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A Comparison of the 1993 and 2006 National Research Council Fluoride Reports

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Health Effects of Ingested Fluoride 1993
Subcommittee on Health Effects of Ingested Fluoride

Committee on Toxicology Board on Environmental Sutdies and Toxicology commission on Life Sciences
National Research Council

Executive Summary:
Fluoridation of drinking water has been a subject of controversy for decades. Over the past 50 years, the incidence of dental caries (cavities) has declined considerably in the United States, an important health advance that most scientists attribute principally to increased access to fluoridated water and dental products. According to the U.S. Centers for Disease Control and Prevention, approximately 132 million Americans now receive drinking water that contains fluoride, either naturally occurring or added, at concentrations of 0.7 milligrams per liter (mg/L) or higher.

 This report deals with the possible toxic effects of ingested fluoride in humans. It does not attempt to weigh fluoride's well-documented health benefits against its possible adverse health effects.

 

http://www.nap.edu/catalog.php?record_id=11571

Fluoride in Drinking Water  –  2006
A Scientific Review of EPA's Standards

Committee on Fluoride in Drinking Water

 National Research Council 

Executive Summary:
Fluoride is one of the drinking-water contaminants regulated by EPA. In 1986, EPA established an MCLG and MCL for fluoride at a concentration of 4 milligrams per liter (mg/L) and an SMCL of 2 mg/L. These guidelines are restrictions on the total amount of fluoride allowed in drinking water. Because fluoride is well known for its use in the prevention of dental caries, it is important to make the distinction here that EPA’s drinking-water guidelines are not recommendations about adding fluoride to drinking water to protect the public from dental caries. Guidelines for that purpose (0.7 to 1.2 mg/L) were established by the U.S. Public Health Service more than 40 years ago. Instead, EPA’s guidelines are maximum allowable concentrations in drinking water  intended to prevent toxic or other adverse effects that could result from exposure to fluoride.

 Because new research on fluoride is now available and because the Safe Drinking Water Act requires periodic reassessment of regulations for drinking-water contaminants, EPA requested that the NRC again evaluate the adequacy of its MCLG and SMCL for fluoride to protect public health.

Dental fluorosis
In general, the evidence supports the conclusion that fluoridation at the recommended concentrations, in the absence of fluoride from other sources, results in a prevalence of mild-to-very-mild (cosmetic) dental fluorosis in about 10% of the population and almost no cases of moderate or severe dental fluorosis. At 5 or more times the recommended concentration, the proportion of moderate-to-severe dental fluorosis is substantially higher.

Dental fluorosis
Since 1993, there have been no new studies of enamel fluorosis in U.S. communities with fluoride at 2 mg/L in drinking water.   Earlier studies indicated that the prevalence of moderate enamel  fluorosis at that concentration could be as high as 15%.

Skeletal Fluorosis
Not addressed in the 1993 review

Skeletal Fluorosis
Skeletal fluorosis is a bone and joint condition associated with prolonged exposure to high concentrations of fluoride. Few clinical cases of skeletal fluorosis in healthy U.S. populations have been reported in recent decades, and the committee did not find any recent studies to evaluate the prevalence of the condition in populations  exposed to fluoride at the MCLG.  …more research is needed to clarify the relationship between fluoride ingestion, fluoride concentrations in bone, and stage of skeletal fluorosis before any conclusions can be drawn.

Bone strength and the risk of bone fracture
In view of the conflicting results and limitations of the current data base on fluoride and the risk of hip or other fractures, the subcommittee concludes that there is no basis at this time to recommend that EPA lower the current standard for fluoride in drinking water for this end point. However, the subcommittee  recommends additional research to improve the current data base.

Bone fractures
Overall, there was consensus among the committee that there is scientific evidence that under certain conditions fluoride can weaken bone and increase the risk of fractures. The majority of the committee concluded that lifetime exposure to fluoride at drinking-water concentrations of 4 mg/L or higher is likely to increase fracture rates in the population, compared with exposure to 1 mg/L, particularly in some demographic subgroups that are prone to accumulate fluoride into their bones (e.g., people with renal disease).  There were few studies to assess fracture risk in populations exposed to fluoride at 2 mg/L in drinking water. The best available study, from Finland, suggested an increased rate of hip fracture in populations exposed to fluoride at concentrations above 1.5 mg/L. However, this study alone is not sufficient to judge fracture risk for people exposed to fluoride at 2 mg/L.  Thus, no conclusions could be drawn about fracture risk or safety at 2 mg/L.

Reproductive effects in animals
The subcommittee concludes that the fluoride concentrations associated with adverse reproductive effects in animals are far higher than those to which human populations are exposed. Consequently, ingestion of fluoride at current concentrations should have no adverse effects on human reproduction.

Reproductive and Developmental Effects
A large number of reproductive and developmental studies in animals have been conducted and published since the 1993 NRC report, and the overall quality of that database has improved significantly. Those studies indicated that adverse reproductive and developmental outcomes occur only at very high contractions that are unlikely to be encountered by U.S. populations. A few human studies suggested that high concentrations of fluoride exposure might be associated with alterations in reproductive hormones, effects on fertility, and developmental outcomes, but design limitations make those studies insufficient for risk evaluation.

Neurotoxicity and Neurobehavioral Effects
Not addressed in the 1993 review

Neurotoxicity and Neurobehavioral Effects
A few epidemiologic studies of Chinese populations have reported IQ deficits in children exposed to fluoride at 2.5 to 4 mg/L in drinking water. Although the studies lacked sufficient detail for the committee to fully assess their quality and relevance to U.S. populations, the consistency of the results appears significant enough to warrant additional research on the effects of fluoride on intelligence. More research is needed to clarify the effect of fluoride on brain chemistry and function.

Endocrine Effects
Not addressed in the 1993 review

Endocrine Effects
The chief endocrine effects of fluoride exposures in experimental animals and in humans include decreased thyroid function, increased calcitonin activity, increased parathyroid hormone activity, secondary hyperparathyroidism, impaired glucose tolerance, and possible effects on timing of sexual maturity. Some of these effects are associated with fluoride intake that is achievable at fluoride concentrations in drinking water of 4 mg/L or less, especially for young children or for individuals with high water intake. Many of the effects could be considered subclinical effects, meaning that they are not adverse health effects. However, recent work on borderline hormonal imbalances and endocrine-disrupting chemicals indicated that adverse health effects, or increased risks for developing adverse effects, might be associated with seemingly mild imbalances or perturbations in hormone concentrations. Further research is needed to explore these possibilities.

Effects on the gastrointestinal system
The subcommittee concludes that the available data show that the

concentrations of fluoride found in drinking water in the United States are not likely to produce adverse effects in the gastrointestinal system 

Effects on Other Organ Systems
The committee also considered effects on the gastrointestinal system, kidneys, liver, and immune system.
There were no human studies on drinking water containing fluoride at 4 mg/L in which gastrointestinal, renal, hepatic, or immune effects were carefully documented. Case reports and in vitro and animal  studies indicated that exposure to fluoride at concentrations greater than 4 mg/L can be irritating to the gastrointestinal system, affect renal tissues and function, and alter hepatic and immunologic parameters. Such effects are unlikely to be a risk for the average individual exposed to fluoride at 4 mg/L in drinking water. However, a potentially susceptible subpopulation comprises individuals with renal impairments who retain more fluoride than healthy people do.

.• Effects on the renal systems
However, human epidemiological studies have found no increase in renal disease in populations with long-term exposure to fluoride at concentrations of up to 8 mg/L of drinking water.  The subcommittee concludes that available evidence shows that the threshold dose of fluoride in drinking water for renal toxicity in animals is approximately 50 mg/L. The subcommittee therefore believes that ingestion of fluoride at currently recommended concentrations is not likely to produce kidney toxicity in humans.

Effects on the immune systems
Reports of hypersensitivity reactions in humans resulting from exposure to sodium fluoride are mostly anecdotal.  The weight of evidence shows that fluoride is unlikely to produce hypersensitivity and other immunological effects.

Genotoxicity
The subcommittee concludes that the genotoxicity of fluoride should not be of concern at the concentrations found in the plasma of most people in the United States.

Genotoxicity
Many assays have been performed to assess the genotoxicity of fluoride. Since the 1993 NRC review, the most significant additions to the database are in vivo assays in human populations and, to a lesser extent, in vitro assays with human cell lines and in vivo experiments with rodents. The results of the in vivo human studies are mixed. The results of in vitro tests are also conflicting and do not contribute significantly to the interpretation of the existing database. Evidence on the cytogenetic effects of fluoride at environmental concentrations is contradictory.

Carcinogenicity in animals and humans
The subcommittee concludes that the available laboratory data are insufficient to demonstrate a carcinogenic effect of fluoride in animals. The subcommittee also concludes that the weight of the evidence from the epidemiological studies completed to date does not support the hypothesis of an association between fluoride exposure and increased cancer risk in humans. Nonetheless, the subcommittee recommends conducting one or more carefully designed analytical epidemiological (case-control or cohort) studies to more fully evaluate the relation between fluoride exposure and cancer,

 

Genotoxicity and Carcinogenicity
A relatively large hospital-based case-control study of osteosarcoma and fluoride exposure is under way at the Harvard School of Dental Medicine and is expected to be published in 2006. That study will be an important addition to the fluoride database, because it will have exposure information on residence histories, water consumption, and assays of bone and toenails. The results of that study should help to identify what future  research will be most useful in elucidating fluoride’s carcinogenic potential.

Conclusions
Based on its review of available data on the toxicity of fluoride, the subcommittee concludes that EPA's current MCL of 4 mg/L for fluoride in drinking water is appropriate as an interim standard. At that level, a small percentage of the U.S. population will exhibit moderate or even severe dental fluorosis. However, the question of whether to consider dental fluorosis a cosmetic effect or an adverse health effect and the balancing of the health risks and health benefits of fluoride are matters to be determined by regulatory agencies and are beyond the charge or expertise of this subcommittee. The subcommittee found inconsistencies in the fluoride toxicity data base and gaps in knowledge. Accordingly, it recommends further research in the areas of fluoride intake, dental fluorosis, bone strength and fractures, and carcinogenicity. The subcommittee further recommends that  EPA's interim standard of 4 mg/L should be reviewed when results of new research become available and, if necessary, revised accordingly.

Conclusions
The committee’s conclusions regarding the potential for adverse effects from fluoride at 2 to 4 mg/L in drinking water do not address the lower exposures commonly experienced by most U.S. citizens. Fluoridation is widely practiced in the United States to protect against the development of dental caries; fluoride is added to public water supplies at 0.7 to 1.2 mg/L. The charge to the committee did not include an examination of the benefits and risks that might occur at these lower concentrations of fluoride in drinking water.