1. General description
Identity
Fluoride is a fairly common element that does not occur in the elemental state in nature because of its high reactivity. It accounts for about 0.3 g/kg of the earth's crust and exists in the form of fluorides in a number of minerals, of which fluorspar, cryolite, and fluorapatite are the most common. The oxidation state of the fluoride ion is -1.
| Property | Sodium fluoride (NaF) | Hydrogen fluoride (HF) |
| Physical state | White, crystalline powder | Colourless liquid or gas with biting smell |
| Melting point (°C) | 993 | -83 |
| Boiling point (°C) | 1695 at 100 kPa | 19.5 |
| Density (g/cm3) | 2.56 | ? |
| Water solubility | 42 g/litre at 10 °C | Readily soluble below 20 °C |
| Acidity | ? | Strong acid in liquid form; weak acid dissolved in water |
Major uses
Inorganic fluorine compounds are used in aluminum production, as a flux in the steel and glass fiber industries, and in the production of phosphate fertilizers (which contain an average of 3.8% fluorine), bricks, tiles, and ceramics. Fluosilicic acid is used in municipal water fluoridation schemes (1).
Environmental fate
Although sodium fluoride is soluble in water (1), aluminum, calcium, and magnesium fluorides are only sparingly so (3).
2. Analytical methodsFluoride is usually determined by means of an ion-selective electrode, which makes it possible to measure the total amount of free and complex-bound fluoride dissolved in water. The method can be used for water containing at least 20 g/litre (2). For rainwater in which fluoride was present at a concentration of 10 �g/litre, a detection limit of 1 g/litre was reported (4).
A method using a fluoride-elective electrode and an ion analyzer to determine fluoride at levels of 0.05?.4 mg/litre has been described (5). With a slight modification, the method can be used to measure fluoride at 0.4?.0 mg/litre.
3. Environmental levels and human exposure
Air
Natural background concentrations are of the order of 0.5 ng/m3. If anthropogenic emissions are included, worldwide background concentrations are of the order of 3 ng/m3. In the Netherlands, concentrations in areas without sources are 30?0 ng/m3, rising to 70 ng/m3 in areas with many sources (2). In a survey of fluoride in the air of some communities in the USA and Canada, concentrations were in the range 0.02?.0 g/m3 (6). In some provinces of China, fluoride concentrations in indoor air ranged from 16 to 46 �g/m3 owing to the indoor combustion of high-fluoride coal for cooking and for drying and curing food (7).
Water
Traces of fluorides are present in many waters; higher concentrations are often associated with underground sources. In seawater, a total fluoride concentration of 1.3 mg/litre has been reported (2). In areas rich in fluoride-containing minerals, well-waters may contain up to about 10 mg of fluoride per litre. The highest natural level reported is 2800 mg/litre. Fluorides may also enter a river as a result of industrial discharges (2). In groundwater, fluoride concentrations vary with the type of rock the water flows through but do not usually exceed 10 mg/litre (3). In the Rhine in the Netherlands, levels are below 0.2 mg/litre. In the Meuse, concentrations fluctuate (0.2?.3 mg/litre) as a result of industrial processes (2).
Fluoride concentrations in the groundwater of some villages in China were greater than 8 mg/litre (8,9). In Canada, fluoride levels in drinking-water of <0.05?.2 mg/litre (non-fluoridated) and 0.6?.1 mg/litre (fluoridated) have been reported in municipal waters; in drinking-water prepared from well-water, levels up to 3.3 mg/litre have been reported. In the USA, 0.2% of the population is exposed to more than 2.0 mg/litre (3). In the Netherlands, year-round averages for all drinking-water plants are below 0.2 mg/litre (2). In some African countries where the soil is rich in fluoride-containing minerals, levels in drinking-water are relatively high (e.g., 8 mg/litre in the United Republic of Tanzania) (3).
Removal from WaterSimple technologies for household "point-of-use" removal of Fluoride from water are few which includes reverse osmosis and have proven to be sustainable in each different setting.
Food
Virtually all foodstuffs contain at least traces of fluorine. All vegetation contains some fluoride, which is absorbed from soil and water. The highest levels in field-grown vegetables are found in curly kale (up to 40 mg/kg fresh weight) and endive (0.3?.8 mg/kg fresh weight) (2). Other foods containing high levels include fish (0.1?0 mg/kg) and tea (2,3). High concentrations in tea can be caused by high natural concentrations in tea plants or by the use of additives during growth or fermentation. Levels in dry tea can be 3?00 mg/kg (average 100 mg/kg), so 2? cups of tea contain approximately 0.4?.8 mg (2,6). In areas where water with a high fluoride content is used to prepare tea, the intake via tea can be several times greater.
Dental uses
For dental purposes, fluoride preparations may contain low (0.25? mg per tablet; 1000?500 mg of fluorine per kg of toothpaste) or high concentrations (liquids containing 10 000 mg/litre and gels containing 4000?000 mg/kg are used for local applications) (2).
Estimated total exposure and relative contribution of drinking-water
Levels of daily exposure to fluoride depend mainly on the geographical area. In the Netherlands, the total daily intake is calculated to be 1.4?.0 mg of fluoride. Food seems to be the source of 80?5% of fluoride intake; intake from drinking-water is 0.03?.68 mg/day and from toothpaste 0.2?.3 mg. For children, total intake via food and water is decreased because of lower consumption. Intake of food and water relative to body weight is higher, however, and is further increased by the swallowing of toothpaste or fluoride tablets (up to 3.5 mg of fluoride per day) (2).
Daily intakes ranging from 0.46 to 3.6?.4 mg/day have been reported in several studies (6). Daily exposure in volcanic areas (e.g. the United Republic of Tanzania) may be as high as 30 mg for adults, mainly from drinking-water intake (J.E.M. Smet, personal communication, 1990). In areas with relatively high concentrations in groundwater, drinking-water becomes increasingly important as a source of fluoride. In some counties in China where coal has a high fluoride content, the average daily intake of fluoride ranged from 0.3 to 2.3 mg via air and from 1.8 to 8.9 mg via food (10).
4. Kinetics and metabolism in laboratory animals and humans
After oral uptake, water-soluble fluorides are rapidly and almost completely absorbed in the gastrointestinal tract. Fluorides less soluble in water are absorbed to a lesser degree. Absorbed fluoride is transported via the blood; with prolonged intake of fluoride from drinking-water, concentrations in the blood are the same as those in drinking-water, a relationship that remains valid up to a concentration in drinking-water of 10 mg/litre. Distribution of fluoride is a rapid process. It is incorporated into teeth and bones; there is virtually no storage in soft tissues. Incorporation into teeth and skeletal tissues is reversible: after cessation of exposure, mobilization from these tissues takes place. Fluoride is excreted via urine, faeces, and sweat (3,6,11).
5. Effects on laboratory animals and in vitro test systems
Long-term exposure
Most long-term studies are limited. In drinking-water studies with sodium fluoride, effects on skeletal tissues were observed. In a 2-year study in rats and mice (25 or 175 mg of sodium fluoride per litre of drinking-water), dentine discoloration and dysplasia developed at both dose levels; osteosclerosis in the long bone was seen in the high-dose females only (12). In another recent 2-year oral study in rats, there were effects on the teeth (ameloblastic dysplasia, fractured and malformed incisors, enamel hypoplasia) and bones (subperiosteal hyerkeratosis) at all dose levels, including the lowest of 4 mg of sodium fluoride per kg of body weight per day (13).
Mutagenicity and related end-points
Many mutagenicity studies have been carried out with fluorides (usually sodium fluoride). Tests in bacteria and insects were negative, as were in vivo studies (11,12,14). In mammalian cells in vitro, fluoride causes genetic damage (including chromosomal aberrations) at cytotoxic concentrations only (=10 mg/litre), the mechanism for which is not known. This genetic effect is probably of limited relevance for practical human exposures (11).
Carcinogenicity
IARC evaluated the available studies in 1987 and concluded that the limited data provide inadequate evidence of carcinogenicity in experimental animals (14). In a recent study in which rats and mice were given sodium fluoride in drinking-water at 11, 45, or 79 mg/litre (as fluoride ion), only the incidence of osteosarcomas in the bones of male rats increased (incidences 0/80, 0/51, 1/50, and 3/80 in the controls, low-, mid-, and high-dose groups, respectively). This increase was considered to provide equivocal evidence for a carcinogenic action in male rats; the study yielded no evidence for such an action in female rats or in male or female mice (12). In another recent study, no carcinogenic effect was observed in rats given sodium fluoride in the diet at dose levels of 4, 10, or 25 mg/kg of body weight per day for 2 years (13).
6. Effects on humans
Fluorine is probably an essential element for animals and humans. For humans, however, the essentiality has not been demonstrated unequivocally, and no data indicating the minimum nutritional requirement are available. To produce signs of acute fluoride intoxication, minimum oral doses of at least 1 mg of fluoride per kg of body weight were required (11).
Many epidemiological studies of possible adverse effects of the long-term ingestion of fluoride via drinking-water have been carried out. These studies clearly establish that fluoride primarily produces effects on skeletal tissues (bones and teeth). Low concentrations provide protection against dental caries, especially in children. This protective effect increases with concentration up to about 2 mg of fluoride per litre of drinking-water; the minimum concentration of fluoride in drinking-water required to produce it is approximately 0.5 mg/litre.
Fluoride may give rise to mild dental fluorosis (prevalence: 12?3%) at drinking-water concentrations between 0.9 and 1.2 mg/litre (15). This has been confirmed in numerous subsequent studies, including a recent large-scale survey carried out in China (16), which showed that, with drinking-water containing 1 mg of fluoride per litre, dental fluorosis was detectable in 46% of the population examined. As a rough approximation, for areas with a temperate climate, manifest dental fluorosis occurs at concentrations above 1.5? mg of fluoride per litre of drinking-water. In warmer areas, dental fluorosis occurs at lower concentrations in the drinking-water because of the greater amounts of water consumed (3,6,10). It is also possible that, in areas where fluoride intake via routes other than drinking-water (e.g. air, food) is elevated, dental fluorosis develops at concentrations in drinking-water below 1.5 mg/litre (10).
Fluoride can also have more serious effects on skeletal tissues. Skeletal fluorosis (with adverse changes in bone structure) is observed when drinking-water contains 3? mg of fluoride per litre. Crippling skeletal fluorosis develops where drinking-water contains over 10 mg of fluoride per litre (6). The US Environmental Protection Agency considers a concentration of 4 mg/litre to be protective against crippling skeletal fluorosis (17).
Several epidemiological studies are available on the possible association between fluoride in drinking-water and cancer rates among the population. IARC evaluated these studies in 1982 and 1987 and considered that they provided inadequate evidence of carcinogenicity in humans (1,14). The results of several epidemiological studies on the possible adverse effects of fluoride in drinking-water on pregnancy outcome are inconclusive (3,6,11).
It is known that persons suffering from certain forms of renal impairment have a lower margin of safety for the effects of fluoride than the average person. The data available on this subject are, however, too limited to allow a quantitative evaluation of the increased sensitivity to fluoride toxicity of such persons (3,11).
7. Guideline value
In 1987, IARC classified inorganic fluorides in Group 3 (14). Although there was equivocal evidence of carcinogenicity in one study in male rats, extensive epidemiological studies have shown no evidence of it in humans (12).
There is no evidence to suggest that the guideline value of 1.5 mg/litre set in 1984 needs to be revised. Concentrations above this value carry an increasing risk of dental fluorosis, and much higher concentrations lead to skeletal fluorosis. The value is higher than that recommended for artificial fluoridation of water supplies (18). In setting national standards for fluoride, it is particularly important to consider climatic conditions, water intake, and intake of fluoride from other sources (e.g. from food and air). In areas with high natural fluoride levels, it is recognized that the guideline value may be difficult to achieve in some circumstances with the treatment technology available.
Information extracted from:Guidelines for drinking-water quality, 2nd ed.
Vol. 2. Health criteria and other supporting information.
Geneva, World Health Organization, 1996. pp. 231-237.
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