In 1941, influenced by a similar policy introduced in St. Louis four years earlier, the city of Pittsburgh passed a law designed to reduce coal production in pursuit of cleaner air. Not willing to cripple such an important part of the local economy, it promised to clean the air by using treated local coal. The new policy ended up not being fully enacted until after World War II.
While the idea was a small step in the right direction, other factors ultimately helped improve Pittsburgh's notorious air quality. Natural gas was piped into the city. Regional railroad companies switched from coal to diesel locomotives. And, ultimately, the collapse of the iron and steel production industries in the 1980s led to rapidly improved air quality leading into the 21st century.
Control of coal smoke made it possible to clean soot-covered buildings and to re-plant hillsides, helping provide the city a look it could hardly envision in the depths of its industrial heyday.
Below, a look at downtown Pittsburgh between 1940 and 1945, courtesy of the University of Pittsburgh's Smoke Control Lantern Slide Collection:
"Corner of Liberty and Fifth Avenues 10:55 AM" 1940