Drinking Water Contaminants – Fluoride

　　Water Contaminant – Fluoride

1. 　　Overview

   • Identity: Fluoride is a reactive element not found freely in nature, composing about 0.3 g/kg of the earth’s crust and present in minerals like fluorspar and fluorapatite. It has an oxidation state of -1.
   • Properties: Sodium Fluoride (NaF) is a white crystalline powder, while Hydrogen Fluoride (HF) is a colorless liquid or gas with a sharp odor. NaF has a melting point of 993°C and a boiling point of 1695°C at 100 kPa, with a density of 2.56 g/cm3. It is highly soluble in water at 42 g/liter at 10°C. HF is a strong acid in liquid form and a weak acid in water. It boils at 19.5°C.
   • Uses: Inorganic fluorine compounds are used in aluminum production, steel and glass fiber industries, manufacturing phosphate fertilizers, and in ceramics. Fluosilicic acid is used for water fluoridation. Sodium fluoride has limited solubility in water, while aluminum, calcium, and magnesium fluorides are less soluble.

2. 　　Analytical Techniques

   • Fluoride is usually measured with an ion-selective electrode, quantifying total free and complex-bound fluoride in water. This method applies to water with at least 20 mg/liter fluoride. For rainwater with 10 μg/liter fluoride, a detection limit of 1 mg/liter has been reported. A technique using a fluoride-selective electrode can detect fluoride from 0.05-0.4 mg/liter, and with modifications, from 0.4-1 mg/liter.

3. 　　Environmental Levels and Human Exposure

   • Atmosphere: Natural concentrations are around 0.5 ng/m3, rising to 3 ng/m3 with anthropogenic emissions. In the Netherlands, concentrations without sources are 30-70 ng/m3. In the US and Canada, air fluoride levels range from 0.02-0.2 μg/m3. Indoor air in Chinese provinces with high-fluoride coal can range from 16 to 46 μg/m3.
   • Aquatic Environment: Fluoride is commonly found in water, with higher concentrations in underground sources. Seawater contains about 1.3 mg/liter fluoride. Well water in fluoride-rich areas can contain up to 10 mg/liter, with the highest natural concentration at 2800 mg/liter. River concentrations vary, with the Rhine River below 0.2 mg/liter and the Meuse River between 0.2-0.3 mg/liter due to industrial activities. Chinese village groundwater has exceeded 8 mg/liter. Canadian drinking water ranges from less than 0.05 to 0.2 mg/liter (non-fluoridated) to 0.6-1 mg/liter (fluoridated), with well-water supplies reaching up to 3.3 mg/liter. In the US, 0.2% of the population is exposed to over 2.0 mg/liter. Annual averages in the Netherlands are below 0.2 mg/liter. In African countries with fluoride-rich soils, levels can be high, such as 8 mg/liter in Tanzania.
   • Water Purification: Household removal methods for fluoride are limited, with reverse osmosis being a sustainable option.
   • Food Consumption: All foods contain trace amounts of fluorine, absorbed from soil and water. High levels are found in curly kale and endive, as well as in fish and tea. Dry tea can contain up to 300 mg/kg, with 2-3 cups providing approximately 0.4-0.8 mg of fluoride.
   • Dental Applications: Dental products contain varying fluoride concentrations, from 0.25 mg per tablet in low doses to 10,000 mg/liter in liquids for topical applications.
   • Estimated Daily Exposure: Daily fluoride intake varies geographically, with food contributing 80-95%, drinking water 0.03-0.68 mg/day, and toothpaste 0.2-0.3 mg. Children’s intake is lower but relatively higher per body weight. Swallowing toothpaste or fluoride tablets can increase intake to up to 3.5 mg/day. Studies report daily intakes from 0.46 to 3.6 mg/day. In volcanic regions, exposure can reach up to 30 mg/day from drinking water.

4. 　　Kinetics and Metabolism

   • Following ingestion, water-soluble fluorides are almost completely absorbed in the gastrointestinal tract, while less soluble fluorides are absorbed to a lesser extent. Absorbed fluoride is distributed throughout the body, incorporated into teeth and bones with minimal storage in soft tissues. Incorporation is reversible, with excretion via urine, feces, and sweat.

5. 　　Effects on Lab Animals and In Vitro Test Systems

   • Chronic Exposure: Studies with sodium fluoride in drinking water have shown effects on skeletal tissues, including dentine discoloration, dysplasia, and osteosclerosis. In rats, effects on teeth and bones were observed at all dose levels, including the lowest of 4 mg/kg/day.
   • Mutagenicity: Fluoride induces genetic damage in vitro only at cytotoxic concentrations, with limited relevance to human exposure.
   • Carcinogenicity: IARC concluded that there is inadequate evidence of carcinogenicity in experimental animals. A recent study found an increase in osteosarcomas in male rats exposed to high fluoride concentrations, providing equivocal evidence of a carcinogenic effect.

6. 　　Effects on Humans

   • Fluorine may be essential for animals and humans, though its essentiality for humans is not conclusively demonstrated. Acute toxicity requires at least 1 mg/kg of body weight. Long-term ingestion primarily affects skeletal tissues, with protective effects against dental caries at low concentrations. Dental fluorosis occurs at concentrations between 0.9 and 1.2 mg/liter, with visible fluorosis at concentrations above 1.5 mg/liter in temperate climates. Skeletal fluorosis is observed at 3 mg/liter and crippling fluorosis at over 10 mg/liter. The US EPA considers 4 mg/liter protective against crippling fluorosis. Epidemiological studies on the association between fluoride and cancer or pregnancy outcomes are inconclusive. Individuals with renal impairment may have a lower tolerance for fluoride toxicity.

7. 　　Guideline Value

   • In 1987, IARC classified inorganic fluorides as Group 3, with no evidence of carcinogenicity in humans. The guideline value of 1.5 mg/liter set in 1984 does not need revision, as higher concentrations increase the risk of dental and skeletal fluorosis. National standards should consider climatic conditions, water intake, and other fluoride sources. In areas with high natural fluoride levels, achieving the guideline value may be challenging with available technology.