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Methodology

Federal Fluoride Cap Too High : Methodology

August 16, 2007

Summary

EWG developed estimates of the distribution of fluoride exposures for children under 2 years of age, considering aggregate exposures from food, incidental toothpaste ingestion, and fluoridated drinking water, including water used to reconstitute powdered or concentrated infant formula.

In our analysis we relied on data from the Centers for Disease Control and Prevention (CDC) National Health and Nutrition Exposure Survey (NHANES) to represent the distribution of food and water intake and body weight; and we incorporated estimates of toothpaste ingestion and fluoride levels in food from the open scientific literature.

From the distribution of fluoride exposure we developed using these parameters, we calculated the proportion of children under age 2 predicted to be exposed to fluoride in excess of the federal government's maximum safe dose of 0.1 mg/kg/day. This dose is consistent with 0.1 mg/kg tolerable upper intake level established by the Institute of Medicine of the National Academy of Sciences (NAS) and endorsed by the American Dental Association, and with the allowable daily dose established for drinking water by EPA (0.11 mg/kg/d).

Data sources

Our exposure model relies on data for 672 children under the age of 2 from the CDC's 2001-2002 National Health and Nutrition Examination Survey (NHANES). The data includes the distribution of body weight, types of food eaten and amounts consumed, drinking water consumption, and water content for reconstituted infant formula (http://www.cdc.gov/nchs/about/major/nhanes/nhanes01-02.htm). We compiled data on fluoride levels in food from the scientific published literature (USDA 2005, Buzalaf et al. 2004, Fein and Cerklewski 2001, Heilman et al. 1997, Jiménez-Farfán et al. 2004, Pang et al. 1992, Warnakulasuriya et al. 2002). We used data on toothbrushing frequency from the Iowa Fluoride Study (IFS), generously provided by the director of the IFS, Dr Steven Levy. We compiled data on toothpaste ingestion as a function of a child's age from a range of peer-reviewed sources (Naccache et al. 1992; Simard et al. 1989; Baxter 1980).

Analysis

We estimated the distribution of fluoride exposure for children under 2 years of age by modeling exposures for the range of individual children represented in the underlying data, with each "child" in our exposure model defined by a body weight, age, and daily food and water consumption pattern (food types and quantities consumed) detailed in the government survey called NHANES. For each child we matched the reported foods consumed to fluoride concentrations in those foods as detailed in the scientific literature. For all ages, the NHANES data report 4268 different foods consumed; for 548 of these EWG found fluoride contamination data in the scientific literature. These foods constitute 75 percent by weight of food consumed for by children under 2 years of age. In calculating fluoride exposures from water, we assumed that the water consumed by each child would contain fluoride at a concentration of 0.8 parts per million, consistent with the Metropolitan Water District's fluoridation plan. We determined fluoride exposure from tooth brushing by combining the mean toothpaste ingestion rate by age for each brushing, with data on how often children brush. The concentration of fluoride ion in the toothpaste was assumed to be 1000 mg/kg (CDC 2001). We assumed a brushing frequency for each modeled child by constructing a distribution of tooth-brushing frequency by age from the IFS data, where the data are categorized as 0, <1, 1, 2, or 3 times/day. Each child in the model was randomly matched with a frequency from this data set. For those who fell within the <1 times brushing/day category, half were assumed not to brush, and half were assumed to brush once a day. To calculate children's total fluoride exposure, we added fluoride exposures from tooth brushing to water and food exposures calculated for each individual in the NHANES survey as described above, and then divided this sum by the individuals’ recorded weight in the NHANES database to derive a total daily fluoride exposure in units of milligrams (mg) of fluoride per kilogram (kg) of body weight per day (d), or mg/kg/d. We ran the model in Monte Carlo fashion until results stabilized. We produced final model results by averaging results from successive model simulations, each of which randomly matched modeled children to tooth brushing frequencies in the IFS distribution. Final model results represent the average summary statistics for each age including the mean; 10-99th percentiles of mg/kg/day; percent contributions from water, food, and tooth-brushing; and percent of individuals over the EPA acceptable dose and IOM/ADA level.

Limitations

Our analysis likely underestimates children's fluoride exposures. Our estimates do not include fluoride exposures from the 25% of food consumed for which we found no fluoride testing data in the scientific literature. We do not include exposures to residues of the pesticide sulfuryl fluoride on food. This fluoride-based fumigant was recently approved for use on food by EPA, but data are not yet available on the levels at which it now contaminates the food supply. We also did not include exposures to fluoride supplements. Although CDC and ADA recommend that children in areas with fluoridated water not take supplements, many dentists disregard this advice (Levy and Carrell 1987).