On Jan. 12, 1970, Seattle begans fluoridation of the water supply. The controversial action came more than a year after oters approved fluoridation in Nov. 1968, after proposals in 1952 and 1963 were defeated. Supporters argued that adding fluoride to water will harden teeth and reduce tooth decay in children.

From 1970 to 2011, the fluoride level was 1.0 parts per million. From 2011 to 2016, while the new federal recommendation was being reviewed and the state requirements were being updated, the fluoride level was 0.8 parts per million.