Excerpt from pages 8-9:
Inorganic fluorides are released naturally into the environment through the weathering of minerals, emissions from volcanoes, and as marine aerosols (Government of Canada 1993). The most important natural source of inorganic fluorides in the environment is bedrock, from which inorganic fluoride-containing minerals are leached by groundwater, and thence into surface and sea water. Another major natural source of inorganic fluorides is volcanoes which release gases and ash into the atmosphere either by explosive eruptions or by continual low energy release (Kawaratani and Fujita 1990). Estimates of annual global releases of gaseous fluoride compounds (primarily HF) from volcanic sources (passive degassing and eruptions) range from 60,000 to 5,000,000 tonnes, 10% of which are injected directly into the stratosphere (Symonds et al. 1988); Environment Canada and Health Canada 1996). Kawaratani and Fujita (1990) measured mean levels of various chemical species in precipitation around the Sakurajima volcano in Japan and found there was a significant increase in levels of inorganic fluorides in the vicinity of the volcano. In the case of volcanoes with a volcanic eruption index of four or greater, significant amounts (5,000 to 50,000 tonnes) of HF are deposited directly into the stratosphere (Symonds et al. 1988). At present, the consequences of increased stratospheric levels of fluoride ion are not known. Estimated emissions of sulphur dioxide, chloride, and fluoride from volcanoes are summarised in Table 4. Finally, about 0.02 x 10(-6) tonnes of inorganic fluorides may be released annually from marine aerosols (i.e., mixture between water and gas) (Symonds et al. 1988).
Government of Canada. 1993. Canadian Environmental Protection act Priority Substances List Assessment Report. Inorganic Fluorides. Environment Canada and Health Canada.
Kawaratani RK, Fujita S. 1990. Wet deposition of volcanic gases and ash in the vicinity of Mount Sakurajima. Atmos. Environ. 24A(6):1487-92.
Symonds RB, Rose WI, Reed MH. 1988. Contribution of Cl- and F- bearing gases to the atmosphere by volcanoes. Nature 334(4)415-9.