3. What are the possible health effects of fluoridation, and what is the latest evidence?
- 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?
3.1 What is the evidence regarding dental fluorosis?
The SCHER opinion states:
Dental fluorosis is a well-recognised condition and an indicator of overall fluoride absorption from all sources at a young age. Initially, fluorosis appears as white opaque striations across the enamel surface, and in more severe cases the porous areas increase in size and pitting occurs with secondary discoloration of the surface. The symptoms appear in a dose-response manner. For classification of fluorosis, see Appendix I. The severity and prevalence of dental fluorosis has been shown to be directly related to the fluoride concentration in drinking water. It is the daily total fluoride intake over a prolonged period of time, but only during the developmental phase of the teeth that results in fluorosis.
The pre-eruptive developments of the deciduous and permanent teeth are critical phases for dental fluorosis. Early ossification of the jaw and development of deciduous tooth buds occurs between 4-6 months in utero. Mineralisation of the permanent tooth buds starts at the time of birth and continues slowly for 12-14 years.
Numerous studies have demonstrated that exposure to fluoride levels during tooth development can result in dental fluorosis. Excess absorbed fluoride may impair normal development of enamel in the pre-eruptive tooth. This will not be apparent until tooth eruption, which will be more than 4-5 years after exposure. The development and severity of fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure during the period of enamel formation.
Fluorosed enamel is composed of hypomineralized sub-surface enamel covered by well- mineralized enamel. The exact mechanisms of dental fluorosis development have not been fully elucidated. It seems that fluoride systemically can affect the ameloblasts, particularly at high fluoride levels, while at lower fluoride levels, the ameloblasts may respond to the effects of fluoride on the mineralizing matrix (Bronckers et al. 2009).
The EFSA NDA panel considered that an intake of less than 0.1 mg F/kg BW/day in children up to 8 years old corresponds to no significant occurrence of “moderate” forms of fluorosis in permanent teeth (EFSA 2005). Figure 1 shows a plot of the Community Fluorosis Index versus the daily fluoride dose/kg bodyweight (Butler et al. 1985, Fejerskov et al. 1996, Richard et al. 1967). The plot shows a linear dose–response relationship and indicates that fluorosis may occur even at very low fluoride intake from water.
Enamel fluorosis seen in areas with fluoridated water (0.7–1.2 mg/L F) has been attributed to early tooth brushing behaviours, and inappropriate high fluoride intake (Ellewood et al. 2008), i.e. use of infant formula prepared with fluoridated drinking water (Forsman 1977). Similarly, enamel fluorosis may occur in non-fluoridated areas, in conjunction with the use of fluoride supplements and in combination with fluoridated toothpaste (Ismail and Hasson 2008). Fluoridated toothpaste has been dominating the European toothpaste market for more than 30 years.
3.2 What is the evidence regarding skeletal fluorosis?
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).
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:
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.
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.
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.
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:
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).
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.
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:
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.
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.
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.