Fluorine

Fluorine link (chemical element, poison)
Highly reactive chemical element. Flouride has been put in drinking water for decades, nominally toprevent tooth decay, but is widely claimed to have a deleterious effect on IQ.

Fluorine is a highly reactive chemical element. Its ions are referred to as fluorides and do occur as part of the earth's crust.

Water fluoridation

Full article: Water/Fluoridation

After uranium hexafluoride was used to enrich uranium, fluorides began to be added to drinking water, reportedly in a bid to tackle tooth decay. A 2019 study showed that exposure of pregnant women to fluoride decreases the IQ of male children.