1 Introduction

Background

Fluoride is not considered to be essential for human growth and development but it is considered to be beneficial in the prevention of dental caries (tooth decay). As a result, intentional fluoridation of drinking water and the development of fluoride containing oral care products (toothpastes and mouth rinses), foods (fluoridated salts) and supplements (fluoride tablets) have been employed since the early 20th century in several parts of the world as a public health protective measure against tooth decay. Additional exposure to fluoride comes from naturally occurring water (tap and mineral), beverages, food, and to a lesser extent, from other environmental sources.

A body of scientific literature seems to suggest that fluoride intake may be associated with a number of adverse health effects. dental fluorosis and effects on bones (increased fragility and skeletal fluorosis) are two well documented adverse effects of fluoride intake. Systemic effects following prolonged and high exposure to fluoride have also been reported and more recently effects on the thyroid, developing brain and other tissues, and an association with certain types of osteosarcoma (bone cancer) have been reported.

Individual and population exposures to fluoride vary considerably and depend on the high variability in the levels of fluoride found in tap (be it natural or the result of intentional fluoridation of drinking water) and mineral waters, and on individual dietary and oral hygiene habits and practices. The emerging picture from all risk assessments conducted on fluoride is that there exists a narrow margin between the recommended intakes for the prevention of dental caries and the upper limits of exposure. Invariably, all assessments to-date call for continued monitoring of the exposure of humans to fluoride from all sources and an evaluation of new scientific developments on its hazard profile.

Exposure assessment was conducted in the most recent evaluations by the European Food Safety Authority (EFSA), setting upper tolerable intake levels (UL) related to concentration limits for fluoride in natural mineral waters (EFSA 2005) and on calcium fluoride and sodium monofluorophosphate as a source of fluoride (EFSA 2008a, EFSA 2008b), and by the Commission Scientific Committee on Consumer Products (fluoride in dental care products (SCCP 2009)). A similar approach was taken by the United States National Academies of Science in its 2006 review of the United States Environmental Protection Agency’s water standards for fluoride (NRC 2006).

There is a continuous controversy over the benefit of fluoride and, in particular, the practices of intentional water fluoridation in tooth decay prevention. This has led to several countries discontinuing drinking water fluoridation and others expanding it.

Besides questioning the practice of intentional water fluoridation itself as being unnecessary or superfluous in the light of the high exposure to fluoride from other sources, opponents of water fluoridation have pointed to reports showing that the health and environmental risks of the most commonly used fluoridating agents, silicofluorides (e.g. (hydro)fluorosilicic acid, sodium silicofluoride, disodium hexafluorosilicate or hexafluorosilicate or hexafluorosilicic acid), have not been properly assessed. Furthermore, they suggest that the presence of these chemicals in drinking water may cause adverse effects on the health of humans and exert possible exacerbating effects on fluoride disposition in bone.
The debate over water fluoridation has prompted several questions from the European Parliament, from Ireland and the United Kingdom where intentional water fluoridation is still practiced.

In order to obtain updated advice on the issue, the Commission considers it necessary to seek the advice of its Scientific Committee on Health and Environmental Risks (SCHER) who should work in close collaboration with the Scientific Committee on Consumer Products (SCCP), EFSA’s panel on dietetic products, nutrition and allergies (EFSA NDA) and EFSA’s panel on contaminants in the food chain (EFSA CONTAM) who have previously delivered opinions on fluoride.

In the preparation of this opinion, SCHER considered research articles and reviews published in peer-reviewed journals, reports from regulatory agencies and other organizations, as well as all papers submitted by different stakeholders following a public call on the internet for submission of relevant scientific information. The preliminary opinion was published for public consultation for a period of three months; it was discussed at a public hearing, and additional material was received. The scientific information available to the committee was evaluated using the weight-of-evidence approach developed by the EU Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). In general, the health risks of fluoridation of drinking water have been investigated within different areas such as epidemiologic studies, experimental studies in humans, experimental studies in animals, and cell culture studies. A health risk assessment evaluates the evidence within each of these areas and then weighs together the evidence across the areas to produce a combined assessment. The general rules of the weight-of-evidence approach were used to evaluate the documents on which the opinion is based.

The Scientific Committee on Health and Environmental Risks (SCHER) is requested to:

1. Taking into consideration the SCCP opinion of 20.09.05(SCCP2005) on the safety of fluorine compounds in oral hygiene products, the EFSA NDA opinion of 22.2.05 on the Tolerable Upper Intake Level of Fluoride, and the EFSA CONTAM panel opinion of 22.06.05,

a. Critically review any information that is available in the public domain on the hazard profile and epidemiological evidence of adverse and/or beneficial health effects of fluoride. In particular the Committee should consider evidence that has become available after 2005, but also evidence produced before which was not considered by the SCCP and EFSA panels at the time.

b. Conduct an integrated exposure assessment for fluoride covering all known possible sources (both anthropogenic and natural). In doing so, and in the case of uncertainties or lack of actual exposure data, the SCHER is requested to conduct a sensitivity analysis that includes a range of possible exposure scenarios (e.g. sources, age group), and describe using appropriate quantitative or qualitative means the weight-of-evidence behind each scenario, the uncertainties surrounding each scenario, and the probability of it occurring in real life.

c. On the basis of its answers above, the SCHER is also asked:

c1 – To evaluate the evidence of the role of fluoride in tooth decay prevention and rank the various exposure situations as to their effectiveness in offering a potential tooth decay preventive action.

c2 – To make a pronouncement as to whether there may be reasons for concern arising from the exposure of humans to fluoride and if so identify exposure scenarios that may give rise to particular concern for any population subgroup.

d. Identify any additional investigative work that needs to be done in order to fill data gaps in the hazard profile, the health effects and the exposure assessment of fluoride.

2. Assess the health and environmental risks that may be associated with the use of the most common drinking water fluoridation agents, silicofluorides (e.g. (hydro)fluorosilicic acid, sodium silicofluoride, disodium hexafluorosilicate or hexafluorosilicate or hexafluorosilicic acid), taking into account their hazard profiles, their mode of use in water fluoridation, their physical chemical behaviour when diluted in water, and the possible adverse effects they may have in exacerbating fluoride health effects as reported in some studies.

Scientific Rationale

Fluoride, whether naturally present or intentionally added to water, food, consumer and medical products, is considered beneficial to prevent dental caries (tooth decay). However, the cause of dental caries is multi-factorial, and the causal factors include microorganisms in dental plaque, fermentable carbohydrates (particularly sucrose), time, the individual’s health status and level of oral hygiene, which depends on socioeconomic and educational status.

Fluorides are ubiquitous in air, water and the lithosphere. Fluorine as an element is seventh in the order of frequency of occurrence, accounting for 0.06-0.09% of the earth’s crust and occurs as fluoride, e.g. cryolite (Na3AlF6). Cryolite (used for the production of aluminium) and rock phosphates (used for the production of fertilizers) have fluoride contents up to 54%. Most of this fluoride is insoluble and not biologically available. Availability of fluoride from soil depends on the solubility of the compound, the acidity of the soil and the presence of water. Fluoride has been detected in the ash from the Icelandic volcano eruption, but EFSA has concluded that based upon available information, the potential risk posed by the fluoride for human and animal health through food and feed is not considered to be of concern in the EU.

The concentration of fluoride in ground water in the EU is generally low, but there are large regional differences due to different geological conditions. Surface water usually has lower fluoride contents than ground water (most often below 0.5 mg/L) and sea water (between 1.2 and 1.5 mg/L). There are no systematic data on the concentration of fluoride in natural drinking water in EU Member States, but rudimentary data show large variations between and within countries, e.g. Ireland 0.01-5.8 mg /L, Finland 0.1- 3.0 mg/L, and Germany 0.1-1.1 mg/L.

Bottled natural mineral water is increasingly being used as a major source of water for drinking. A large variation in the level of fluoride has been observed reaching up to 8 mg/L (EFSA 2005). Commission Directive 2003/40/EC of 16th May 2003 establishing the list, concentration limits and labelling requirements for the constituents of natural mineral waters and the conditions for using ozone-enriched air for the treatment of natural mineral waters and spring waters requires that waters which contain more than 1.5 mg/L must be labelled as not suitable for the regular consumption by infants and children under 7 years of age and that by 1st January 2008, natural mineral waters shall, at the time of packaging, comply with the maximum concentration limit set out in Annex I for fluorides of 5 mg/L.

WHO established a guidance value for naturally occurring fluoride in drinking water of 1.5 mg/L based on a consumption of 2 L water/day, and recommended that artificial fluoridation of water supplies should not exceed the optimal fluoride levels of 1.0 mg/L (WHO 2006). In Europe, only Ireland and selected regions in the UK and Spain currently fluoridate drinking water at concentrations ranging from 0.8 to 1.2 mg/L (Mullen 2005). The Council Directive 98/83/EC of 3rd November 1998 (Council Directive 98/83/EC) determined a fluoride level (both natural and as a result of fluoridation) for water intended for human consumption of less than 1.5 mg/L. Recently, the US Department of Health and Human Services recommended a fluoride level in water of 0.7 mg/L “to balance the benefit of preventing tooth decay while limiting any unwanted health effects” (http://www.hhs.gov/news/press/2011pres/01/20110107a.html).The parametric value refers to the residual monomer concentration in the water as calculated according to specifications of the maximum release from the corresponding polymer in contact with the water.

Fluoride intake from food is generally low, except when food is prepared with fluoridated water or salt. However, some teas (e.g. Camellia sinensis) represent a significant source of fluoride intake. Fruit and vegetables, milk and milk products, bread and cereals contain between 0.02-0.29 mg/kg (EFSA 2005). Recently, EFSA (2008a, 2008b) has permitted CaF2 and Na2PO3F as a source of fluoride in food supplements.

Dental products (toothpaste, mouthwashes and gels) contain fluoride at different concentrations up to 1,500 mg/kg (1,500 ppm). The mean annual usage of toothpaste in EU Member States in 2008 was 251 mL (range 130-405 mL) per capita. The extent of systemically available fluoride from toothpaste depends on the percentage of toothpaste swallowed per application.

Fluoride is widely distributed in the atmosphere, originating from the dust of fluoride containing soils, industry and mining activities, and the burning of coal. The fluoride content in the air in non-industrialized areas has been found to be low and is not considered to contribute more than 0.01 mg/day to the total intake.

An upper tolerable intake level (UL) of 0.1 mg/kg BW/day for fluoride has been derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) (EFSA 2005) based on a prevalence of less than 5% of moderate dental fluorosis in children up to the age of 8 years as the critical endpoint, i.e. 1.5 mg/day for children 1-3 years of age, and 2.5 mg/day for children aged 4-8 years. For adults, an UL of 0.12 mg/kg BW/day was based on a risk of bone fracture, which converts on a body weight basis into 7 mg/day for populations aged 15 years and older, and 5 mg/day for children 9-14 years of age.

Tolerable upper intake levels for fluoride have not been established for infants. For infants up to 6 months old, the UK Department of Health (UK DoH 1994) concluded that 0.22 mg F/kg BW/day was safe.

Several pathologies have been linked to high levels of fluoride exposure but are mostly based upon circumstantial evidence. Thus, this opinion will focus on fluorosis of teeth and bones, osteosarcoma, neurotoxicity and reprotoxicity.

3 What happens to fluoride in your body?

The SCHER opinion states:

Hexafluorosilicic acid and hexafluorosilicates are the most commonly used agents in drinking water fluoridation and it has been claimed that incomplete dissociation of these agents in drinking water may result in human exposure to these chemicals. The toxicology of these compounds is incompletely investigated. Recent studies have addressed the equilibrium of the free fluoride ion and fluorosilicate species in aqueous solutions over a wide concentration and pH range. In the pH-range and at the concentrations of hexafluorosilicates/fluoride relevant for drinking water, hydrolysis of hexafluorosilicates to fluoride was rapid and the release of the fluoride ion was essentially complete. Residual fluorosilicate intermediates were not observed by sensitive 19F-NMR. Other hydrolysis products of hexafluorosilicate such as Si(OH)4 are rapidly transformed to colloidal silica (Finney et al. 2006). Si(OH)4 is present naturally in drinking water in large quantities and is not considered a risk. In summary, these observations suggest that human exposure to fluorosilicates due to the use of hexafluorosilicic acid or hexafluorosilicate for drinking water fluoridation, if any, is very low as fluorosilicates in water are rapidly hydrolyzed to fluoride, as illustrated in the following equation:
H 2 SiF6 ( aq ) + 6OH ? ( aq ) ? 6 F ? ( aq ) + Si( OH )4 ( aq ) + 2 H 2 O( l ) Studies on Na2SiF6 and H2SiF6, compounds used to fluoridate drinking water, show a pharmacokinetic profile for fluoride identical to that of sodium fluoride (NaF) (Maguire et al. 2005, Whitford et al. 2008). It therefore seems unlikely that the rate and degree of absorption, fractional retention, balance and elimination of fluoride will be affected if these fluoride compounds are added artificially in low concentrations, or if fluoride is naturally present in drinking water.

Hexafluorosilicic acids used as fluoridating agents may contain some impurities. Concerns have been raised about several heavy metals present as low-concentration impurities in commercial hexafluorosilicic acid. The average concentrations of arsenic, mercury, lead and cadmium present in hexafluorosilicic acid are low – between 10 and 400 mg/kg H2SiF6 (CEN 12175-2006). Therefore, fluoridation of drinking water only contributes to a limited extent to the total exposure to these contaminants (expected drinking water concentrations are between 3.0 and 16.2 ng/L). These calculated concentrations are at least two orders of magnitude below drinking water guideline values for these metals established by WHO and other organizations, and therefore are not regarded as an additional health risk.

It has been claimed that fluoridated drinking water increases human exposure to lead due to solubilisation of lead from drinking water pipes by formation of highly soluble lead complexes. The claim was based on relationships of drinking water fluoridation and blood lead concentrations observed in a case study (Coplan et al. 2007).

Based on the available chemistry of fluoride in solution, the chemistry of lead and lead ions, and the concentrations of fluoride in tap water, it is highly unlikely that there would be an increased release of lead from pipes due to hexafluorosilicic acid. The added concentrations of hexafluorosilicic acid do not influence the pH of tap water, and do not form soluble lead complexes at the low concentrations of hexafluorosilicic acid present in the gastrointestinal tract after consumption of fluoridated drinking water (Urbansky and Schock 2000).

Physico-chemical properties

the main substance of concern is the fluoride ion (F-) and therefore the identification and the physico-chemical properties of sodium fluoride (NaF) given in Table 1 are considered applicable.

Table 1: Main physico-chemical properties of sodium fluoride (NaF). SCHER agreed to use these physico-chemical properties where relevant in this opinion.

Substance – Sodium fluoride
Elemental symbol – NaF
Ionic form – Na+, F-
CAS-number 7681-49-4
EINECS-number 231-667-8
Molecular weight (M) 42 g/mol (Na: 23; F: 19)
Melting point (MP) ca. 1,000°C
Boiling point (BP) 1,700°C
Vapour pressure (VP) 133 Pa at 1077°C
Vapour pressure at 25°C (VP) 1.97E-5 Pa (conversion by EUSES)
Water solubility (WS) 40,000 mg/L at 20°C
Water solubility at 25°C (WS) 42,900 mg/L (conversion by EUSES)
Octanol-water partition (log Kow) Not appropriate
Henry’s Law constant (H) 1.93E-8 Pa.m3/mol (calculation by EUSES)
Sorption capacity (Kd) 0.0006–0.03 dm3/kg (estimation) (Bégin et al. 2003) (see 3.1)
Removal rate (R) 1.39E-06 d-1 at 12°C (default)
Bioconcentration factor (BCF) Not relevant

Oral uptake

In humans and animals, ingested fluoride occurs as hydrogen fluoride (HF) in the acidic environment of the stomach and is effectively absorbed from the gastrointestinal tract, although there is no proved absorption from the oral cavity. Peak plasma levels are typically seen within 30–60 minutes after ingestion. Highly soluble fluoride compounds, such as NaF present in tablets, aqueous solutions and toothpaste are almost completely absorbed, whereas compounds with lower solubility, such as CaF2, MgF2, and AlF3, are less well absorbed. Ingestion of fluoride with milk or a diet high in calcium will decrease fluoride absorption.

Dermal absorption

No experimental data on the extent of dermal absorption of fluoride from dilute aqueous solutions are available. As fluoride is an ion it is expected to have low membrane permeability and limited absorption through the skin from dilute aqueous solutions at near neutral pH (such as water used for bathing and showering). This exposure pathway is unlikely to contribute to the fluoride body burden.

Inhalation

No systematic experimental data on the absorption of fluoride after inhalation are available. A few older occupational studies have shown uptake of fluoride in heavily exposed workers from fluoride-containing dusts, but it is unlikely that inhalation exposure will contribute significantly to the body burden of fluoride in the general population.

Fluoride distribution, metabolism and excretion

Once absorbed, fluoride is rapidly distributed throughout the body via the blood. The short term plasma half-life is normally in the range of 3 to 10 hours. Fluoride is distributed between the plasma and blood cells, with plasma levels being twice as high as blood cell levels. The saliva fluoride level is about 65% of the level in plasma (Ekstrand 1977). Plasma fluoride concentrations are not homeostatically regulated, but rise and fall according to the pattern of fluoride intake. In adults, plasma fluoride levels appear to be directly related to the daily exposure of fluoride. Mean plasma levels in individuals living in areas with a water fluoride concentration of 0.1 mg/L or less are normally 9.5 ?g /L, compared to a mean plasma fluoride level of 19-28.5 ?g/L in individuals living in areas with a water fluoride content of 1.0 mg/L. In addition to the level of chronic fluoride intake and recent intake, the level of plasma fluoride is influenced by the rates of bone accretion and dissolution, and by the renal clearance rate of fluoride. Renal excretion is the major route of fluoride removal from the body. The fluoride ion is filtered from the plasma by the glomerulus and then partially reabsorbed; there is no tubular secretion of fluoride. Renal clearance rates of fluoride in humans average at 50 mL/minute. A number of factors, including urinary pH, urinary flow, and glomerular filtration rate, can influence urinary fluoride excretion. There are no apparent age related differences in renal clearance rates (adjusted for body weight or surface area) between children and adults. However, in older adults (more than 65 years of age), a significant decline in renal clearance of fluoride has been reported consistent with the age-related decline in glomerular filtration rates.

Approximately 99% of the fluoride in the human body is found in bones and teeth. Fluoride is incorporated into tooth and bone by replacing the hydroxyl ion in hydroxyapatite to form fluorohydroxyapatite. The level of fluoride in bone is influenced by several factors including age, past and present fluoride intake, and the rate of bone turnover. Fluoride is not irreversibly bound to bone and is mobilized from bone through bone remodelling.

Soft tissues do not accumulate fluoride, but a higher concentration has been reported for the kidney due to the partial re-absorption. The blood-brain barrier limits the diffusion of fluoride into the central nervous system, where the fluoride level is only about 20% that of plasma. Human studies have shown that fluoride is transferred across the placenta, and there is a direct relationship between fluoride levels in maternal and cord blood. In humans, fluoride is poorly transferred from plasma to milk. The fluoride concentration in human milk is in the range of 3.8–7.6 ?g/L.

What are the possible health effects of fluoridation, and what is the latest evidence?

The SCHER opinion states:

A number of mechanisms are involved in the toxicity of fluoride to bone. Fluoride ions are incorporated into bone substituting hydroxyl groups in the carbonate-apatite structure to produce fluorohydroxyapatite, thus altering the mineral structure of the bone. Unlike hydroxyl ions, fluoride ions reside in the plane of the calcium ions, resulting in a structure that is electrostatically more stable and structurally more compact. Because bone strength is thought to derive mainly from the interface between the collagen and the mineral (Catanese and Keavney 1996), alteration in mineralization affects bone strength.
Skeletal fluorosis is a pathological condition resulting from long-term exposure to high levels of fluoride. Skeletal fluorosis, in some cases with severe crippling, has been reported in individuals residing in India, China and Africa, where the fluoride intake is exceptionally high, e.g. due to high concentration of fluoride in drinking water and indoor burning of fluoride-rich coal resulting in a high indoor fluoride air concentration. In Europe, skeletal fluorosis has only been reported in workers in the aluminium industry, fluorospar processing and superphosphate manufacturing (Hodge and Smith 1977). The study design for most of the available studies is not suitable for estimating the dose- response relationship and development of a N/LOAEL for skeletal fluorosis because of other factors such as nutritional status and climate influence water intake (IPCS 2002).

Effect on bone strength and fractures

A large number of epidemiological studies have investigated the effect of fluoride intake on bone fractures. The amount of fluoride taken up by bone is inversely related to age. During the growth phase of the skeleton, a relatively high proportion of ingested fluoride will be deposited in the skeleton: up to 90% during the first year of life, which gradually decreases to 50% in children older than 15 years of age. There is no clear association of bone fracture risk with water fluoridation (McDonagh et al. 2000), and fluoridation at levels of 0.6 to 1.1 mg/L may actually lower overall fracture risk (AU-NHMRC 2007). It has been postulated that a high level of fluoride can weaken bone and increase the risk of bone fractures under certain conditions, and a water concentration ?4 mg fluoride/L will increase the risk of bone fracture (NRC 2006).

Conclusion

SCHER acknowledges that there is a risk for early stages of dental fluorosis in children in EU countries. A threshold cannot be detected.
The occurrence of endemic skeletal fluorosis has not been reported in the EU. SCHER concludes that there are insufficient data to evaluate the risk of bone fracture at the fluoride levels seen in areas with fluoridated water.

 

3.3 What is the evidence regarding osteosarcoma?

The SCHER opinion states:

Genotoxicity studies

In general, fluoride is not mutagenic in prokaryotic cells, however sodium and potassium fluoride (500-700 mg/L) induced mutations at the thymidine kinase (Tk) locus in cultured cells at concentrations that were slightly cytotoxic and reduced growth rate. In contrast, fluoride did not increase the mutation frequency at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) locus (200-500 mg F/L). Chromosomal aberrations, mostly breaks/deletions and gaps, following exposure to NaF have been investigated in many in vitro assays, but no significant increase in frequency was observed in human fibroblasts at concentrations below 4.52 mg F/L and for Chinese hamster ovary (CHO) cells below 226 mg F/L.
Positive genotoxicity findings in vivo were only observed at doses that were highly toxic to animals, while lower doses were generally negative for genotoxicity. Chromosomal aberrations and micronuclei in bone marrow cells were observed in Swiss Webster mice (up to 18 mg F/kg BW), however no effects were observed in Swiss Webster mice following oral exposure for at least seven generations compared to low fluoride exposure (EFSA 2005). Fluoride has only been reported to be positive in genotoxicity tests at high concentrations (above 10 mg/L), and this effect is most likely due to a general inhibition of protein synthesis and enzymes such as DNA polymerases. There are conflicting reports on genotoxic effects in humans. An increase in sister chromatid exchanges (SCE) and micronuclei has been reported in peripheral lymphocytes from patients with skeletal fluorosis or residents in fluorosis-endemic areas in China and India, while no increased frequency of chromosomal aberrations or micronuclei were observed in osteoporosis patients receiving sodium fluoride treatment. The quality of the former studies is questionable.

Carcinogenicity studies

Carcinogenesis studies have been conducted by the US National Toxicology Program (NTP). Male rats (F344/N) receiving 0.2 (control), 0.8, 2.5 or 4.1 mg F/kg BW in drinking water developed osteosarcoma with a statistically significant dose-response trend. However, a pair-wise comparison of the incidence in the high dose group versus the control was not statistically significant (p=0.099). No osteosarcoma was observed in female rats. Thus NTP concluded that there was “equivocal evidence of carcinogenic activity of NaF in male F344/N rats”.
In male Sprague Dawley (SD) rats receiving up to 11.3 mg F/kg BW/day, no osteosarcoma was observed, but only one fibroblastic sarcoma (1/70) at the highest dose level, and no tumours in female rats. In a bioassay in B6C3F1 mice receiving the high doses of 8.1 and 9.1 mg F/kg BW/day for males and females, respectively, a total of three osteosarcomas occurred, but no osteosarcomas occurred in the medium or high-dose groups.

On the basis of the results from the most adequate long-term carcinogenicity studies, there is only equivocal evidence of carcinogenicity of fluoride in male rats and no consistent evidence of carcinogenicity in mice (ATSDR 2003). No carcinogenicity studies have been conducted using (hydro)fluorosilicic acid, sodium silicofluoride, disodium hexafluorosilicate or hexafluorosilicate or hexafluorosilicic acid.

Epidemiological studies

Early epidemiological studies did not find a consistent relationship between mortality from all types of cancer and exposure for fluoride, including the consumption of fluoride- containing drinking water. Concerns regarding the potential carcinogenic effect of fluoride have been focused on bone cancer due to the known accumulation of fluoride in bones. osteosarcoma.htm” class=”link-glossary”>Osteosarcoma is a rare form of cancer making it difficult to analyse risk factors using epidemiology.

Two studies from the US found a higher incidence of osteosarcoma among males less than 20 years of age living in fluoridated communities compared with non-fluoridated communities (Cohn 1992, Hoover 1991). However, two case-control studies did not find an increase in osteosarcoma in young males consuming fluoridated drinking water (above0.7 mg/L) (Eyre et al. 2009).

A recent study in the UK performed by McNally et al. did not find a statistically significant difference in osteosarcoma rates between areas with fluoride levels of 1 mg/L and those with lower fluoride levels. However, these results are described only in an abstract and the data cannot be assessed. In addition, the relevant age group does not seem to have been studied.

One case-control study found an association between fluoride exposure during childhood and the incidence of osteosarcoma among males, but not among females (Bassin 2006). The Harvard Fluoride osteosarcoma.htm” class=”link-glossary”>Osteosarcoma study was conducted as a hospital based case- control study in 11 hospitals in the USA and was limited to subjects below the age of 20. The study consisted of 103 cases and 215 controls matched to the cases. The level of fluoride in drinking water was the primary exposure of interest, and the estimated exposure was on the source of the drinking water (municipal, private well, bottled) and the subject’s age(s) while at each address. The level of fluoride in drinking water was obtained from local, regional and national registries. For well water, water samples were analyzed in the laboratory, while a value of 0.1 mg/L was assumed for bottled water. As water consumption may vary based on the local climate, the fluoride exposure estimates were based on Centers for Disease Control and Prevention (CDC) recommendations for optimal target levels for the fluoride level in drinking water. The CDC target level for a warmer climate was 0.7 mg/L and for colder climate was 1.2 mg/L. The exposure estimate was expressed as the percentage of climate-specific target levels in drinking water at each age, and grouped into less than 30%, between 30-99% and above 100%. Information on the use of fluoride supplements and mouth rinses was also obtained. However, it is of concern that the exposure assessment is based on retrospectively collected data. A statistically significant increased risk was only observed for males exposed at the highest level (above100%) of the CDC optimal target level and when this exposure took place between 6 and 8 years of age. This coincides with the mid-childhood growth spurt in boys. The increased risk remained after adjustment, e.g. socioeconomic factors, use of fluoride products. No increased risk was observed in females. A preliminary conclusion was based upon an intermediate evaluation and further research was recommended to confirm or refute the observation that fluoride exposure was associated with development of osteosarcoma.

Conclusion

SCHER agrees that epidemiological studies do not indicate a clear link between fluoride in drinking water, and osteosarcoma and cancer in general. There is no evidence from animal studies to support the link, thus fluoride cannot be classified as carcinogenic.

 

3.4 What is the evidence regarding neurological effects?

The SCHER opinion states:

Neurotoxicity

Animal studies

There are only limited data on the neurotoxicity of fluoride in experimental animals. One study in female rats exposed to high doses of fluoride (7.5 mg/kg BW/day for 6 weeks) resulted in alterations of spontaneous behaviour, and the authors noted that the observed effects were consistent with hyperactivity and cognitive deficits (ATSDR 2003). In a recent study, in which female rats were given doses of fluoride up to 11.5 mg/kg BW/day for 8 months, no significant differences among the groups in learning or performance of the operant tasks were observed. Tissue fluoride concentrations, including seven different brain regions, were directly related to the levels of exposure (Whitford et al. 2009). The authors concluded that ingestion of fluoride at levels more than 200 times higher than those experienced by humans consuming fluoridated water, had no significant effect on appetitive-based learning in female rats.
Some animal studies have suggested a potential for thyroid effects following fluoride exposure. The available information is inconsistent and no effects on the thyroid were observed in long-term studies with fluoride in rats. Apparently, fluoride does not interfere with iodine uptake into the thyroid. However, after long-term exposure to high fluoride content in food or water, the thyroid glands of some animals have been found to contain increased fluoride levels (EFSA 2005).

Human Studies

There are limited data on neurotoxicity of fluoride in humans. It has been demonstrated that degenerative changes in the central nervous system, impairment of brain function, and abnormal development in children are caused by impaired thyroid function. Increases in serum thyroxine levels without significant changes in T3 or thyroid stimulating hormone levels were observed in residents of regions in India and China, with high levels of fluoride in drinking water, but these data are inconclusive due to the absence of adequate control for confounding factors. Thus, fluoride is not considered to be an endocrine disruptor (ATSDR 2003).

A series of studies on developmental effects of fluoride were carried out mostly in China in areas where there are likely to be less stringent controls over water quality. Thus it cannot be excluded that the water supply may be contaminated with other chemicals such as arsenic, which may affect intelligence quotient (IQ). The studies consistently show an inverse relationship between fluoride concentration in drinking water and IQ in children. Most papers compared mean IQs of schoolchildren from communities exposed to different levels of fluoride, either from drinking water or from coal burning used as a domestic fuel. All these papers are of a rather simplistic methodological design with no, or at best little, control for confounders, e.g. iodine or lead intake, nutritional status, housing condition, and parents level of education or income.

Tang et al. (2008) published a meta-analysis of 16 studies carried out in China between 1998 and 2008 evaluating the influence of fluoride levels on the IQ of children. The authors conclude that children living in an area with high incidence of fluorosis and high ambient air fluoride levels have five times higher odds of developing a low IQ than those who live in a low fluorosis area. However, the paper does not follow classical methodology of meta-analysis and only uses un-weighted means of study results without taking into account the difference between cross-sectional and case-control studies. Thus it does not comply with the general rules of meta-analysis. Furthermore the majority of these studies did not account for major confounders, a problem that cannot be solved in a summary.

Wang et al. (2007) carried out a study on the intelligence and fluoride exposure in 720 children between 8 and 12 years of age from a homogenous rural population in the Shanxi province, China. Subjects were drawn from control (fluoride concentration in drinking water 0.5 mg/L, n=196) and high fluoride (8.3 mg/L) areas. The high fluoride group was sub-divided according to arsenic exposure; low arsenic (n=253), medium arsenic (n=91), and high arsenic (n=180). The IQ scores in the high-fluoride group were significantly reduced compared to the control group, independent of arsenic exposure. The influence of socio-economic and genetic factors cannot be completely ruled out, but is expected to be minimal.

In a cross-sectional design, Rocha-Amador et al. (2007) studied the link between fluoride in drinking water and IQ in children from three rural communities in Mexico with different levels of fluoride (0.8 mg/L, 5.3 mg/L and 9.4 mg/L; in the latter setting children were supplied with bottled water) and arsenic in drinking water. The children’s IQ was assessed blind as regards fluoride or arsenic levels in drinking water. Socio-economic status was calculated according to an index including household flooring material, crowding, potable water availability, drainage, and father’s education. Additional information about the type of water used for cooking (tap or bottled), health conditions, etc., was obtained by questionnaire. An inverse association was observed between fluoride in drinking water and IQ after adjusting for relevant confounding variables, including arsenic.

Conclusion

Available human studies do not clearly support the conclusion that fluoride in drinking water impairs children’s neurodevelopment at levels permitted in the EU. A systematic evaluation of the human studies does not suggest a potential thyroid effect at realistic exposures to fluoride. The absence of thyroid effects in rodents after long-term fluoride administration and the much higher sensitivity of rodents to changes in thyroid related endocrinology as compared with humans do not support a role for fluoride induced thyroid perturbations in humans. The limited animal data can also not support the link between fluoride exposure and neurotoxicity at relevant non-toxic doses.

SCHER agrees that there is not enough evidence to conclude that fluoride in drinking water at concentrations permitted in the EU may impair the IQ of children. SCHER also agrees that a biological plausibility for the link between fluoridated water and IQ has not been established.

 

3.5 What is the evidence regarding effects on human reproduction?

The SCHER opinion states:

Animal studies

Most of the animal studies on the reproductive effects of fluoride exposure deal with the male reproductive system of mice and rats. They consistently show an effect on spermatogenesis or male fertility. Sodium fluoride administered to male rats in drinking water at levels of 2, 4, and 6 mg/L for 6 months adversely affected their fertility and reproductive system (Gupta et al. 2007). In addition, in male Wistar rats fed 5 mg/kg BW/day for 8 weeks, the percentage of fluoride-treated spermatozoa capable of undergoing the acrosome reaction was decreased relative to control spermatozoa (34 vs. 55%), and the percentage of fluoride-treated spermatozoa capable of oocyte fertilization was significantly lower than in the control group (13 vs. 71%). It was suggested that sub-chronic exposure to fluoride causes oxidative stress damage and loss of mitochondrial trans-membrane potential, resulting in reduced male fertility (Izquierdo- Vega et al. 2008). However, the fluoride doses used in these studies were high and caused general toxicity, e.g. reduced weight gain. Therefore, the effects reported are likely to be secondary to the general toxicity.

Multi-generation studies in mice did not demonstrate reproductive toxicity at doses up to 50 mg F/kg BW. When mice were administered more than5.2 mg F/kg BW/day on days 6-15 after mating, no sign of adverse effect on pregnancy and implantation was observed. Sperm mobility and viability were reduced in both mice and rats after 30 days of administration of 4.5 and 9.0 mg F/kg BW/day (ATSDR 2003).

Serum testosterone increased in rats after drinking water with a fluoride content of 45 and 90 mg/L for 2 weeks. Thereafter the level of serum testosterone decreased and was no different from the controls after 6 weeks. No effect was observed on several reproductive parameters in rats receiving up to 90.4 mg F/L for 14 weeks.

Human studies

The National Health Service (NHS) review on Public Water Fluoridation (McDonagh et al. 2000) did not find any evidence of reproductive toxicity in humans attributable to fluoride. Since then, no new evidence seems to be available other than abstracts without methodological details. There is slight evidence that a high level of occupational exposure to fluoride affects male reproductive hormone levels. A significant increase in follicle-stimulating hormone (p<0.05) and a reduction of inhibin-B, free testosterone, and prolactin in serum (p<0.05), as well as decreased sensitivity in the FSH response to inhibin-B (p<0.05) was found when the high-exposure group was compared with a low-exposure group. Significant correlation was observed between urinary fluoride and serum concentrations of inhibin-B (p<0.028). No abnormalities were found in the semen parameters in either the high- or low-fluoride exposure groups (Ortiz-Pérez et al. 2003). The alteration in the reproductive hormone levels after occupational fluoride exposure is not relevant for drinking water exposure.

Conclusion

There is no new evidence from human studies indicating that fluoride in drinking water influences male and female reproductive capacity. Few human studies have suggested that fluoride might be associated with alterations in reproductive hormones and fertility, but limitations in the study design make them of limited value for risk evaluation. Many experimental animal studies are of limited quality and no reproductive toxicity was observed in a multi-generation study.
SCHER concludes that fluoride at concentrations in drinking water permitted in the EU does not influence the reproductive capacity.

• • 3.1 What is the evidence regarding dental fluorosis?
  • 3.2 What is the evidence regarding skeletal fluorosis?
  • 3.3 What is the evidence regarding osteosarcoma?
  • 3.4 What is the evidence regarding neurological effects?
  • 3.5 What is the evidence regarding effects on human reproduction?
• 4. Are there any reasons for concern about people’s fluoride intake? If so, who is at risk?
• 5. What role does fluoride play in preventing tooth decay?
• 6. What further investigations are needed to improve assessment of exposure and of the health effects of fluoride?
• 7. Does the fluoridation of drinking water specifically lead to adverse ecological impacts?

The GreenFacts Three-Level Structure used to communicate this SCHER Opinion is copyrighted by Cogeneris SPRL.