Which unit expresses the recommended fluoride level in drinking water?

• A. percent
• B. grams per liter
• C. moles per liter
• D. parts per million (ppm)

Answer: (D)

Concentration units for drinking water contaminants are chosen to reflect the tiny amounts present in a liter of water. The standard way fluoride levels are reported is in parts per million (ppm), which directly corresponds to milligrams per liter (mg/L) in water. Since 1 mg/L equals 1 ppm in aqueous solutions, this unit is both intuitive and practical for regulatory guidelines—example: the typical recommended level is about 0.7 mg/L, or 0.7 ppm. Using percent would imply a much larger fraction of the solution, grams per liter would suggest far higher quantities than are present in drinking water, and moles per liter, while a valid chemistry unit, is not the common public-health way to express regulatory concentrations.