Fluoride 

Fluoride can occur naturally in well water as a result of the geological composition of soils and bedrock and is often added to public water supplies to deter dental cavities. However, levels exceeding 2.0 mg/L may cause tooth discoloration and persistent exposure to levels above 4.0 mg/L may cause pits in the tooth enamel of children and increased the likelihood of bone fractures in adults. Point of use reverse osmosis is an effective method for removing excess fluoride from drinking water.