Fluoride   has   been   found   to   have   a   significant

mitigating   effect   against   dental   caries   and   it   is

accepted  that  some  fluoride  presence  in  drinking

water   is   beneficial.   Optimal   concentrations   are

around   1   mg/l.   However,   chronic   ingestion   of

concentrations   much   greater   than   1.5   mg/l   (the

WHO  guideline  value)  is  linked  with  development

of  dental  fluorosis  and,  in  extreme  cases,  skeletal

fluorosis.  High  doses  have  also  been  linked  to

cancer.   Health   impacts   from   long-term   use   of

fluoride-bearing    water    have    been    summarised

(Dissanayake, 1991) as:

<0.5 mg/l:

dental caries

0.5-1.5 mg/l 

promotes dental health

1.5-4 mg/l

dental fluorosis

>4 mg/l

dental, skeletal fluorosis

>10 mg/l

crippling fluorosi

Smedley, Pauline

NERC Open Research Archive

1Water Quality Fact Sheet:FluorideWHO guideline value (recommended limit): 1.5 mg/lTypical range in groundwater: <0.01 mg/l to 4 mg/lHealth effectsFluoride has been found to have a significantmitigating effect against dental caries and it isaccepted that some fluoride presence in drinkingwater is beneficial. Optimal concentrations arearound 1 mg/l. However, chronic ingestion ofconcentrations much greater than 1.5 mg/l (theWHO guideline value) is linked with developmentof dental fluorosis and, in extreme cases, skeletalfluorosis. High doses have also been linked tocancer. Health impacts from long-term use offluoride-bearing water have been summarised(Dissanayake, 1991) as:<0.5 mg/l: dental caries0.5-1.5 mg/l promotes dental health1.5-4 mg/l dental fluorosis>4 mg/l dental, skeletal fluorosis>10 mg/l crippling fluorosisDental fluorosis is by far the most commonmanifestation of chronic use of high-fluoride water.As it has greatest impact on growing teeth, childrenunder age 7 are particularly vulnerable. However, itis important to note that additional factors such asnutrition are also important in determining thecourse of disease. Calcium and vitamin C deficiencyare recognised as important exacerbating factors.Food is an additional source of fluoride.Occurrence in groundwaterMost groundwaters have low or acceptableconcentrations of fluoride (<1.5 mg/l). However,some large groundwater provinces have significantconcentrations which cause prominent healthproblems. Most high-fluoride provinces occur in thedeveloping world, largely because of lack of suitableinfrastructure for treatment.The dominant controls on fluoride build-up inwater are:i) geology;ii) contact times with fluoride minerals;iii) groundwater chemical composition;iv) climate.Fluoride in water derives mainly from dissolution ofnatural minerals in the rocks and soils with whichwater interacts. The most common fluorine-bearingminerals are fluorite, apatite and micas. Fluorideproblems therefore tend to occur where the elementis most abundant in the host rocks. Groundwatersfrom crystalline rocks, especially granites areparticularly susceptible to fluoride build-up becausethey often contain abundant fluoride-bearingminerals. Alkaline granites (deficient in calcium)present a special problem, as in East Africa. Inactive volcanic terrains, fluoride in groundwater mayalso derive from mixing with fluids from hot springsand volcanic gases, which can containconcentrations of several tens to hundreds ofmilligrams per litre. Some sandstones have very lowconcentrations of fluorine and hence residentgroundwaters may also be low.Reaction times with aquifer minerals are alsoimportant. High fluoride concentrations can be builtup in groundwaters which have long residence timesin the host aquifers. Surface waters usually have lowconcentrations, as do shallow groundwaters fromhand-dug wells as they represent young, recentlyinfiltrated, rainwater. Deeper (older) groundwatersfrom tubewells are most likely to contain highconcentrations of fluoride. Exceptions can occurlocally in active volcanic areas where surface waterand shallow groundwaters can have highconcentrations due to hydrothermal inputs.This is one of a series of information sheets prepared for a limited number of inorganic constituents ofsignificant health concern that are commonly found in groundwater. The sheets aim to explain the nature ofthe health risk for each constituent, the origin and occurrence in groundwater, the means of testing andavailable methods of mitigation. The purpose of the sheets is to provide guidance to WaterAid CountryOffice staff on targeting efforts for water-quality testing and to encourage further thinking in theorganisation on water-quality issues.2High fluoride concentrations are also a feature ofarid climatic conditions. Here, groundwater flow isslow and reaction times between water and rocksare therefore enhanced. Fluoride build-up is lesspronounced in the humid tropics because of highrainfall inputs and their diluting effect ongroundwater chemical composition.High-fluoride groundwaters typically (though notalways) have sodium and bicarbonate as thedominant dissolved constituents, with relatively lowcalcium and magnesium concentrations. Such watertypes also generally have high pH values (>7) andthese can be useful proxy indicators of potentialproblems. Bicarbonate (alkalinity) and pH can bereadily measured in the field.High-fluoride groundwaters are found in many partsof the developing world, and many millions ofpeople rely on groundwater with concentrationsabove the WHO guideline value. Worst-affectedareas are arid parts of northern China (InnerMongolia), India, Sri Lanka, West Africa (Ghana,Ivory Coast, Senegal), North Africa (Algeria), SouthTable 1. Removal methods for fluoride from drinking water (after Solsona, 1985; Heidweiller, 1990)RemovalmethodCapacity/doseWorkingpHInterferences Advantages Disadvantages RelativeCostPrecipitationAlum(aluminiumsulphate)150 mg/mg FNon-specific - EstablishedprocessSludge produced,treated water is acidic,residual Al presentMed-highLime 30mg/mg F Non-specific - EstablishedprocessSludge produced,treated water is alkalineMed-highAlum+lime(‘Nalgonda’)150 mgalum+ 7mglime/mg FNon-specific,optimum 6.5- Low-tech,establishedprocessSludge produced,high chemical dose,residual Al presentMed-highGypsum +fluorite5 mg gypsum+ <2 mgfluorite/mg FNon-specific - Simple Requires trained operatorsLow efficiency, high residualCa, SO4Low-medAdsorption/ion exchangeActivatedcarbonVariable <3 Many - Large pH changes beforeand after treatmentHighPlant carbon 300 mg F/kg 7 - Locally available Requires soaking inpotassium hydroxideLow-medZeolites 100 mg F/kg Non-specific - Poor capacity HighDefluoron 2 360 g F/m3 Non-specific Alkalinity Disposal of chemicals usedin resin regenerationMediumClay pots 80 mg F/kg Non-specific - Locally available Low capacity, slow LowActivatedalumina1200 g F/m3 5.5 Alkalinity Effective, well-establishedNeeds trained operators,chemicals not always availableMediumBone 900 g F/m3 >7 Arsenic Locally available May give taste; degeneratesNot universally acceptedLowBone char 1000 g F/m3 >7 Arsenic Locally availableHigh capacityNot universally accepted LowOtherElectrodialysis High Non-specific Turbidity Can removeother ions. Usedfor high salinitySkilled operatorsHigh cost.Not much usedVery highReverseosmosisHigh Non-specific Turbidity Can removeother ions. Usedfor high salinitySkilled operatorsHigh costVery high3Africa, East African Rift (Kenya, Uganda, Tanzania,Ethiopia), northern Mexico and central Argentina.In the early 1980s, it was estimated that around260 million people worldwide (in 30 countries) weredrinking water with more than 1 mg/l of fluoride(Smet, 1990). In India alone, endemic fluorosis isthought to affect around 1 million people (Teotia etal., 1981) and is a major problem in 17 out of thecountry’s 22 states, especially Rajastan, AndraPradesh, Tamil Nadu, Gujarat and Uttar Pradesh. InSri Lanka, fluoride problems have a stronggeographical control linked to climatic conditions,with high-fluoride waters being restricted to the DryZone on the eastern side of the island (Dissanayake,1991).Field testing for fluorideSimple fluoride analysis can be carried out bycolorimetry or ion-selective electrode. Low-cost,pocket colorimeters are available for field testing offluoride and can be supplied as kits with reagentsolutions. Alternatively, the ion-selective electrode isa rapid and accurate test of free fluorideconcentrations and also requires relatively littleequipment (fluoride and reference electrodes, ionmeter, standard solutions). It is not strictly a fieldtest technique as the electrode requires pre-calibration using known standard solutions andsamples are therefore best analysed in batches in alaboratory.Remediation techniquesMany methods of fluoride removal using variousmedia have been tried and are established practice.Some of the common ones are listed in Table 1.Most low-technology methods rely on precipitationor adsorption/ion-exchange processes. Probably themost well-known and established method is theNalgonda technique, commonly used in India,where a combination of alum (or aluminiumchloride) and lime (or sodium aluminate), togetherwith bleaching powder, are added to high-fluoridewater, stirred and left to settle. Fluoride issubsequently removed by flocculation,sedimentation and filtration. The method can beused at domestic scale (in buckets) or communityscale (fill-and-draw type defluoridation plants;Nawlakhe and Bulusu, 1989). It has moderate costsand uses materials which are usually easily available.Other precipitation methods include the use ofgypsum, dolomite or calcium chloride. Mostmethods tested (except gypsum) are capable inprinciple of reducing fluoride in treated water tobelow the WHO guideline value.The most common ion-exchange removal methodstested are activated carbon, activated alumina, ion-exchange resins (e.g. Defluoron 2), plant carbon,clay minerals, crushed bone or bone char. Activatedalumina and bone materials are among the mosteffective appropriate-technology removal methods(with highest removal capacity, Table 1). These alsohave drawbacks however: activated alumina may notalways be available or affordable and bone productsare not readily acceptable in some cultures.Other highly efficient methods of removal includeelectrodialysis and reverse osmosis. These tend tobe higher technology and higher cost methods(Table 1) and are therefore less suitable for manyapplications in developing countries.Most methods designed for village-scale fluorideremoval have some drawbacks in terms of removalefficiency, cost, local availability of materials,chemistry of resultant treated water and disposal oftreatment chemicals. Local circumstances willdictate which methods, if any, are the mostappropriate.Alternative mitigationIn practice, remediation techniques meet withvarying degrees of success, depending on efficacy,user acceptance, ease of maintenance, degree ofcommunity participation, availability and cost of rawmaterials. Alternative methods of water-qualityimprovement can potentially be afforded by carefultubewell siting and groundwater management.Factors worth considering in tubewell siting arelocal geology and variations in groundwater fluorideconcentration with depth. Management includesconsideration of optimum pumping rates (especiallywhere there exists the possibility of mixing ofgroundwater with deep hydrothermal solutions,enhanced at high pumping rates), and possibilitiesfor artificial re-charge of low-fluoride surface water.Data sourcesDissanayake, C. B. 1991. The fluoride problem inthe groundwater of Sri Lanka – environmentalmanagement and health. Intl. J. Environ. Studies,19, 195-203.Heidweiller, V. M. L., 1990. Fluoride removalmethods. In: Proc. Symposium on Endemic Fluorosisin Developing Countries: Causes, Effects and PossibleSolutions, ed: Frencken, J. E., Chapter 6, NIPG-TNO, Leiden, pp51-85.Nawlakhe, W. G. and Bulusu, K. R. 1989. Nalgondatechnique – a process for removal of excessfluoride from water. Water Quality Bull., 14, 218-220.Solsona, F. 1985. Water defluoridation in the RiftValley, Ethiopia. UNICEF Technical Report,Addis Ababa, 27 pp.4Smet, J. 1990. Fluoride in drinking water. In: Proc.Symposium on Endemic Fluorosis in DevelopingCountries: Causes, Effects and Possible Solutions, ed:Frencken, J. E., Chapter 6, NIPG-TNO, Leiden,pp51-85.Teotia, S. P. S., Teotia, M. and Singh, R. K. 1981.Hydrogeochemical aspects of endemic skeletalfluorosis in India – an epidemiological study.Fluoride, 14, 69-74.British Geological Survey 2000© NERC 2000

Water quality fact sheet: Fluoride

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Water quality fact sheet: Fluoride

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