Atmospheric fluorides reduce or interfere with photosynthesis as a result of fluoride accumulation or alteration in plant function, even before the leaves show damage. Exposure to fluoride also causes plants to undergo changes in growth, metabolism, tissue degeneration and inhibition of certain enzyme activity. Prolonged exposure to ambient air concentrations of less than one million parts of air, by volume, may create a hazard to sensitive agricultural crops and forest species or to farm animals feeding on exposed forage. In this respect, fluorides are more than 100 times as toxic as sulphur dioxide.
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.
Plants vary greatly in their sensitivity to injury by fluorides. Pine is among the most sensitive species and cotton among the most resistant.