Governments that do this insist that it's necessary and helps to purify the water and prevent tooth- decay but studies show that fluoride is not safe to ingest.
Before 1933, the study of mottled tooth enamel, later known as fluorosis
1933-1945, the study on the relationship between fluorine and tooth decay
researchers from Harvard School of Public Health (HSPH) and China Medical University in Shenyang for the first time combined 27 studies and found strong indications that fluoride may adversely affect cognitive development in children.