Water fluoridation
Health and dental organizations worldwide have endorsed its safety and effectiveness. Its use began in 1945, following studies of children in a region where higher levels of fluoride occur naturally in the water. Further research showed that moderate fluoridation prevents tooth decay.
Portland, Oregon
As of 2012, Portland was the largest city in the United States that did not add fluoride to its public water supply, and fluoridation has historically been a subject of controversy in the city. Portland voters have four times voted against fluoridation, in 1956, 1962, 1980 (repealing a 1978 vote in favor), and 2013. In 2012 the city council, responding to advocacy from public health organizations and others, voted unanimously to begin fluoridation by 2014. Fluoridation opponents forced a public vote on the issue, and on May 21, 2013, city voters again rejected fluoridation.