For five days in 1948, a Pennsylvania mill town couldn't see the end of its own street.

October 26, 1948.
40.1734° N, 79.8573° W

The Monongahela River makes a tight horseshoe bend south of Pittsburgh. On the inside of that bend, on a narrow shelf of flat land under steep hills, sits Donora. It was laid out at the start of the twentieth century as a company town. Its name was made from the names of two people tied to the Mellon fortune, William Donner and Nora Mellon.

Donora existed for the mills. The American Steel and Wire Company, part of U.S. Steel, ran a steel plant and wire works along the river. In 1915 it added a zinc works, which roasted and smelted zinc ore in long rows of furnaces. The works ran around the clock. Thousands of men from the town worked there. Many were immigrants or the sons of immigrants, from Slovakia, Poland, Italy, Hungary, Ireland and elsewhere. They lived in frame houses stacked up the hillsides above the plants.

By the 1940s, about fourteen thousand people lived in Donora.

The smoke was part of the place. On most days it rose out of the stacks and drifted down the valley. Laundry hung outside picked up grit. Cars needed their headlights at midday on bad days. On the hill above the zinc works, the vegetation had died off years earlier, and the slope was bare dirt and rock. Across the river in the village of Webster, farmers said their crops and orchards had been ruined by fumes from the zinc works. Some of them sued the company in the 1920s. The cases were settled.

People in Donora coughed. It was considered normal. The works meant steady pay, and the town was proud of what it made. Stan Musial, by then a star for the St. Louis Cardinals, had grown up there. So had Ken Griffey Sr.'s father, Buddy Griffey.

In late October 1948 the weather in the Monongahela Valley settled into a pattern that was well known to meteorologists and much less well known to the people living under it. A layer of warm air sat over the valley like a lid, and the cooler air beneath it stopped rising. Whatever went up the stacks of Donora stayed in the valley.

On Tuesday, October 26, a fog came in and did not lift. It was thick and yellowish gray, and it smelled of sulfur. The mills kept running. By Wednesday the fog was heavier. By Thursday people were having trouble seeing across the street. Drivers pulled over because they could not see the road. Some people walked with one hand on a fence or a wall to keep their bearings.

Nobody treated it as an emergency. Fogs had come and gone in the valley before.

The town kept to its schedule. On Friday, October 29, Donora held its Halloween parade. Families lined the streets to watch it pass in the haze.

That same day the sick calls started to pile up. Older people with asthma and heart trouble began struggling to breathe. Then younger, healthy people did too. Doctors' phones rang without stopping. The town's physicians went from house to house on foot, because driving had become impossible. Some were treating people through the night. The worst cases sat up in chairs, because lying flat made it harder to breathe.

The Donora Fire Department went door to door with oxygen tanks. The firemen had a limited supply. They gave each person a few breaths and moved on to the next house.

The first deaths came early Saturday morning, October 30. By the end of the day, people were dying faster than the town's undertakers could keep up. A temporary morgue was set up in the community center.

On Saturday afternoon the Donora High School football team played its scheduled game against Monongahela. The smog was so thick that people in the stands had trouble following the ball. Messages came over the public address system calling players and spectators home because a family member was ill.

The town's board of health was led by a local doctor, William Rongaus. As the deaths mounted, he began urging people with heart and lung trouble to leave Donora. Some residents tried to drive out and found the roads too dark with smog to navigate. Hotels and hospitals in nearby towns took in the sick. The Red Cross came in.

Stan Musial's father was one of the people sickened. The ballplayer drove home from St. Louis to be with him.

The zinc works were still running on Saturday. Pressure grew to shut them down. Early on Sunday morning, October 31, the plant began to bank its furnaces and suspend operations.

That afternoon it rained. The rain broke up the inversion and the smog washed out of the valley. People could see the hills again.

By then twenty people were dead. Health officials later estimated that about six thousand people, close to half the town, had been made sick. Many of those who survived never fully recovered their breathing.

The zinc works started up again soon after.

The story went national. A town of fourteen thousand had been poisoned by its own air, and newspapers and radio across the country covered it. The state of Pennsylvania opened an investigation. So did the United States Public Health Service, which sent a team to Donora to interview residents, examine the sick, measure the air and study the weather. It was the first investigation of its kind by the federal government. U.S. Steel conducted its own study.

The Public Health Service released its report in 1949. It described a combination of industrial pollutants trapped by the weather, and it found that people already suffering from heart and lung disease had been hit hardest. It did not name a single substance or a single plant as the cause. It recommended further study and better monitoring.

U.S. Steel's position was that the disaster had been caused by an unusual weather event. The company's own investigation pointed to a range of possible sources, not just the zinc works.

Residents and the families of the dead brought lawsuits against American Steel and Wire. More than a hundred suits were filed.

Every death in Donora that weekend had come from the air the town breathed every day, made heavier for five days. The only thing that had changed was the weather.

The lawsuits against American Steel and Wire were settled in the early 1950s for a small fraction of what had been sought. The company admitted no fault. No criminal charges were filed.

The disaster became a reference point in the debate over air pollution. In 1955 Congress passed the Air Pollution Control Act, the first federal law on the subject, which funded research. The Clean Air Act followed in 1963, and the much stronger Clean Air Act of 1970 put federal limits on what industry could put into the air. Donora was cited in the arguments for each of them.

The zinc works closed in 1957. The steel and wire works closed in the 1960s. The jobs went with them, and the town shrank. Much of the old mill ground along the river is empty now.

The hill above the old zinc works stayed bare for decades. Vegetation has slowly come back.

The Donora Smog Museum opened in town in 2008, in a storefront on McKean Avenue. It holds photographs, newspaper coverage and the stories of the families who lived through that week. The sign in its window reads: Clean Air Started Here.