WHO guidelines for 1.5mg/L fluoride
Upper limit of 10mg/day (considered to be high by some bodies)
https://www.eatforhealth.gov.au/nutrient-reference-values/nutrients/fluoride-updated-2017
Basic bath: only considering water intake, consuming 6-7 liters in a day (regular occurance working in Australia) puts you over the upper limit without considering major sources like diet, tea and dental products and treatments.
Despite fish being a shell, I thought it was representative of openbsd here (hence the puffer fish)