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Fluoride overfeed at a well site near an elementary school in Michigan. (Features).


A fluoride overfeed occurred on Wednesday, July 3, 1991, at a well site near an elementary school in Portage, a suburb of Kalamazoo, Michigan. The incident was reported to the Michigan Department of Public Health on July 5, 1991, which led to the authors' involvement and active participation. The fluoride overfeed resulted from two circumstances: 1) a problem in the design of the well system circuit and 2) damage to the logic card that controlled the fluoride solution pump. The pump control logic card appeared to have been damaged by an electrical surge or spike. As a result of the damage, the logic card simulated a situation in which the well pump kept running, and the fluoride solution pump continued to pump solution into the well pump discharge pipe. Although the control system had been tested after installation, the problem was not evident because the test procedure did not include detection for this kind of damage.

The overfeed incident resulted in a high concentration of fluoride (92 milligrams per liter [mg/LI) in water that came from a drinking fountain at the school. At least seven students who drank water from the fountain became ill. All summer activities at the school were suspended, and the use of drinking water at the location was immediately stopped. The use of drinking water at the school resumed when the fluoride concentration had been adjusted to optimal levels.

The objectives of this paper are to briefly describe the causes of the fluoride overfeed, the toxicology of fluoride, the reported adverse health effects, and the improvements made to the water system pump controls to prevent recurrence. A summary of this work has appeared in minutes of the Biannual Meeting of the U.S. Environmental Protection Agency (U.S. EPA)-sponsored Federal-State Toxicology and Risk Analysis Committee (Sidhu & Kimmer, 2000).

Benefits and Potential Health Risks of Fluorides

What follows is only a brief description of health benefits and pertinent potential health risks related to ingestion of fluorides. For comprehensive details, the reader is referred to "Review of Fluoride: Benefits and Risks" (Public Health Service, 1991a) and "Toxicological Profile for Fluorides" (Agency for Toxic Substances and Disease Registry [ATSDR], 1993).

Fluoride is a normal constituent of all diets and is an essential nutrient. When the concentration is within optimal limits, no adverse health effects are observed, and the rate of dental caries (destruction of teeth) in children is at least 65 percent below the rate in communities with little or no fluoride in their water supplies. Excessive amounts of fluoride in drinking-water supplies produce adverse health effects and unsightly dental fluorosis (brownish discoloration of teeth), which increase with fluoride concentration (Okun, 1986).

Fluorides have substantial benefits in the prevention of dental cavities. Several studies have clearly established a causal relationship between the use of fluoridated water and the prevention of dental cavities (Public Health Service, 1991a, 1991b, 1991c). Fluoridated drinking water normally contains 0.7 to 1.2 parts per million (ppm) of fluoride. Fluoride concentration in nonfluoridated water usually ranges from negligible levels to less than 0.3 ppm (Public Health Service, 1991a, 1991b, 1991c).

Fluoride forms hydrofluoric acid in the acidic environment of the stomach, which in turn causes irritation of the gastrointestinal tract. The primary adverse health effects of fluorides are sudden onset of nausea, vomiting, cramplike abdominal pain, diarrhea, and pulmonary edema caused by the aspiration of vomitus (ATSDR, 1993). Ingestion of 5 to 10 grams of sodium fluoride by a 70-kilogram (70-kg) man (32-64 mg/kg/day) may cause death (ATSDR, 1993).

Chronic oral exposure to large doses of fluoride mottles teeth (an effect known as dental fluorosis), thickens bones, and may also result in exostosis of bones--the creation of abnormal bony projections (a condition known as skeletal fluorosis) (ATSDR, 1993).

Dental fluorosis occurs during tooth formation and becomes visible during eruption of the teeth. The condition varies from mild to severe. Mild dental fluorosis is characterized by whitish areas on teeth, while the severe form of the condition is characterized by pitting of the enamel and brownish discoloration. Unlike mild fluorosis, severe fluorosis is easily detectable, but it is considered only a cosmetic problem (Public Health Service, 1991a, 1991b, 1991c).

Fluoride has dose-related effects on bones. Crippling skeletal fluorosis has been observed in many parts of the world where the concentration of naturally occurring fluoride in drinking water exceeds 10 ppm. This chronic bone and joint disease is characterized by limitation of joint movements, calcification of ligaments, and deformities of bones and joints. Crippling skeletal fluorosis also is influenced by several other factors, such as nutritional deficiencies, impaired kidney functions, and age at exposure to high levels of fluoride (Public Health Service, 1991c). The disease is extremely rare in the United States (Public Health Service, 1991a).

A study conducted by the National Toxicology Program (NTP) to assess the effect of fluoride on cancer found equivocal evidence of a fluoride-related increase in osteosarcoma in male rats (ATSDR, 1993; NTP, 1990). The same study showed no evidence, however, of any fluoride-related neoplasms in female rats or in male and female mice (ATSDR, 1993; NTP 1990). Equivocal evidence of carcinogenic description means that the study is interpreted as indicating a marginal increase of neoplasms that may be related to fluoride (NTP, 2000).

Drinking-Water Regulations for Fluoride

U.S. EPA develops two types of drinkingwater regulations: (1) National Primary Drinking Water Regulations and (2) National Secondary Drinking Water Regulations. The National Primary Drinking Water Regulations are based on the toxic properties of chemical contaminants, while the National Secondary Drinking Water Regulations (commonly called Secondary Maximum Contaminant Levels, or SMCLs) are based on aesthetic considerations such as taste, odor, and appearance. Sometimes an SMCL for a drinking-water contaminant may be set from the point of view of technology. Using risk assessment procedures along with various decision-making parameters, U.S. EPA develops two tiers of National Primary Drinking Water Regulations: (1) maximum contaminant level goals (MCLGs) and (2) maximum contaminant levels (MCLs). An MCLG, a non-enforceable health goal, is defined as the maximum level of a contaminant in drinking water at which no known or anticipated adverse effects on human health would occur, allowing for an adequate margin of safety An MCL is a legally enforceable standard that regulates levels of a chemical contaminant in public water supplies. Although human health is the primary factor considered for the establishment of an MCL, other decision-making parameters such as the practical quantification limit (PQL), the technology performance limit, cost-benefit analysis, and feasibility factors are brought into consideration. Taking into account these parameters, as well as human health effects, an MCL reflects the best achievable level for regulation of a drinking-water contaminant (Cotruvo, 1988; Ohanian 1992; Sidhu, 1991; Sidhu, 1992a; U.S. EPA, 2000).

Based on prevention of skeletal fluorosis, the MCL for fluoride in drinking water is set at 4 ppm (U.S. EPA, 2000). The SMCL for fluoride in drinking water, based on prevention of dental fluorosis, is set at 2 ppm (U.S. EPA, 2000).

Materials and Methods

The following information about the adverse health effects experienced by some students after ingestion of drinking water contaminated with fluoride was obtained from the Kalamazoo County Department of Human Services, Kalamazoo, Michigan.

Data from analyses of water samples from the elementary school and the neighborhood areas were provided by the city of Portage, Michigan. Chemical analyses of water were conducted by Kar Laboratories, Kalamazoo, Michigan.

The exposure assessment for fluoride was conducted according to U.S. EPA guidelines and procedures (Sidhu, 1992b; U.S. EPA, 1989a). The dose of fluoride to which some students were exposed was compared with the oral Reference Dose (RfD), obtained from the Integrated Risk Information System (U.S. EPA, 1989b); the Minimal Risk Level (ATSDR, 1992; ATSDR, 1993); and the lethal dose for humans (ATSDR, 1993). An oral RfD is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of daily dose exposure to a human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious health effects over a lifetime. The RfD is expressed in units of daily dose (Sidhu, Nash, & McBride, 1995). Minimal Risk Level (MRL) is an estimate of daily exposure of a human being to a chemical (in mg/kg/day) that is likely to be without an appreciable risk of deleterious effects (noncarcinogenic) in a specified period. MRLs are based on human and animal studies and are repo rted for acute (14-day), intermediate (15- to 364-day), and chronic ([[greater than or equal to]365-day) exposures (ATSDR, 1992).

The drinking-water system at the elementary school was flushed several times to lower the concentration of fluoride to optimal levels. Several improvements were made to prevent recurrences and to minimize the impacts if such an incident should recur.

The state of Michigan provided toxicological consultation and addressed concerns of the residents through telephone calls about potential health risks from exposure to high levels of fluoride in the drinking water. Users of the drinking-water system were kept informed about the incident by the city of Portage staff through a news release jointly issued with the Kalamazoo County Human Services Department and the state of Michigan.

Results and Discussion

The only health effects observed in exposed students were nausea and vomiting. It was reported that the drinking water at the elementary school had an unusual taste and was discolored, circumstances that may have prevented many of the students from drinking much of the water, thus limiting exposure. Vomiting also lowered the extent of incidental exposure. The nausea and vomiting may have resulted from irritation of the stomach by hydrofluoric acid formed from the reaction of fluoride with hydrochloric acid (which has a low pH) in the stomach (ATSDR, 1993).

The exposure assessment for fluoride is shown in the sidebar on page 17. The highest concentration of fluoride, in the sample taken on the third day of the incident, was 92 mg/L. The MCL and SMCL of fluoride are set at 4 mg/L and 2 mg/L, respectively No chemical analysis of the drinking water was conducted on the day of the incident. It is possible that the fluoride concentration of drinking water on the first day of the incident may have differed from the recorded concentration (92 mg/L) on the third day after the incident.

The oral-exposure dose for fluoride (0.03 mg/kg/day) was lower (see sidebar) than both the U.S. EPA (1989) RED (0.06 mg/kg/day) and the ATSDR (1993) chronic MRL (0.05 mg/kg/day).

In other words, exposure to fluoride by the students was not sufficient to cause any serious adverse health or cosmetic dental effects. The oral-exposure dose (0.03 mg/kg/day) of fluoride may, however, have been sufficient to form some hydrofluoric acid in the stomach, causing nausea and vomiting in some exposed students.

The oral-exposure dose (0.03 mg/kg/day) of fluoride (see sidebar) was far less than the lethal dose of fluoride (32-64 mg/kg/day). Both oral-exposure assessment results (see sidebar) and the clinical observations of symptoms (nausea and vomiting) in exposed students indicate that the exposure to fluoride was very mild.

It seems likely that the exposure to fluoride experienced in this incident will not increase the risk of cancer in the exposed children, for the following reasons: First, the children were exposed to a single low dose of fluoride. Second, fluoride is a weak carcinogen, as indicated in the results of an experimental study by the National Toxicology Program (1990).

The concentrations of fluoride in different locations at the school and in neighborhood areas, as determined for the third day to the 48th day after the incident, are shown in Table 1. The results indicate two things. First, the impact of the fluoride overfeed was confined to the elementary school. The reason was that the elementary school was next to the well site where the overfeed occurred. At the time, most well systems were not pumping, hut fluoride was being drawn into the school through its well site piping (Figure 1). Second, it is quite evident that the results of fluoride concentrations were variable. After flushing, the fluoride concentrations appeared significantly lower than before flushing. Also, the concentration of fluoride rose when water was allowed to stand overnight. This kind of variability is quite clear from the fluoride determinations made on the fifth, sixth, and eighth days after the incident. There are two possible causes of the variability in fluoride concentration: (1) leaching of fluoride from scale deposits in the pipes and (2) siphoning of fluoride from areas that had little use and were not flushed sufficiently Flushing continued for 48 days before the fluoride concentration in the school drinking water was brought to optimal levels. It is possible that if all outlets had been flushed at the same time the concentration of fluoride would have been brought to optimal levels in a shorter time (i.e., in less than 48 days).

Improvements Made

Several improvements were made to prevent a recurrence of fluoride overfeed, as follows:

1. The electrical circuit energizing the fluoride system was redesigned to prevent the fluoride feeder from operating when the water well was not pumping. The redesign placed the logic card in series with the pump discharge pressure switch and the motor control switch. Each switch has to be closed for the fluoride feeder to operate.

2. The discharge piping from the well field was examined, It was found that the water service to the school was connected upstream to the addition of the treatment chemicals. The water service was relocated downstream to the addition of the treatment chemicals, Relocation of this service to the school will prevent recurrence of the fluoride overfeed and provide the school with properly treated water (Figure 2).

3. Operational changes were also made to further minimize the impact of a fluoride overfeed, including the following alterations and modifications:

(a) Day tanks for the fluoride feeder were installed, allowing for daily inspection of the pumping facility. If for any reason the fluoride solution feed pump was malfunctioning, only a limited supply of solution could be pumped into the system.

(b)The capacity of the fluoride solution pump was reduced. New pumps were installed for which the feed was near capacity. This improvement worked well and reduced the potential for a fluoride overfeed due to an improperly adjusted fluoride solution pump.

After the redesign and modifications were completed, the system was tested, and failure of the pump logic card no longer caused the fluoride feeder to pump.

Risk Communication

Good risk communication is important for the successful implementation of a risk management decision. U.S. EPA recommends adoption of the following seven Cardinal Rules for a successful risk communication approach:

1. accept and involve the public as a legitimate partner;

2. plan carefully and evaluate your efforts;

3. listen to the publics specific concerns;

4. be honest, frank, and open;

5. coordinate and collaborate with other credible sources;

6. meet the needs of the media; and

7. speak clearly with compassion (U.S. EPA, 1988).

Keeping the public fully informed and addressing the public's concerns are important and helpful in the management of risk-based decisions (Sidhu & Chadznyski, 1994; Sidhu & Chadzynski, 1996).

In this case, a news release informed the public that fluoride is added to the city drinking water to achieve the desired concentration ([congruent to]1 ppm), which is optimal for dental health. The community was advised that exposure to fluoride at a higher concentration (above the MCL of 4 mg/L) for an extended period can cause discoloration of teeth and that a short-term exposure to higher concentrations can result in abdominal pain, vomiting, and diarrhea.

The news release reported that mild nausea and vomiting had been experienced by some students at the elementary school. (It was fortunate that the fluoride overfeed occurred when the school was closed for summer vacation and that only a few student activities were in progress during the summer months.) The news release reported that monitoring conducted by the city throughout residential areas adjacent to the school found optimal fluoride concentrations in the water. It was noted that the overfeed impact was observed only at the elementary-school water outlets. It was also emphasized that there is no evidence for any long-term health risk from a short-term exposure of this kind. Moreover, the news release cited the improvements that were made to prevent a recurrence of the fluoride overfeed in the future.

An evaluation of this risk communication revealed that the public-health agencies responsible for it had intuitively followed U.S. EPA'S seven cardinal rules of risk communication (Sidhu & Chadzynski, 1994; U.S. EPA, 1988). The public was kept fully informed about the incident, about potential health risks, and about the measures taken to prevent recurrence of the incident in the future. Also, via personal communications and meetings, the concerns of the public were fully addressed, and the public was assured that there should be no evidence of any long-term health risk from this short-term exposure to fluoride. In this case, the approach taken was found to enhance public trust in state and local government agencies.


On the basis of symptoms, exposure assessment, or both, it was determined by the agencies involved in this incident that in exposed students, the fluoride had irritated the stomach, causing nausea and vomiting. This single, mild exposure to fluoride was, however, too low to cause any appreciable long-term adverse health or cosmetic dental effects. The fluoride dose was sufficient neither to cause mortality nor, realistically to increase the risk of cancer in the exposed students. After repeated system flushings over 48 days, the concentration of fluoride was brought to optimal levels. The electrical system was modified to prevent recurrence of a fluoride overfeed, and operational changes were made to minimize the impact of such an overfeed.

Fluoride Concentration in Drinking Water

Day Location Concentration

1 SF (a) no analysis
3 SF 92.0
3 (after flushing for 30 min.) SF 1.5-2.0
5 SF 17.5 (sample 1)
 11.0 (sample 2)
5 neighborhood (b) 0.9
6 SF 11.9 (sample 1)
 12.3 (sample 2)
6 (after flushing for 1 hour) SF 0.2
6 (after flushing for several various locations 0.8-1.2
 hours) in the school
7 SF 0.4
8 SF 6.5
9 SCF (c) 3.2
12 (after flushing all SF 0.3-0.7
 faucets for 2 days)
17 (after flushing for 1 day) SF 0.2
48 (after several additional SF [congruent to] 1.0

(a)School fountain.

(b)Neighborhood adjacent to theschool area.

(c)School cafeteria fountain.
Exposure Assessment for Fluoride

Data and Assumptions:

Concentration in water = 92 mg/L
Water consumed = 20 mL ( 1 mouthful)
Body weight, 10-year-old child = 32 kg
Possible loss due to vomiting = 50%

Exposure Assessment:
Fluoride taken in mouth = 92 mg/L X 20mL/day/1000 mL/L
 = 1.84 mg/day
Fluoride remained after vomiting = 1.84 mg/day X 0.50
 = 0.92 mg/day
Oral exposure dose = 0.92 mg/day/32 kg
 = 0.03 mg/kg/day

Comparison Dose:
Oral reference dose = 0.06 mg/kg/day
Minimal risk level, chronic = 0.05 mg/kg/day
Lethal dose = 32-64 mg/kg/day

Data and Assumptions: Reference

Concentration in water
Water consumed
Body weight, 10-year-old child a,b
Possible loss due to vomiting c

Exposure Assessment:
Fluoride taken in mouth

Fluoride remained after vomiting

Oral exposure dose

Comparison Dose:
Oral reference dose d
Minimal risk level, chronic e
Lethal dose f

(a)Behrman, Kliegman, & Jenson (1992).

(b)Sidhu et al. (1995).

(c)Sidhu & Sidhu (1999).

(d)U.S. EPA (1989b). The oral reference dose is based on prevention of
dental fluorosis in children.

(e)Riggs, Hodgson, & O'Fallon (1990): ATSDR (1993).

(f)Hodge & Smith (1965). ATSDR (1993).

Acknowledgements: The authors express sincere thanks to Jack Hartman, of the city of Portage; to Patrick Krause, of the Kalamazoo County Human Services Department; to Robert Green and Richard Benzie, of the Michigan Department of Environmental Quality, for their excellent cooperation; to Peter Kliejunas for technical assistance; and to Lawrence Chadzynski, R.S., MPH., for his critical review of the manuscript.


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Corresponding Author: Kirpal S. Sidhu, Toxicologist, Division of Environmental and Occupational Epidemiology, Michigan Department of Community Health, 3423 North Martin Luther King Jr. Blvd., P.O. Box 30195, Lansing, MI 48909. E-mail: <>.
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Author:Kimmer, Robert O.
Publication:Journal of Environmental Health
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Date:Oct 1, 2002
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