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Fluoride in most groundwaters occurs as the anion F–. Waters with high fluoride content are found mostly in calcium-deficient ground waters in many basement aquifers, such as granite and gneiss, in geothermal waters and in some sedimentary basins. Groundwaters with high fluoride concentrations occur in many areas of the world including large parts of Africa, China, the Middle East and southern Asia (India, Sri Lanka). One of the best known high fluoride belts on land extends along the East African Rift from Eritrea to Malawi. There is another belt from Turkey through Iraq, Iran, Afghanistan, India, northern Thailand and China. The Americas and Japan have similar belts.

Fluoride is found in vegetables, fruit, tea and other crops. although drinking water is usually the largest contributor to the daily fluoride intake. Fluoride is also found in the atmosphere, originating from the dusts of fluoride-containing soils, from gaseous industrial wastes, from the burning of coal fires in populated areas and from gases of volcanic activity. Thus fluoride, in varying concentrations, is freely available in nature. Most of the studies of fluoride intake have been done in developed countries. In temperate climates, daily exposure is about 0.6mg/adult/day if the water is not fluoridated. The WHO guideline value for fluoride is 1.5mg/litre, with a target of between 0.8–1.2mg/l to maximise benefits and minimise harmful effects. Acceptable levels depend on climate, volumes of water intake and the likely intake of fluoride in other sources. Much depends on whether other sources, such as those mentioned above, also have high levels.

The effects of too little—and too much—Fluoride

Fluoride is a desirable substance: it can prevent or reduce dental decay and strengthen bones, thus preventing bone fractures in older people. Where the fluoride level is naturally low, studies have shown higher levels of both dental caries (tooth decay) and fractures. Because of its positive effect, fluoride is added to water during treatment in some areas with low levels. But you can have too much of a good thing; and in the case of fluoride, water levels above 1.5mg/litre may have long-term undesirable effects (Table 1: see also fact file on fluorosis). Much depends on whether other sources, such as vegetables, also have high levels. The risk of toxic effect rises with the concentration. It only becomes obvious at much higher levels than 1.5mg/l. The natural level can be as high as 95mg/l in some waters, such as in Tanzania where the rocks are rich in fluoride-containing minerals. Severe effects of excess fluoride have recently been reported from the Assam state in India (Box).

Table 1. Fluoride effects

Level in water Effects
0.8–1.2 mg/l Prevention of tooth decay, strengthening of skeleton
Above 1.5 mg/l Fluorosis: pitting of tooth enamel and deposits in bones
Above about 10 mg/l Crippling skeletal fluorosis

Box 2 : Too much natural fluoride in India

Nearly 100,000 villagers in the remote Karbi Anglong district in the north-eastern state of Assam were reported to be affected by excessive fluoride levels in groundwater in June 2000. Many people have been crippled for life. The victims suffer from severe anaemia, stiff joints, painful and restricted movement, mottled teeth and kidney failure. The first fluorosis cases were discovered in the middle of 1999 in the Tekelangiun area of Karbi Anglong. Fluoride levels in the area vary from 5-23 mg/L, while the permissible limit in India is 1.2 mg/L. Local authorities launched a scheme for the supply of fluoride-free water and painted polluted tube-wells red: they also put up notice boards warning people not to drink the water from these wells. (Times of India / UNI, 2 Jun 2000)

Removing excess fluoride

It is difficult and expensive to reduce a high natural level of fluoride in water. This means that the first option should be to find an alternative source with lower Fluoride levels. If there is no other possible or cost-effective source, de-fluoridation must be attempted to avoid the toxic effects. The best method depends on local circumstances (Table 2)

Table 2: Ways of removing fluoride

Method How it works
Bone charcoal; Filters out the fluoride, then column of charcoal disposed of
Precipitation method, for example, Nalgonda Aluminium sulphate and lime added daily and fluoride-rich sludge then removed
Activated alumina Filters out the fluoride, then column removed

Only the water for drinking and cooking needs to be de-fluoridated. The entire water demand is often ten times higher and to de-fluoridate this would be too expensive, as well as producing a large amount of toxic sludge.

Multiple fluoride exposure

In addition to the natural sources of fluoride, it may also be present in toothpaste and a range of other products aimed to reduce dental decay. Fluoride may reach high levels in fish because in builds up in the fish bones: a problem where soft bones are eaten, such as in salmon and sardines. Milk levels of Fluoride are typically low.

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