Water Fluoridation and Cancer Risk

More than 60 years after fluoride was first added to drinking water in some parts of the United States, there is still controversy about the possible health effects of drinking water fluoridation. Many people hold strong views either for or against water fluoridation. Their concerns are based on everything from legitimate scientific research, to freedom of choice issues, to government conspiracy theories.

Here we will explore the possible link between fluoridation and cancer. We will not address in detail other possible health effects of water fluoridation (positive or negative). This is not intended as a position statement of the American Cancer Society.

What is fluoride?

Fluorides are compounds that combine the element fluorine with another substance, usually a metal. Examples include sodium fluoride, stannous fluoride, and fluoride monofluorophosphate (MFP fluoride).

Once in the body, fluorides are absorbed into the blood through the digestive tract. They travel through the blood and tend to collect in areas high in calcium, such as the bones and teeth.

Where is fluoride found?

Some fluorides occur naturally in soil, air, or water, although the levels of fluoride can vary widely. Just about all water contains some level of fluoride.

The major sources of fluoride for humans are water and other beverages, food, and fluoride-containing dental products (toothpastes, mouth rinses, etc.). Because dental products are generally not swallowed (except, perhaps, by younger children), they may be less of a concern with regard to possible health issues.

Fluoride in drinking water

Water fluoridation began in some parts of the United States in 1945, after scientists noted that people living in areas with higher water fluoride levels had fewer cavities.

The United States Public Health Service (PHS) has, since 1962, recommended that public water supplies contain between 0.7 and 1.2 milligrams of fluoride per liter (mg/L) of drinking water to help prevent tooth decay. (Some natural water sources have fluoride levels within this range, or even higher.)

Fluoride is now used in the public drinking water supplied to about 2 out of 3 Americans. The types of fluoride added to different water systems include fluorosilicic acid, sodium fluorosilicate, and sodium fluoride.

The US Environmental Protection Agency (EPA) has set a maximum amount of fluoride allowable in drinking water of 4.0 mg/L. Long-term exposure to levels higher than this can cause a condition called skeletal fluorosis, in which fluoride accumulates in the bones. This can eventually result in joint stiffness and pain, and can lead to weak or brittle bones in older adults.

The EPA also set a secondary standard of no more than 2.0 mg/L to help protect children (under the age of 9) from dental fluorosis. In this condition, fluoride collects in developing teeth, preventing tooth enamel from forming normally and resulting in permanent staining or pitting of teeth.

Some states have maximum fluoride levels in drinking water that are lower than the national 4.0 mg/L standard.

Does fluoride cause cancer?

People have raised questions about the safety and effectiveness of water fluoridation since it first began. Over the years, many studies have looked at the possible link between fluoride and cancer.

Some of the controversy concerning the possible link stems from a study of lab animals reported by the US National Toxicology Program in 1990. The researchers found "equivocal" (uncertain) evidence of cancer-causing potential of fluoridated drinking water in male rats, based on a higher than expected number of cases of osteosarcoma (a type of bone cancer). There was no evidence of cancer-causing potential in female rats or in male or female mice.

Osteosarcoma seems to be the cancer about which the most concern has been raised. One theory on how fluoridation might affect the risk of osteosarcoma is based on the fact that fluoride tends to collect in parts of bones where they are growing. These areas, known as growth plates, are where osteosarcomas typically develop. The theory is that fluoride might somehow cause the cells in the growth plate to grow faster, which might make them more likely to eventually become cancerous.

What have studies in humans found?

More than 50 population-based studies looking at the potential link between water fluoride levels and cancer have been reported in the medical literature. Most of these have not found a strong link to cancer. Just about all of the studies have been retrospective (looking back in time). They have compared, for example, the rates of cancer in a community before and after water fluoridation, or compared cancer rates in communities with lower levels of fluoride in drinking water to those with higher levels (either naturally or due to fluoridation). Some factors are hard to control for in these types of studies (that is, the groups being compared may be different in ways other than just the drinking water), so the conclusions reached by any single study must be looked at with caution.

And there are other issues that make this topic hard to study. For example, if fluoridation is a risk factor, is the type of fluoride used important? Also, is there a specific level of fluoride above which the risk is increased?

Osteosarcoma is a rare cancer. Only about 400 cases are diagnosed in children and teens each year in the United States. This means it can be hard to gather enough cases to do large studies. Smaller studies can usually detect big differences in cancer rates between 2 groups, but they may not be able to detect a smaller difference. If fluoride increased the risk only slightly, it might not be picked up by these types of studies.

Small studies by themselves may not provide the answers, but taken as a whole they tend to have more weight. Several systematic reviews over the past 25 years have looked at all of the studies published on this subject.

In its review published in 1987, the International Agency for Research on Cancer (IARC), part of the World Health Organization, labeled fluorides as "non-classifiable as to their carcinogenicity [ability to cause cancer] in humans." While they noted that the studies "have shown no consistent tendency for people living in areas with high concentrations of fluoride in the water to have higher cancer rates than those living in areas with low concentrations," they also noted that the evidence was inadequate to draw conclusions one way or the other.

In 1991, the US Public Health Service issued a report on the benefits and risks of fluoride. When looking at a possible link with cancer, they first reviewed the results of studies done with lab animals. They concluded that the few studies available "fail[ed] to establish an association between fluoride and cancer." They also looked at population-based studies, including a large study conducted by the National Cancer Institute. They concluded: "Optimal fluoridation of drinking water does not pose a detectable cancer risk to humans as evidenced by extensive human epidemiological data available to date, including the new studies prepared for this report."

The National Research Council (NRC), part of the National Academies, issued a report titled "Health Effects of Ingested Fluoride" in 1993. Its conclusion was that "the available laboratory data are insufficient to demonstrate a carcinogenic effect of fluoride in animals." They also concluded that "the weight of the evidence from the epidemiological [population-based] studies completed to date does not support the hypothesis of an association between fluoride exposure and increased cancer risk in humans." The report recommended that additional well-designed studies be done to look at the possible link to cancers, especially osteosarcomas.

In the United Kingdom, the National Health Service (NHS) Centre for Reviews and Dissemination, University of York, published a systematic review of water fluoridation in the year 2000. After searching through the medical literature, they included 26 studies in their analysis, all of which were considered to be of "low" to "moderate" quality. They concluded, "Overall, no clear association between water fluoridation and incidence or mortality of bone cancers, thyroid cancer, or all cancers was found." However, they also noted, "Given the level of interest surrounding the issue of public water fluoridation, it is surprising to find that little high quality research has been undertaken."

The National Research Council issued an update of its 1993 review in early 2006. While the review included some new data, the results of this report were essentially the same: "On the basis of the committee's collective consideration of data from humans, genotoxicity assays, and studies of mechanisms of actions in cell systems, the evidence on the potential of fluoride to initiate or promote cancers, particularly of the bone, is tentative and mixed." The report also noted that an ongoing study from the Harvard School of Public Health would add important information to the current body of research.

A partial report from the Harvard study, published in 2006, found that exposure to higher levels of fluoride in drinking water was linked to a higher risk of osteosarcoma in boys but not in girls. However, researchers linked to the study noted that early results from a second part of the study did not appear to match those of the report. They therefore advised caution in interpreting the report until the full results of the study become available. The full study has not yet been published.

The US Centers for Disease Control and Prevention (CDC) has issued a statement on water fluoridation and osteosarcoma in response to the study, noting that "at this time, the weight of the scientific evidence, as assessed by independent committees of experts, comprehensive systematic reviews, and review of the findings of individual studies does not support an association between water fluoridated at levels optimal for oral health and the risk for cancer, including osteosarcoma." The statement also noted that further results from the Harvard study should "provide further information as to whether and to what extent an association may exist between osteosarcoma and exposure to fluoride."

The general consensus among the reviews done to date is that there is no strong evidence of a link between water fluoridation and cancer. However, these reviews were all done before the partial results of the Harvard study were published in 2006. Several of the reviews noted that further studies, including the full results of the Harvard study, are needed to clarify the possible link.

Can you reduce your fluoride exposure?

Even without fluoridation, the natural levels of fluoride in water in some places can be even higher than 4 mg/L. Community water systems in such areas are required to lower the fluoride level below the acceptable standard. Private water sources, however, may still be higher.

For people concerned that they or their families may be exposed to too much fluoride, there are some steps that can be taken to reduce exposure.

First, people should know the level of fluoride in their drinking water. If your drinking water comes from a public source, you can find out about the levels of certain substances in your drinking water, including fluoride, by contacting your local community water system. Each system is also required to provide its customers with an annual report on water quality known as a Consumer Confidence Report. This report lists the levels of certain chemicals and other substances in the water, including fluoride. You can also contact the EPA's Safe Drinking Water Hotline at 1-800-426-4791 for more general information about drinking water safety. Those who get their drinking water from a private source such as a well can consider having fluoride levels tested by a reputable laboratory.

People who live in areas with high levels of fluoride in the water may consider using alternative sources of drinking water, such as bottled water. Most bottled water contains at least some fluoride, with natural spring waters tending to be the lowest. You may want to contact the bottler to find out about fluoride levels. (The US Food and Drug Administration (FDA) sets the standards for allowable levels of fluoride in bottled water.) There are also several methods to filter fluoride out of water, although these can be expensive.

Parents with concerns should give small children only a pea-sized amount of toothpaste for brushing, and should do their best to ensure their children are not swallowing, as this can be a significant source of fluoride. Speak to your child's dentist before using toothpaste in children under 2 years of age. Low- and no-fluoride toothpastes and other dental products are also available.

Additional resources

More information from your American Cancer Society

The following related information may also be helpful to you. These materials may be viewed on our Web site or ordered from our toll-free number, at 1-800-227-2345.

Known and Probable Human Carcinogens

National organizations and Web sites*

In addition to the American Cancer Society, other sources of information include:

Centers for Disease Control and Prevention (CDC)
Toll-free number: 1-800-CDC-INFO (1-800-232-4636)
Web site: www.cdc.gov

Community water fluoridation page: www.cdc.gov/fluoridation

Environmental Protection Agency
Toll-free number (Safe Drinking Water Hotline): 1-800-426-4791
Web site: www.epa.gov

Ground water & drinking water page: www.epa.gov/safewater/index.html

National Cancer Institute
Toll-free number: 1-800-4-CANCER (1-800-422-6237)
Web site: www.cancer.gov

Fluoridated water page: www.cancer.gov/cancertopics/factsheet/Risk/fluoridated-water

No matter who you are, we can help. Contact us anytime, day or night, for information and support. Call us at 1-800-227-2345 or visit www.cancer.org.

References

Agency for Toxic Substances and Disease Registry, Centers for Disease Control and Prevention. ToxFAQs for Fluorine, Hydrogen Fluoride, and Fluorides. 2007. Accessed at www.atsdr.cdc.gov/tfacts11.html on December 30, 2009.

Bassin EB, Wypij D, Davis RB, Mittleman MA. Age-specific fluoride exposure in drinking water and osteosarcoma (United States). Cancer Causes Control. 2006;17:421–428.

Centers for Disease Control and Prevention. CDC Statement on Water Fluoridation and Osteosarcoma. Accessed at www.cdc.gov/fluoridation/safety/osteosarcoma.htm on December 29, 2009.

Douglass CW, Joshipura K. Caution needed in fluoride and osteosarcoma study. Cancer Causes Control. 2006;17:481–482.

International Agency for Research on Cancer. Fluorides (Inorganic, Used in Drinking-water). 1987; Supp 7: 208-210. Accessed at http://monographs.iarc.fr/ENG/Monographs/suppl7/Suppl7-83.pdf on December 30, 2009

McDonagh MS, Whiting PF, Wilson PM, et al. Systematic review of water fluoridation. BMJ. 2000;321:855-859. (Full report available online at: www.york.ac.uk/inst/crd/CRD_Reports/crdreport18.pdf. Accessed December 30, 2009.)

National Cancer Institute. Fluoridated Water: Questions and Answers. 2005. Accessed at www.cancer.gov/cancertopics/factsheet/Risk/fluoridated-water on December 29, 2009.

National Research Council. Fluoride in Drinking Water: A Scientific Review of EPA's Standards. 2006. Accessed at http://books.nap.edu/catalog.php?record_id=11571 on December 30, 2009.

National Research Council. Health Effects of Ingested Fluoride. 1993. Accessed at http://www.nap.edu/catalog.php?record_id=2204 on December 30, 2009.

National Toxicology Program. Toxicology and Carcinogenesis Studies of Sodium Fluoride (CAS No. 7681-49-4) in F344/N Rats and B6C3F1 Mice (Drinking Water Studies). 1990. Abstract accessed at http://ntp.niehs.nih.gov/index.cfm?objectid=0709411C-E355-A12E-DBB6666806CB8DB2 on December 30, 2009.

United States Public Health Service. Review of Fluoride: Benefits and Risks. 1991. Accessed at www.health.gov/environment/ReviewofFluoride/default.htm on December 30, 2009.