Prolonged exposure to high fluoride concentrations over several years increases the risk of developing crippling skeletal fluorosis. This condition is characterised by pain and stiffness in the backbone and joints, accompanied by increased bone density (osteosclerosis). In its later stages, crippling deformities of the spine and joints arise, together with neurological defects, muscle wasting and paralysis (Oszvath, 2009). Studies on occupational and endemic fluorosis have shown that the extent of the symptoms is related to the duration and level of exposure and that skeletal fluorosis is at least partially reversible over a number of years. (e.g. Grandjean, 1982; Krishnamachari, 1986; Susheela and Bhatnagar, 2000). In addition to these more obvious symptoms, there are others grouped under the term “non-skeletal fluorosis”. Non-skeletal fluorosis includes a reported decrease in cognitive capacity (measured by IQ tests), lethargy, an impaired ability to concentrate and possibly even the onset of dementia (USRC, 2006). Whether these effects could be due to enzymatic changes or impaired function of the thyroid gland is unclear. Fluoride may also disturb the endocrine system, acting as an inhibitor of secretions from the parathyroid glands, which regulate extracellular calcium and phosphate concentrations. Possible effects on the gastrointestinal, renal, hepatitic and immune systems have also been reported (USRC, 2006). There has been difficulty in proving observed health effects (in scientific studies) to be the result of elevated fluoride intake, and more rigorous epidemiological studies have been recommended.
The WHO guideline value of 1.5 mg/L might not be suitable for hot, arid areas where people have a higher daily water intake (Brouwer et al., 1988). The recommended maximal daily fluoride intake for children younger than 8 years (to prevent dental and skeletal fluorosis) is 0.1 mg/day per kg of body weight (SCSEDRI, 1997, Table 3.3). For adults, a daily intake of 14 mg leads to an excess risk of adverse skeletal fluorosis, and there is evidence for increased risk of an effect on the skeleton at an intake of 6 mg/day (Fawell et al., 2006). Not only drinking water, but also cooking water and fluoride contained in food products can contribute considerably to a person’s daily fluoride uptake (Malde et al., 2011). Skeletal effects in fluorotic areas may vary in severity depending not only on the daily fluoride uptake, but also on the intake of other essential nutrients important for bone formation, such as calcium, zinc, iron and magnesium. Deficient nutrition will therefore increase the risk of bone deformation when excess fluoride is consumed (Chakma et al., 2000).