“We want people to realize Donora was a big part of the environmental movement,” said Don Pavelko, a Donora councilman who came up with idea for the museum. “The smog in Donora over the years had been looked upon as a black eye. The older folks just didn’t want to talk about it because they thought it was bad publicity.”

The museum, in a former Chinese restaurant, brings together photographs, old newspapers, maps and copies of studies of the smog. Brian Charlton, a high school history teacher, has been appointed archivist and is pursuing oral histories of that week.

Paul C. Brown, 81, who worked in the steel mills then, remembered going to work at the mill that Saturday.

“We all thought it would lift eventually, because we were used to the fog in the valley,” he said. “Then I started to hear about people getting sick.”

Smog was not unusual in Donora, a town of 14,000 then that was home to the American Steel & Wire Company and the Donora Zinc Works plants  both run by the United States Steel Company  that sat along the river and employed 5,000 people.

But this was different. The thick, yellowish, acrid smog was the result of an unusual weather inversion  a pocket of warm, stagnant air  that sat over the valley for five days.

Underneath what was essentially a lid on the valley were sulfuric acid, nitrogen dioxide and other poisonous gases, including fluorine, that would normally rise into the atmosphere. Most researchers blame the zinc plant, which had long been a source of complaints and was responsible for the denuding of almost all vegetation within a half mile of the plant.