The fluoride added to the public water supply of more than 73 percent of the U.S. population isn’t naturally occurring, and that creates added risk. This fluoride is derived primarily as a chemical byproduct of the phosphate fertilizer and aluminum production industries. It would be deemed as pollution if it escaped into the air or was dumped into a landfill.
Rebecca Hanmer, former deputy assistant administrator at the Environmental Protection Agency (EPA) Office of Water, noted that reality in a letter she wrote in 1983 (pdf):