About how many Americans took part in the first Earth Day?

About 20 million Americans took part in the first Earth Day on April 22, 1970.

The nationwide event grew from Wisconsin Senator Gaylord Nelson’s proposal for a large environmental teach-in. Denis Hayes coordinated the effort, expanding it beyond college campuses into rallies, marches, lectures, and demonstrations across the United States. The gatherings protested pollution and environmental damage at a time when issues such as smog, oil spills, pesticide use, and contaminated waterways were gaining public attention.

The figure is an estimate, not a headcount of every individual. Organizers and historians use it to describe the combined participation at thousands of events. That scale made the first Earth Day one of the largest public demonstrations in American history and helped create political momentum for stronger environmental laws.

Earth Day began as a largely U.S.-focused event. It became international in 1990, when organizers coordinated activities in 141 countries. Today, April 22 is observed worldwide, but the 1970 estimate refers specifically to American participants.