"Whatever it's called, the major portion of the fluorosilic acid added to the nation's water supply comes from Florida's phosphate fertilizer industry. Here's how:
Florida's phosphate rock is about 3.5 percent fluorine. To make phosphoric acid for fertilizer, the rock is mixed with sulfuric acid. The mixture produces a gas called silicon tetrafluoride. The gas is sent through ductwork and a water scrubber to create fluorosilic acid, a clear liquid that in high concentrations is toxic. The acid is what fertilizer companies sell as a fluoride additive. It's diluted to what's considered a safe level when pumped into water supplies.
Source: Florida Institute of Phosphate Research"