regulation of the air pollution caused by the industries of Donora.

But, on October 27, 1948, something began to change.

The plants continued to pump out their emissions of hydrogen fluoride and sulfur dioxide, as usual. What wasn’t usual was the weather. The Monongahela Valley was trapped by a phenomenon known as a “temperature conversion,” which occurs when warmer air aloft confines pollution in a layer of colder air near the surface. This meant that the sulfuric acid, nitrogen dioxide, fluorine, and other poisonous gases that usually dispersed into the atmosphere were unable to escape the valley, and the trapped pollutants in the air mixed with fog to form a yellowish, acrid smog that began to thicken over Donora on Wednesday. By Thursday, it was causing coughing and other signs of respiratory distress for many residents of the community.

On Saturday, October 30, around 2 a.m., the first death occurred.

Terrified residents began calling doctors and hospitals to report difficulty breathing, sending physicians scrambling through the unnavigable streets. Hundreds flooded the hospitals, gasping for air, while hundreds more with respiratory or cardiac conditions were advised to evacuate the city, but the heavy smog and increased traffic made leaving difficult.

It wasn’t until 5 days later, on October 31, when rain arrived at midday, finally dissipating the fog finally.

Sadly, for many, it was already too late. Twenty people from Donora and Webster were dead. Thousands more were seriously ill. Faced with the carnage, Dr. William Rongaus, the head of the local Board of Health, clinging to optimism, said the rain was blessing. Without it, he believed, ““the casualty list would have been 1,000 instead of 20.”

The Donora smog disaster received national attention when it was reported by Walter Winchell on his radio show. The Federal Public Health Agency sent 25 investigators to Donora and Webster, where they took health surveys from residents, inspected crops and livestock, measured different sources of air pollution, and monitored wind speed and meteorological conditions. They found that the health effects went far beyond the 5-day smog event. More than 5,000 of the 14,000 locals had experienced symptoms ranging from moderate to severe, and that the American Steel & Wire Plant and the Donora Zinc Works emitted a combination of poisonous gases, heavy metals and fine particulate matter.

“If you looked at the X-rays of their lungs, they looked like the survivors of poison gas warfare,” said Devra Davis, the founder of Environmental Health Trust.

In the aftermath, air pollution finally became a matter of public concern, although the companies involved refused to take responsibility. When lawsuits were filed against the American Steel & Wire Company, it deflected, claiming the smog was “an act of God.”

In the end, American Steel & Wire settled without accepting blame for the incident. Although no further research was done into the incident in the years immediately after it, a 1961 study found the rate of death from cancer and cardiovascular disease in Donora from 1948 to 1957 was significantly elevated. Davis believes that, in the months and years after the incident, there were likely thousands more deaths than the ones officially attributed to the fog incident.

But there were some positives to come out of the whole thing. The incident led to the passage of 1955 Clean Air Act. The Donora Zinc Works shuttered operations in 1957. Most of the types of heavy visible pollutants responsible for the deaths in Donora have now been mostly outlawed and eliminated, although other types of pollutants still remain.