Maximilien Ringelmann Smoke Charts
Science History Institute

The complex instruments used by atmospheric scientists detect particulates in the air to the most minuscule amounts—even to parts per trillion. But our eyes have been an important tool in the detection of certain types of air pollution—and they remain important even with the advent of more complex instrumentation and monitoring.

In the late 19th century smoke plumes billowing from stacks were not only a symbol of industrial progress but a source of concern. As coal burning reached a high, so too did the smoke engulfing many U.S. cities. The smoke was soon tied to a variety of health problems, and city governments took action with smoke ordinances to curb pollution.

If the government was to enforce new regulation for the health of its citizens, new measurement techniques were necessary. In 1897 architectural engineer Maximilien Ringelmann developed smoke charts to allow observers to contextualize observable smoke into a scale of known gray. Lighter smoke indicated fewer particulates and more water, while the darkest of smoke was of grave concern. The charts were posted outside of factories, providing a very public method of environmental monitoring and awareness.

But our eyes are not foolproof. Some were critical of the method, which lacked precision and could be influenced by such factors as the position of natural light. Government inspectors’ use of the method allowed for human error—and even allowed for the possibility of being influenced by factory owners.

Before the 1970s, when a plethora of new instrumentation and methodology propelled the study of the environment to new levels of specificity, the Ringelmann chart was one of the primary methods used by government environmental and regulatory agencies to detect levels of industrial smoke. The Ringelmann scale’s visual method of identifying concentrations of smoke is still in use today, providing a baseline measurement for the visible threats to clean air. The environmental shades of gray are still perceivable without any extra instrument.