Water Fluoridation
tl;dr: Helps with stronger teeth, but more and more studies these days point to it being neurotoxic -- especially important during prenatal stage, but definitely possible as an adult too. Especially the elderly.
Notes
WHO and US support optimal levels
Some like EU don’t necessarily find it beneficial yet
Modern toothpaste might have enough fluoride for what we need to prevent tooth decay
Opponents say not enough data, it’s mass medication, stripping liberties
Seems like in US, might not be optimal to drink filtered water
Just the right amount can significantly improve dental health in the population. (source)
It's been pretty thoroughly vetted by the scientific and health communities. (source)
Toxicant exposures in early life can have much more serious consequences than exposures occurring later in life, and the developing brain is known to be particularly vulnerable
Topical fluoride is nowhere near as effective at preventing cavities as systemic fluoride that is provided to children while their teeth are forming.
US recommended levels are .7-1.2 mg per liter of water
1.3 mg of fluoride in a serving (quarter teaspoon) of toothpaste
A 2016 review by the National Toxicology Program (NTP) focused on fluoride neurotoxicity in regard to learning and memory [55]. At water concentrations higher than 0.7 mg/L, NTP found a low-to-moderate level of evidence. The evidence was the strongest (moderate) in animals exposed as adults and weaker (low) in animals exposed during development, where fewer studies were available at relevant exposure levels. Most experimental studies had used concentrations exceeding the levels added to water in fluoridation programs, but the NTP recognized that rats require about five times more fluoride in their water to achieve the same serum-fluoride concentrations as humans (source)
Additional community studies in adults have focused on cognitive problems and neurological symptoms in subjects with skeletal fluorosis. Using neuropsychological tests, including the Wechsler scale, 49 adult fluorosis patients were compared with controls and showed deficits in language fluency, recognition, similarities, associative learning, and working memory (source)
Dementia diagnosis in North Carolina was more common at higher water-fluoride concentrations [81], and similar findings for fluoride (and aluminum) have recently been reported from Scotland (source)
Excess occurrence of neurological symptoms (i.e., headaches, insomnia, and lethargy) have also been recorded in both adults and children from waterborne fluorosis areas (source)
However, these studies are hard to evaluate due to uncertainty about past fluoride exposure levels and the possible influence of other risk factors. (source)
Neurotoxicity occurs when the exposure to natural or manmade toxic substances (neurotoxicants) alters the normal activity of the nervous system. This can eventually disrupt or even kill neurons, key cells that transmit and process signals in the brain and other parts of the nervous system. (source)
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