Health Stream Article - Issue 44 December 2006

Health Stream Article - Issue 44 - December 2006

WHO Fluoride Monograph

The World Health Organisation (WHO) has released a monograph on Fluoride in Drinking-water to provide member countries with detailed information and guidance on prevention of adverse health effects from excessive levels of fluoride in water. The publication incudes chapters on the occurrence and geochemistry of fluoride in the environment, the sources of human exposure, the evidence for adverse health effects, and application of the current WHO guideline value to local conditions. There is an extensive section on methods used to remove excess fluoride from drinking water, and a description of five commonly used analytical methods for fluoride determination. The final chapter summarises the available data from 28 countries on dental and skeletal fluorisis associated with exposure to fluoride through drinking water. Indices for scoring dental fluorisis are described in an Appendix.

The element fluorine is highly electronegative and exists in nature predominantly as the negatively charged fluoride ion (F - ) which forms complexes with a number of cations. Fluorides are a component of many low solubility minerals including fluorspar, rock phosphate, cryolite, apatite, mica, and hornblende. Fluoride is also associated with volcanic activity and geothermal waters. The concentration of fluoride in seawater is about 1 mg/L while surface waters generally have levels below 0.5 mg/L. The concentration of fluoride in groundwater supplies varies enormously depending on the geological characteristics of the aquifer and the presence of other minerals such as calcium which may limit fluoride solubility. The highest levels reported in groundwaters serving as drinking water sources are around 50mg/L.

Other sources of human exposure to fluoride include air, dental products, and foods and beverages. In non-industrial areas, concentrations in air are generally low. However, industrial production of phospate fertilisers, dust from high fluoride minerals, volcanic activity and burning of high fluoride coal in both industrial and domestic settings make air a significant exposure route in some regions of the world. Dental products such as fluoridated toothpaste, topical treatments and tablets contribute to total exposure where these products are used. Fruit and vegetables generally contain low levels of fluoride (less than 0.4 mg/Kg), although somewhat higher levels have been reported in barley, rice, taro, yams and cassava. Fish (2-4 mg/Kg) contains more fluoride than meat (up to 1 mg/Kg). Both human milk (0.02 mg/L) and cow's milk (0.02-0.05 mg/L) have very low fluoride levels. Western-style diets appear to be relatively low in fluoride but significant exposures from dietary sources may occur in some regions due to local dietary customs or airborne exposures that contaminate crops. The overall composition of the diet may affect fluoride excretion, with a high protein diet reported to result in greater retention of fluoride in the body. There is also some evidence that a nutrient deficient diet is associated with higher levels of both dental and skeletal fluorosis. In most circumstances drinking water is the largest single contributor to total fluoride intake. WHO has previously produced a rough estimate that total daily exposure for adults in a temperate climate without fluoridated water is about 0.6 mg/day, while with fluoridated water the daily intake would be about 2 mg/day.

Ingested fluoride is absorbed from both the stomach and the gut, with the amount available for absorption being affected by cations such as aluminium, calcium and magnesium which form insoluble complexes with fluoride. Fluoride is retained primarily in the bone tissue and the teeth. Fluoride is not considered to be carcinogenic, and early reports of adverse developmental or reproductive effects in animals have not been confirmed by recent studies. Acute poisoning episodes have been associated with overdosing of fluoride in water supplies at levels of 30 mg/L or higher. Adverse effects on teeth and bone are considered to be the most significant effect of chronic exposure to excessive levels of fluoride. Dental fluorosis occurs when children are exposed to excessive levels of fluoride at the time when teeth are forming. The effects can range from mild forms which are not cosmetically significant to severe pitting and discolouration of the teeth. Skeletal fluorosis also ranges from a mild form which is detectable only on X-ray to severe cases where the effects on bone structure are crippling.

The WHO Guideline Value of 1.5 mg/L for fluoride was set in 1984 and confirmed by subsequent reviews of the scientific evidence in 1996 and 2004. This level was set to confer a protective effect against dental decay but avoid significant amounts of dental fluorosis. Clinically significant skeletal fluorosis is believed to occur only with chronic ingestion of water with fluoride levels of 10 mg/L or higher. The guideline is not intended as a universally applicable value; rather it should be modified in the light of local knowledge about water consumption, exposure to other significant sources of fluoride, and other factors such as altitude that are known to affect fluoride retention in the body.

Fluoride can be removed from water by some filtration media and other water treatment methods, however successful application of these interventions requires technical skills that may be difficult to provide and maintain in rural and remote locations. Therefore as a first option for reducing fluoride exposure the possibility of using alternative drinking water sources or blending with low fluoride supplies should be explored. The monograph describes five water treatment methods that are considered to be potentially suitable for developing countries. These methods are based on three basic treatment processes:

• sorption media such as bone charcoal, activated alumina and clay are generally used in the form of packed columns through which water is passed. The media eventually become saturated with fluoride and need to be replaced or regenerated.

• co-precipitation uses chemicals such as aluminium sulphate and lime, or polyaluminium chloride and lime to remove fluoride in a daily batch treatment process. These methods generate sludge which must be disposed of.

• contact precipitation chemicals such as calcium or phophate compounds are added to the water prior to passing through a catalytic filter bed causing the precipitation of calcium fluoride or fluorapatite.

Advanced methods such as reverse osmosis, electrodialysis and distillation, and use of patented media are not considered in the monograph due to their high cost and technical demands.

While there are a number of documented case studies where the relatively low-tech treatments described in the monograph have been successfully applied, there are also many instances where such schemes have failed due to lack of social acceptability, high cost, and inability or failure of users to maintain equipment. Careful consideration is needed to address specific circumstances at the local level including water quality factors, social acceptability, proper system design and knowledge of responsible officials, ensuring availability and affordability of media and spare parts, and provision of continuing efforts to motivate and train users of the system.

High fluoride concentrations occur in groundwaters in three geological settings; in the presence of sediments of marine origin in mountainous areas, in strata of volcanic origin, and in granite and gneissic rocks. Elevated fluoride levels in groundwater occur in areas of Africa, China , the eastern Mediterranean, southern Asia, the Americas and Japan .

In China it has been estimated that over 26 million people have dental fluorosis as a result of excess fluoride in drinking water while a further 16.5 million people have developed dental fluorosis as a result of pollution from burning high-fluoride coal. About two million people in China are believed to be suffering from skeletal fluorosis with about half of the cases attributable to drinking water and half to coal. Fluoride of volcanic origin affects large areas of several African countries lying in the East African Rift system, and endemic fluorosis occurs in more than half of India 's 32 states. In the developed world, excessive fluoride levels in groundwater have been most extensively documented in the United States where significant dental fluorosis was once widespread in many states. Indeed it was during investigations of dental fluorosis that the link between low levels of fluoride in drinking water and protection from dental caries was first recognised. In the developing world, skeletal fluorosis remains a significant cause of illness, and further efforts are required to reduce population exposures.

Fluoride in Drinking-water (2006) Fawell J et al. ISBN 92 4 156319 2 WHO Press and ISBN 1900222965 (IWA Publishing). Available from the WHO website: