Fluoride is naturally present in all water. Community water fluoridation is the precise adjustment of fluoride that occurs naturally in a community's water supply to the level recommended for optimal dental health. The U.S. Public Health Service, a division of the U.S. Department of Health and Human Services, has established the optimal concentration for fluoride in water supplies to be 0.7 parts per million (ppm). At 1 ppm, one part of fluoride is diluted in a million parts of water.