This is a fairly common lichen (symbiotic organism consisting of a fungal matrix with photosynthetic algae embedded in it) that has been used historically to produce a purple-brown dye. It is also a useful pollution indicator, as it is tolerant of sulfur dioxide, but particularly sensitive to fluoride exposure (“Pollution monitoring with lichens” by D.H.S Richardson, 1992). ECOS magazine, from the CSIRO, has an interesting article “Fluoride and Trees” describing the effects of fluoride exposure on vegetation, including chlorosis (yellowing caused by failure to produce chlorophyll) and necrosis (cell death). It should be noted that some plants are more sensitive to fluoride emissions that mammals and therefor act as indicator or ‘sentinel’ species.
The following key information is from the “World Health Organisation Environmental Health Criteria for Fluorides” produced by a panel of expert scientists -
Physical and chemical characteristics: Hydrogen fluoride (HF) is a colourless, pungent liquid or gas that is highly soluble in organic solvents and in water, in which it forms hydrofluoric acid. Calcium fluoride (CaF2) is a colourless solid that is relatively insoluble in water and dilute acids and bases. Sodium fluor- ide (NaF) is a colourless to white solid that is moderately soluble in water. Sulfur hexafluoride (SF6) is a colourless, odourless, inert gas that is slightly soluble in water and readily soluble in ethanol and bases.
Sources: Fluorides are released into the environment naturally through the weathering and dissolution of minerals, in emissions from volcanoes and in marine aerosols. Fluorides are also released into the environment via coal combustion and process waters and waste from various industrial processes, including steel manufacture, primary aluminium, copper and nickel production, phosphate ore processing, phosphate fertilizer production and use, glass, brick and ceramic manufacturing, and glue and adhesive production. The use of fluoride-containing pesticides as well as the controlled fluoridation of drinking-water supplies also contribute to the release of fluoride from anthropogenic sources. Based on available data, phosphate ore production and use as well as aluminium manufacture are the major industrial sources of fluoride release into the environment.
Transport mechanisms: Fluorides in the atmosphere may be in gaseous or particulate form. Atmospheric fluorides can be transported over large distances as a result of wind or atmospheric turbulence or can be removed from the atmosphere via wet and dry deposition or hydrolysis. The transport and transformation of fluoride in water are influenced by pH, water hardness and the presence of ion-exchange materials such as clays. The transport and transformation of fluoride in soil are influenced by pH and the formation of predominantly aluminium and calcium complexes. Fluoride is not readily leached from soils.
Human health effects: Epidemiological investigations on the effects of fluoride on human health have examined occupationally exposed workers employed primarily in the aluminium smelting industry and populations consuming fluoridated drinking-water. In a number of analytical epidemiological studies of workers occupationally exposed to fluoride, an increased incidence of lung and bladder cancer and increased mortality due to cancer of these and other sites have been observed. In general, however, there has been no consistent pattern; in some of these epidemiological studies, the increased morbidity or mortality due to cancer can be attributed to the workers’ exposure to substances other than fluoride.
Fluoride has both beneficial and detrimental effects on tooth enamel. The prevalence of dental caries is inversely related to the concentration of fluoride in drinking-water. The prevalence of dental fluorosis is highly associated with the concentration of fluoride, with a positive dose–response relationship.
Cases of skeletal fluorosis associated with the consumption of drinking-water containing elevated levels of fluoride continue to be reported. A number of factors, such as nutritional status and diet, climate (related to fluid intake), concomitant exposure to other sub- stances and the intake of fluoride from sources other than drinking-water, are believed to play a significant role in the development of this disease. Skeletal fluorosis may develop in workers occupationally exposed to elevated levels of airborne fluoride; however, only limited new information was identified.
Environmental health effects: The uptake of fluoride by biota is determined by the route of exposure, the bioavailability of the fluoride and the uptake/excretion kinetics in the organism. Soluble fluorides are bioaccumulated by some aquatic and terrestrial biota. However, no information was identified concerning the biomagnification of fluoride in aquatic or terrestrial food-chains. Terrestrial plants may accumulate fluorides following airborne deposition and uptake from soil. Fluoride accumulates in the bone tissue of terrestrial vertebrates, depending on factors such as diet and the proximity of fluoride emission sources.For example,mean fluoride concentrations of 7000– 8000 mg/kg have been measured in the bones of small mammals in the vicinity of an aluminium smelter.
The original findings of fluoride effects on mammals were from studies in the field on domestic animals such as sheep and cattle. Fluoride can be taken up from vegetation, soil and drinking-water. Tolerance levels have been identified for domesticated animals, with the lowest values for dairy cattle at 30 mg/kg feed or 2.5 mg/litre drinking-water. Incidents involving domesticated animals have originated both from natural fluoride sources, such as volcanic eruptions and the underlying geology,and fromanthropogenic sources, such as mineral supplements, fluoride-emitting industries and power stations. Symptoms of fluoride toxicity include emaciation, stiffness of joints and abnormal teeth and bones. Other effects include lowered milk production and detrimental effects on the reproductive capacity of animals. Investigations of the effects of fluoride on wildlife have focused on impacts on the structural integrity of teeth and bone. In the vicinity of smelters, fluoride-induced effects, such as lameness, dental disfigurement and tooth damage, have been found.