A colleague (an analytical chemist) has done the maths for us on the amount of water someone would have to drink in order to achieve a fluoride load commensurate with chronic fluorosis (around 20mg/kg dry matter in the diet).

Food varies quite a bit in its water content, but they’ve assumed an average of 50% water content and an average daily intake (from the USDA figure for average Americans) of 2.18kg wet weight, so 1.06kg dry weight of food per day.

On that diet our hypothetical someone would then need to take in 21.3mg F- each day to experience chronic fluorosis. Until fluoridation was discontinued, Hamilton’s town supply water had 0.7-1ppm fluoride ie a maximum of 1mg/L. This means our someone would need to take in 21.3 L of water a day, and maintain that intake, for fluorosis to become a reality. (At this level water intoxication would be a real threat.)

The estimated mean highest daily F- intake in New Zealanders – any age group, & from a combination of diet and town supply fluoridated water, is around 2.1mg/day (Cressey et al, 2010, J.Pub.Health.Dent 70: 327-336).