At what ppm does water undergo defluoridation?

• A. 0.7 ppm
• B. 1 ppm
• C. 5 ppm
• D. 2 ppm

Answer: (D)

Defluoridation is used to lower fluoride levels in drinking water to protect against fluorosis while keeping enough fluoride for dental health. The practical trigger is when fluoride reaches about two milligrams per liter (2 mg/L), which is the level above the commonly recommended range for drinking water. The aim is to reduce from around 2 mg/L (or higher) down to roughly 0.7 mg/L, the target that balances caries prevention with minimal fluorosis risk. So the threshold at which defluoridation is considered is about 2 ppm. The other numbers are targets or much higher concentrations, not the usual trigger point.