April 22, 2019

Hoover et al, Fluoridation of Drinking Water and Subsequent Cancer Incidence and Mortality


For over 40 years, fluoridation of municipal drinking water supplies to prevent dental caries has sparked controversy. The issues involved have been political and public policy in nature, with claims of possible health effects sometimes evoked to buttress the arguments. Among the many adverse effects raised by opponents of fluoridation as being possibly related to fluoride ingestion is the risk of malignancy. For this reason, the relationship between natural and artificial fluoridation of drinking water and cancer incidence and mortality has been the subject of numerous epidemiologic studies. These investigations, employing a variety of methods, have been conducted in many regions around the world. The results have been intensively reviewed by a number of expert panels (1-3), and the conclusions reached have been remarkably consistent. That is, when appropriate methods are applied to high-quality data, the resulting analyses have not found any evidence of an association between fluoridation of drinking water and an increased risk of cancer.

In March of 1990, a preliminary report of a bioassay study conducted in rodents under the auspices of the National Toxicology Program suggested a relationship between the dose of fluoride and the risk of osteosarcomas and possibly the risk of oral tumors (4). The Draft Technical Report (5) prepared subsequently for a peer review committee concluded that: “Under the conditions of these 2-year dosed water studies, there was equivocal evidence of carcinogenic activity of sodium fluoride in male F344/N rats, based on the occurrence of a small number of osteosarcomas in dosed animals. There was no evidence of carcinogenic activity in female F344/N rats receiving sodium fluoride at concentrations of 25, 100, or 175 ppm (11, 45, or 79 ppm fluoride) in drinking water for 2 years. There was no evidence of carcinogenic activity of sodium fluoride in male or female mice receiving sodium fluoride at concentrations of 25, 100, or 175 ppm in drinking water for 2 years.” The suggestion of a relationship with oral cancer was discounted in this report: “The squamous cell neoplasms in rats receiving sodium fluoride were not considered chemical related because a squamous cell carcinoma was observed in one control male (paired control group) and in one control female, the incidence rates in the dosed groups were not significantly greater than in concurrent controls and were within the range of historical controls, and there was no supporting evidence of focal hyperplasia of the oral mucosa.”

In 1976, the National Cancer Institute (NCI) evaluated 20 years of U.S. cancer mortality and a limited amount of cancer incidence data, and found no pattern consistent with an adverse effect of fluoridation (6). In the face of the new laboratory findings, we felt it was appropriate to update this study. The passage of time has provided 16 additional years of mortality data, enabling an evaluation of the effects of up to 35 years of fluoridation. In addition, the establishment in 1973 of the NCI Surveillance, Epidemiology and End Results (SEER) program, a network of population-based cancer incidence registries, has resulted in more relevant incidence data than were previously available. Incidence statistics are especially important for the evaluation of bone osteosarcoma, since histology-specific information is not generally available in mortality data. In addition, mortality data for bone cancer are limited since bone is a frequent site of metastases for a number of common malignancies, which may be misclassified as bone cancer deaths.

Hoover et al, Fluoridation Of Drinking Water And Subsequent Cancer Incidence And Mortality, 1991