However, communities sometimes exceed the recommended levels because fluoride occurs naturally at higher levels in certain water sources. In 2011, officials reported that 2 in 5 U.S. adolescents had at least mild tooth streaking or spottiness because of too much fluoride.
Since 2015, federal health officials have recommended a fluoridation level of 0.7 milligrams per liter of water. For five decades before that, the recommended upper range was 1.2 milligrams per liter. The World Health Organization has set a safe limit for fluoride in drinking water of 1.5 milligrams per liter.
The addition of low levels of fluoride to drinking water has long been considered one of the greatest public health achievements of the past century — one of the most cost-effective ways to prevent tooth decay on a large scale.