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.