Fluorides are widely distributed in the earth's crust, in combined form, in gases, such as hydrogen fluoride and silicon tetrafluoride, and in particular substances, such as cryolite, fluorapatite, calcium fluoride, and aluminium fluoride. Airborne fluorides are derived from natural sources, from industrial processes, and from fluoride-containing materials used by the public. Once fluorides have entered the atmosphere, their dispersal and their continued suspension in or loss from the atmosphere are determined not only by meterologic conditions but also by the chemical stability and physical state of the fluorides and by their interaction with water vapor in the atmosphere. The normal concentration of fluorides in the ambient air is low. Of a large number of air samples analyzed, 97% from nonurban areas had no detectable fluorides, and the highest concentration was 0.16 μg/m3. Of samples from urban areas, 87% had less than 0.05 μg/m3, and the concentration ranged from 0.05 to 1.89 μm3 in the others. Higher levels occurred in areas close to industries that emit fluorides in their processes.
