Fluoride in well water: limits for kids and adults
Above 2 mg/L, fluoride can cause white spots on children's teeth. Above 4 mg/L (US) or 1.5 mg/L (Ontario), it's over the health limit.
- US (EPA)
- 4.0 mg/L health limit; 2.0 mg/L cosmetic guideline.
- Ontario
- 1.5 mg/L.
What it is
Fluoride occurs naturally in some groundwater. A little protects teeth; too much over many years can cause tooth and bone problems.
What your number means
- Within limitsBelow 1.5 mg/L
Under the limits.
- Worth a look1.5 to 4 mg/L
Over Ontario's standard and EPA's guideline for teeth above 2. Use another source for young children.
- Act nowAbove 4 mg/L
Over the EPA health limit. Use another source and look at reverse osmosis.
What to do
- Use another source for children's drinking water if fluoride is above 2 mg/L.
- Tell your dentist your well's fluoride level before using fluoride supplements.
- Ask about reverse osmosis if you're over the limit.
How often to test
With a full panel every 3 to 5 years.
Common questions
Should my kids take fluoride supplements?
Only after your dentist knows your well's fluoride level.
Sources: EPA drinking water standards; EPA secondary standards; Ontario Drinking Water Quality Standards, O. Reg. 169/03. Limits checked October 2026.