is it found in any food consumed by humans?
Seafood, tea, and gelatin.
Fluoride
 Water and food sources of fluoride include community water fluoridation, seafood, tea, and gelatin.
when did they start putting fluoride in water 1945 what was the objective of doing this? Water fluoridation is the controlled adjustment of fluoride to a public water supply to reduce tooth decay. what is this substance mentioned here? It is an inorganic, monatomic anion, whose salts are typically white or colorless. how is it associated with teeth? Fluoridated water operates on tooth surfaces: in the mouth, it creates low levels of fluoride in saliva, which reduces the rate at which tooth enamel demineralizes and increases the rate at which it remineralizes in the early stages of cavities.
wiki:1206867
Fluoride [SEP] Ingestion
According to the U.S. Department of Agriculture, the Dietary Reference Intakes, which is the "highest level of daily nutrient intake that is likely to pose no risk of adverse health effects" specify 10 mg/day for most people, corresponding to 10 L of fluoridated water with no risk. For infants and young children the values are smaller, ranging from 0.7 mg/d for infants to 2.2 mg/d. Water and food sources of fluoride include community water fluoridation, seafood, tea, and gelatin. Soluble fluoride salts, of which sodium fluoride is the most common, are toxic, and have resulted in both accidental and self-inflicted deaths from acute poisoning.