Fluorine is the most chemically aggressive element. In pure form it is a pale, yellow-green F2 gas. It is extremely toxic and reacts with nearly everything, in most cases very violently. At contact with water, it forms the very caustic hydrofluoric acid, HF.
Its salts (fluorides), especially fluorite (calcium fluoride, CaF2) frequently occur in nature as minerals. Fluoride is needed for bones and teeth, but quickly becomes poisonous if the dose is too high.
Fluoride, the most consumed drug in the USA, is deliberately added to 2/3 of public water supplies theoretically to reduce tooth decay, but with no scientifically-valid evidence proving safety or effectiveness. Fluoride is in many foods, beverages and dental products. The ubiquity of the cavity-fighting chemical can result in overconsumption, particularly among young children.
There is no universally accepted optimal level for daily intake of fluoride. Some researchers even wonder whether the 1 mg/L added into drinking water is too much.
Fluoride can subtly alter endocrine function, especially in the thyroid — the gland that produces hormones regulating growth and metabolism. A series of epidemiological studies in China have associated high fluoride exposures with lower IQ. Studies and tests on lab animals suggest that high fluoride exposure increases the risk of bone fracture.