Fluoride in drinking water at low concentrations can prevent tooth decay, but at higher concentrations can be harmful to human health. Fluoride concentrations above the Maximum Contaminant Level (MCL) drinking-water standard of 4 mg/L in untreated groundwater from wells that supply drinking water in the U.S. were infrequent: 1.6% of 16,441 drinking-water wells studied (public-supply and domestic wells combined) and 0.6% of the 11,032 of the drinking-water wells that are domestic wells. On the other hand, more than 85% of drinking-water wells had fluoride concentrations below the optimal concentration of 0.7 mg/L for preventing tooth decay recommended by the U.S. Public Health Service.

Scientists took a closer look at groundwater from domestic wells because such groundwater typically is not treated to lower elevated fluoride concentrations nor is fluoride added if concentrations are low. An estimated 172,000 people in the conterminous 48 United States are served by domestic wells with fluoride concentrations that exceed the MCL, and another estimated 522,000 are served by domestic wells with concentrations that exceed the secondary MCL of 2.0 mg/L for cosmetic effects such as tooth discoloration. On the other hand, more than 28 million people are estimated to get their drinking water from domestic wells with fluoride concentrations less than the optimal fluoride concentration of 0.7 mg/L recommended to prevent tooth decay.

Fluoride concentrations in untreated groundwater in the western U.S. were generally higher than in the east. The study identified eight factors that affect fluoride concentrations, including mean annual precipitation, pH, and well depth. Knowledge of these factors can be used in models to estimate fluoride concentrations in unmonitored areas, which could benefit domestic-well owners and others concerned with human consumption of well water with elevated fluoride concentrations.