On this day in 1945, Grand Rapids became the first city in the world to add fluoride to its public water supply. The city, along with the U.S. Public Health Service, the Michigan Department of Health, and the University of Michigan School of Dentistry, began a ten-year study to determine the effectiveness of fluoride in the prevention of tooth decay. By 1955 the study had shown a sixty-five percent reduction in tooth decay and led to the adoption of fluoridation as a accepted public health measure.