Some of this fluoride combines with hydrogen to form hydrogen fluoride. Some coal-burning power plants use pollution control devices to remove sulfur from gases leaving their stacks. These devices also remove some of the hydrogen fluoride from stack gas before it reaches the air. Fluoride-bearing wastes captured by pollution control devices are usually sent to ash ponds or land disposal sites. Almost all hydrogen fluoride from power plants comes from burning coal.
Most hydrogen fluoride released into the air by natural sources comes from volcanoes.
Smaller amounts come from ocean spray, forest fires, weathering rocks, and dust. Hydrogen fluoride released into the environment by human activities outside the utility industry comes mainly from industrial boilers that burn coal and oil, facilities that fabricate aluminum and stainless steel, and agricultural use of phosphate fertilizers. Industries reporting to EPA released 6,257 tons of hydrogen fluoride into the environment in 1996. Nearly all was released into the air. Hydrogen fluoride released into the air from power plants easily dissolves in airborne water to form dilute hydrofluoric acid.