With electronic configuration 1S² 2S² 2P⁵, elemental fluorine is just one electron short of configuration of inert gases and as such, is most reactive of all elements. On account of this high chemical reactivity, it occurs as F in minerals. Rarely, it is also involved in the formation of fluoro-anions such as BF₄ and SiF₆. On account of its high chemical reactivity, fluorine is one of the most dispersed elements in the environment. Fluorine is very toxic for both plant and animal life.

The importance of fluoride compounds dates back to the time when man first learned to chemically modify the materials for his environment. The fluorine containing compounds are still used to increase the fluidity of melts and slags in the glass and ceramic industries. Fluorspar is used to reduce the viscosity of the slag in the metallurgy of iron. Cryolite is involved in the formation of electrolyte in the metallurgy of iron wherein Aluminium oxide is dissolved in this electrolyte and the metal is reduced electrically from the melt. The fluorocarbon polymers are among the most versatile and valuable compounds. The chemical applications of fluorine and its derivatives are extensive. Industrial plants manufacturing hydrofluoric acid, aluminium, super-phosphate, enamel, bricks and industries consuming high sulphur non-coking coal like thermal power plants are the main sources of fluoride pollution (Griffin et al, 1980; Deshmukh et al, 1995). These days large amount of industrial effluent containing fluoride are generated from high-tech industries such as those manufacturing semiconductors and integrated circuits.

Fluoride dust and fumes pollute the environment. Inhaling dust and fumes is as dangerous as consuming fluoride containing food, water or drugs. Industrial fluorosis is a serious problem in the developed western and other industrialised countries. However, in India, the problem of industrial fluorosis is also reaching an alarming state due to rapid industrialisation.

Except for evaporites, fluorine is the most abundant halogen in the sedimentary rocks (Wedepohl, 1974). Fluorite (48.7 %F), apatite (3.5 %F), mica (0.14-0.22 %F), illite (0.11-0.26 %F) are the chief fluorine bearing minerals in sedimentary rocks (Koritnig; 1963). Therefore, the kind and distribution of fluorine bearing minerals ultimately determine the fluorine content of rocks.

The fluorine content of hot spring varies from 0.15 to 55.4 mg/l (Matuura and Kokubu, 1955; Sugawara, 1967) and is seen to increase with increasing temperature, but the mole ratio F/Cl remains approximately constant. In India hot springs (35-100^o C) are mostly distributed along major lineaments and rifts (Ravi Shankar, 1986). Their fluorine content varies from 10-17 mg/l (Banerjee, 1967; Chowdhury et al, 1964; Chowdhury and Handa, 1973). In rainwater, fluorine may originate from sea and mostly varies from 0-0.089 mg/l, but near cities and industrial areas, values more than 1 mg/l also have been reported (Handa, 1977).

Irrespective of its primary source, the element fluorine is ultimately dispersed in the environment and is found in the atmosphere, soil and water. Therefore, fluorides reach the living organisms through water, air and soil, though principal source of fluoride for human and cattle population is drinking water. It has been reported that levels of fluoride in water in some parts of India was as high as 39 mg/l (Mangla, 1988).

Fluoride enters into the body through variety of sources.

1. Water
2. Food
3. Air
4. Medicine
5. Cosmetics.

The solubility of fluor-apatite (Ca₅F(PO₄)₃) and fluorite (CaF₂) in natural waters is very low (Appelo and Postma, 1993). Even in natural waters, on account of the ionic strength of the complex forming ions, the solubility of CaF₂ is further drastically reduced causing CaF₂ precipitation (Handa, 1977). In groundwater, the distribution of Ca and F is therefore antipathic (Handa, 1977). On the other hand, higher alkalinity in natural waters leads to increased fluoride solubility. It is for this reason that, despite higher average fluorine concentration in calcareous shales and limestones,

Table-3: Reported range of fluoride concentration in certain common
food items.

* Based on work of Nanda and Kapoor, 1971; Sengupta and Pal 1971; Lakdawala and Punekar, 1973; Chari et al. 1975) as reported in course manual on performance evaluation of defluoridation plants by Gujarat Jalseva Training Institute (GJTI).

the groundwater hosted in such aquifers is relatively poor in its fluorine content (Kodate et al, 1995). In arid and semi-arid regions, the fluorine content of groundwater is higher, compared to groundwater from humid areas. This is probably due to higher TDS in groundwater resulting in increased ionic strength and consequent higher CaF₂ solubility in groundwater.

The data reveals that safe water sources (<1.5 ppm) do co-exist with excess fluoride contaminated water within the same area. Therefore, checking fluoride in a few samples and generalising the information based on that may not be the right step to ensure safe water quality.

Fluorine content of plants, mostly cultivated plants, is generally low, except for tea, which contains upto 440 mg/l fluorine (Deshmukh et al, 1995). Sea food contains significantly higher amounts of fluorine compared to freshwater food. In animals, including man, the fluorine ingestion is primarily from drinking water but considerable amount of fluorides are also ingested through food and polluted atmosphere (Srikantia, 1977; Batra et al, 1995). It is observed that fluorine content of food items grown in fluorosis endemic areas is anomalously high and, therefore, fluoride ingestion of affected population through food is also significantly large (Jyothi Kumari et al, 1995). It is observed that fluorine intake in jowar (Sorghum julgare), wheat (Triticum aestivam), rice (Oryza sativa), red gram Dal (Cajanus cajan) and red chillies (Capsicum annuum) is in proportion to the fluorine distribution in their rooting media (Batra et al, 1995; Jyothi Kumari, 1995).

Ingesting food and consuming water containing fluoride over a period of time is likely to result in toxic manifestations. It is well recognised that consuming fluoride contaminated food or water for a period of 6 months to 1 year is adequate to have ill effects on the health. Fluorine ingestion from all sources below 2.75 mg/day in adults leads to dental caries while above 7.75 mg/day results in various types of fluorosis.

The prolonged use of drugs containing sodium fluoride is known to cause skeletal fluorosis. During 1982, two cases of drug induced skeletal fluorosis were reported from Switzerland. Patients of rheumatoid arthritis received uninterrupted and prolonged treatment with niflumic acid .

In India, a patient of dental fluorosis was prescribed fluoride mouth rinse (Proflo) to render the teeth stronger. Fluoride mouth rinse does not make a fluorosed teeth stronger. On the contrary, the patient developed severe complaints of non-ulcer dyapepsia, became severely anaemic which were early signs of fluoride toxicity. Timely intervention by withdrawal of ëprofloí mouth wash and providing adequate calcium and vitamin C prevented the warning signs of fluoride toxicity, and further deterioration towards skeletal fluorosis.

Although, considerable amount of fluoride get bound in the body tissues, some amount is excreted through sweat, urine and stool. The extent of excretion is determined by the level of different hormones and efficiency of kidney function, age of the individual, nutritional status, climate condition etc.