Mineral
Chemical formula
Fluorine concentration (%)
Sellaite
MgF2
61
Villiaumite
NaF
55
Fluorspar
CaF2
49
Cryolite
Na3AlF6
45
Bastnaesite
(Ce, LaY)(CO3)F
9
Fluorapatite
Ca5(PO4)3F
3.5
In water, inorganic fluorides (F) usually remain in ionic form under conditions of relatively low pH and hardness and in the presence of ion-exchange materials such as bentonite clays and humic acids. In unpolluted freshwater, fluoride concentration usually varies from 0.01 to 0.3 mg/L, whereas in unpolluted seawater, it varies from 1.2 to 1.5 mg/L (Camargo 2003). However, wide variations in groundwater fluoride concentration may occur naturally without any human intervention, depending upon geological, chemical, and physical characteristics of the water-supplying area, consistency of the soil, porosity of rocks, pH, temperature, complexing action of other elements, and the depth of wells (WHO 2002).
Fluoride concentration in the atmosphere in unpolluted areas usually varies between 0.02 and 2.0 µg/m3 (USEPA 1980). Atmospheric fluoride may be in gaseous or particulate forms. Gaseous forms include hydrogen fluoride, sulfur hexafluoride, silicon tetrafluoride, and hexafluorosilicic acid. Particulate forms include sodium aluminum fluoride, calcium phosphate fluoride, sodium hexafluorosilicate, aluminum fluoride, calcium fluoride, and lead fluoride. Hydrogen fluoride and inorganic fluoride particulates (sodium and calcium fluoride) are major inorganic fluorides present in the atmosphere, accounting for nearly 75 and 25 %, respectively (WHO 2002).
1.2 Is Fluoride Essential for Health?
Whether fluoride is essential for maintenance of health is an issue of much debate. An expert World Health Organization committee included fluorine in the list of 14 elements that are essential for life (WHO 1973). The American Dietetic Association considers that fluorine is an important element for all mineralized tissues in the body (Palmer and Wolfe 2005). The Federal Register of the US Food and Drug Administration also declared fluorine as an essential nutrient for human health. However, it is almost impossible to provide experimental evidence supporting the essentiality of fluoride for maintenance of human and animal health. The Food and Nutrition Board of the National Research Council (NRC) has therefore, withdrawn the term “essential” and is now using terms “beneficial element” (BE) and “apparent beneficial intake” (ABI) for fluoride.
1.2.1 Fluoride and Human Health
Fluoride is required for mineralization of bone and teeth, maintenance of fertility, hematopoiesis, and activation of certain enzymes, such as adenylate cyclase, acid and alkaline phosphatases, and isocitrate dehydrogenase (Kirck 1991). In vitro studies suggest that fluorine helps in activities of histone methyltransferase, stabilizes the interaction between guanosine triphosphatase (GTPase) and GTPase-activating proteins, and affects the posttranslational assembly of glycosaminoglycan chains in mineralizing bone cells (Kirck 1991).
It is widely accepted that a low level of fluoride intake decreases susceptibility to dental caries in human beings (Treasure and Dever 1992). Anticaries action of the fluoride is mediated by its incorporation into the tooth enamel as fluor(hydroxyl) apatite which has high levels of fluoride and low levels of carbonate and high acid resistance. In addition, fluoride can inhibit bacterial metabolism of carbohydrates, hence it decreases production of acids (Bowden 1990). A few decades back, use of fluorinated toothpaste for prevention of dental caries was a great vogue, but soon it became obsolete due to increasing reports of ill effects on oral health such as the occurrence of perioral dermatitis (Mellete et al. 1983; McCaffery 2003). Daily intake of 1–3 mg F/kg body weight has a potential effect on dental caries prevention over a short time span, however, prolonged intake of equal doses may have deleterious health effects. Earlier, fluoride was also used in the treatment of osteoporosis, although the therapeutic window was known to be narrow (Dequeker and Declerck 1993). Some investigators even reported increased risk of fracture in osteoporotic women treated with fluoride (Hedlund and Gallagher 1989; Kurttio et al. 1999). Therefore, fluoride is not used at present for prevention and treatment of osteofluorosis (Danielson 1992).
1.2.2 Fluoride and Animal Health
Many biologists doubt the essentiality of fluoride for maintenance of animal health. Perhaps the fluoride requirement is so small that deficiency is never observed spontaneously or could hardly be produced in natural and/or experimental conditions (Wheeler and Fell 1983; Alberts 1998). In a study on the water flea (Daphnia magna), it was observed that the growth rate is enhanced at extremely low concentrations of <0.007 mg F/L water and the authors concluded that the essential F level for the flea is 0.004 mg F/L, which is lower than the level found anywhere in the natural environment (Dave 1984).
Attempts to demonstrate fluoride deficiency disorders in experimental and laboratory animals, by and large, failed despite several decades of research. In a study, mice fed low F diets developed anemia and impaired reproduction (Messer et al. 1972, 1973). However, later on, it was found that anemia and impaired reproduction were due to low dietary iron and no such symptoms developed when mice were fed a diet containing low fluoride and sufficient iron (Tao and Suttie 1976). Another study reported skeletal abnormalities in female goats and poor growth in their offspring when they were fed a diet containing less than 0.3 ppm fluoride (on a dry matter basis) for 10 generations (Anke et al. 1997). However, no further study documenting similar findings appeared in the literature. Some experimental studies documented the beneficial effects of fluoride supplementation on animal health and performance. For example, an increase in growth rate of broiler chickens was recorded after supplementation of 80 µg F/g diet (Gutierrez et al. 1993). Nevertheless, evidence arising from scientific studies to date has not been sufficient to conclude that fluoride is essential for maintenance of animal health and production; rather its toxic effects appear more important.
1.3 Fluoride Toxicity
Large doses of fluoride exert toxic effects in almost all living creatures. Fluoride is a general tissue poison, even more poisonous than lead and just slightly less poisonous than arsenic. The toxic potential of fluoride can be very well assessed by the fact that a single dose of 5–10 mg/kg body weight causes acute toxic effects and death may occur following a single oral fluoride intake at the rate of 16 mg/kg body weight (WHO 2000).
Depending upon the quantity of intake and the chemical form of the fluoride compound, fluoride toxicity can occur in acute, subacute, and chronic form. Acute fluoride toxicity, although rare in occurrence, results mostly after accidental ingestion of large doses of fluorine compounds, such as sodium fluoroacetate and fluoroacetamide used as a rodenticide, sodium fluorosilicate used as an insecticide, and sodium fluoride used as an acaricide. Because of the decrease in use of these fluoride compounds for various household and agricultural purposes, acute fluoride poisoning is now seldom reported. Prolonged intake of low but toxic doses of fluoride compounds induces chronic fluoride toxicity, often referred as “fluorosis”. Chronic fluoride toxicity is more common and important for human and domestic animals and is often characterized by pathological changes in teeth (dental fluorosis) and bone (osteofluorosis). Ingestion of fluoride compounds (through food and water) appears the major route of fluoride uptake. When drinking water contains excess fluoride and serves as a major source of excess fluoride intake, the chronic fluoride toxicity produced is referred as “hydrofluorosis”. Water fluoride concentrations above 1.5 ppm, have been reported to cause osteo and dental fluorosis in a wide range of domestic animals including cattle (Bos taurus), buffalo (Bubalus bubalis), horse (Equus caballus), donkey (Equus asinus), dromedary camel (Camelus dromedarius), sheep (Ovis aries), and goat (Capra hircus) (Ranjan et al. 2009; Choubisa 2013, 2014).
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