Fluoride

“In point of fact, fluoride causes more human cancer death, and causes it faster than any other chemical.”–Dean Burk — Congressional Record 21 July 1976   

And:

The National Research Council (NRC) report of 2006 and the U.S. Environmental Protection Agency (USEPA) Fluoride is considered by toxicologists to be more toxic than lead and only slightly less toxic than arsenic. (16)