The United States' lead in instituting artificial water fluoridation led to its acceptance by the World Health Organization as an effective oral health intervention. At least 30 nations instituted artificial water fluoridation policies. However, a number of countries including Sweden, The Netherlands, Germany, and Switzerland stopped fluoridating their water supplies due to concerns about safety and effectiveness [
8,
9].
Currently, only about 5% of the world's population—350 million people—(including 200 million Americans) consume artificially fluoridated water globally. Only eight countries—Malaysia, Australia, USA, New Zealand, Singapore, and Ireland, more than 50% of the water supply artificially fluoridate. Over the past two decades many communities in Canada, the USA, Australia, and New Zealand have stopped fluoridating their water supplies and in Israel the Minister for Health announced in April 2013 the end of mandatory water fluoridation. However, public health authorities continue to try and develop new community water fluoridation schemes.
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The inability to control individual dose renders the notion of an “optimum concentration” obsolete.
In the USA, a study in Iowa found that 90% of 3-month-olds consumed over their recommended upper limits, with some babies ingesting over 6 mg of fluoride daily, above what the Environmental Protection Agency and the WHO say is safe to avoid crippling skeletal fluorosis [41]. Most recently a study in the UK of fluoride levels found in tea concluded that “… fluoride concentrations can exceed the recommended DRI of 4 mg/day…, in certain tea commodities, under the minimal brewing time of 2 min…” [42, page 569]. This study used nonfluoridated water but supports earlier findings by Koblar et al. who report that the adequate intake of fluoride from a 70 kg adult consuming five cups of tea daily ranges from 25 to 210% depending upon tea brand and whether the water is fluoridated [
43].
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In a meta-analysis of 27 mostly China-based studies on fluoride and neurotoxicity, researchers from Harvard School of Public Health and China Medical University in Shenyang found strong indications that fluoride may adversely affect cognitive development in children [50]. All but one study suggested that high fluoride content in water may negatively affect cognitive development. The average loss in intelligence quotient (IQ) was reported as a standardized weighted mean difference of 0.45, which would be approximately equivalent to seven IQ points for commonly used IQ scores with a standard deviation of 15 [
50]. While fluoride's effect on IQ in this meta-analysis did not reach statistical significance, the combined effect at population level is remarkable. A particular concern of the NRC committee was the impact of ingested fluoride on the thyroid gland [
49]. In a 2005 study, it was found that 47% of children living in a New Delhi neighbourhood with average water fluoride level of 4.37 ppm have evidence of clinical hypothyroidism attributable to fluoride. They found borderline low FT
3levels among all children exposed to fluoridated water [
51].
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There are 66 enzymes which are affected by fluoride ingestion, including P450 oxidases, as well the enzyme which facilitates the formation of flexible enamel [
65]. A recent study of the effects of inorganic fluoride compounds on human cellular functions revealed that fluoride can interact with a wide range of enzyme-mediated cellular processes and genes modulated by fluoride including those related to the stress response, metabolic enzymes, the cell cycle, cell-cell communications, and signal transduction [
66].
Due to high negativity of fluoride, it interacts actively with positively charged ions such as calcium and magnesium. In industrial settings, hydrofluoric acid poisoning is usually treated with intravenous calcium gluconate as such poisoning is associated with acute hypocalcaemia [67]. As with calcium, magnesium plays important roles in optimal bone and teeth formation. By competing with magnesium and calcium in teeth and bones, fluoride deranges the delicate bone formation and bone resorption processes. Such derangements, and consequent intensity of fluoride's adverse effects on bone and teeth, are amplified in malnutrition, calcium deficiency, and magnesium deficiency [
68,
69]. Chronic fluoride ingestion is commonly associated with hyperkalaemia and consequent ventricular fibrillation [
70].
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