Municipalities began using the Ringelmann Chart to measure coal smoke in the early 1900s. The technique, developed by Maximillian Ringelmann of France in the late 1800s, measured the darkness of smoke with four different black grids on a white background, which was placed a distance away from smokestacks for a set time. Smoke control efforts in Chicago and other cities quickly adopted the technique, defining air pollution as visible smoke in need of control. Engineering developments joined with public pressure to keep smoke control a priority in local politics.
- Goldfield, David. Encyclopedia of American Urban History. SAGE Publications, 2007.