The major sources of atmospheric fluorides from industrial operations are the manufacture of aluminium metal products and phosphate fertilizers. However, waste products from smelting, production of pig iron, clay and ceramic industries, brick and glass manufacture also contain fluorides. In fact, any process that involves new materials taken from the earth's crust and subjected to heating at high temperatures will liberate fluorides. Both gaseous and particulate fluorides may be discharged as waste products. Of these, the most injurious to vegetation is the gaseous hydrogen fluoride. Particulate fluorides are much less toxic, and calcium fluoride, because of its insolubility, is the least toxic.