Drinking Water Contaminants - Fluoride Page 2

Fluoride is usually determined by means of an ion-selective electrode, which makes it possible to measure the total amount of free and complex-bound fluoride dissolved in water. The method can be used for water containing at least 20 g/litre (2). For rainwater in which fluoride was present at a concentration of 10 �g/litre, a detection limit of 1 g/litre was reported (4).

A method using a fluoride-elective electrode and an ion analyzer to determine fluoride at levels of 0.05?.4 mg/litre has been described (5). With a slight modification, the method can be used to measure fluoride at 0.4?.0 mg/litre.

3. Environmental levels and human exposure

Air

Natural background concentrations are of the order of 0.5 ng/m3. If anthropogenic emissions are included, worldwide background concentrations are of the order of 3 ng/m3. In the Netherlands, concentrations in areas without sources are 30?0 ng/m3, rising to 70 ng/m3 in areas with many sources (2). In a survey of fluoride in the air of some communities in the USA and Canada, concentrations were in the range 0.02?.0 g/m3 (6). In some provinces of China, fluoride concentrations in indoor air ranged from 16 to 46 �g/m3 owing to the indoor combustion of high-fluoride coal for cooking and for drying and curing food (7).

Water

Traces of fluorides are present in many waters; higher concentrations are often associated with underground sources. In seawater, a total fluoride concentration of 1.3 mg/litre has been reported (2). In areas rich in fluoride-containing minerals, well-waters may contain up to about 10 mg of fluoride per litre. The highest natural level reported is 2800 mg/litre. Fluorides may also enter a river as a result of industrial discharges (2). In groundwater, fluoride concentrations vary with the type of rock the water flows through but do not usually exceed 10 mg/litre (3). In the Rhine in the Netherlands, levels are below 0.2 mg/litre. In the Meuse, concentrations fluctuate (0.2?.3 mg/litre) as a result of industrial processes (2).

Fluoride concentrations in the groundwater of some villages in China were greater than 8 mg/litre (8,9). In Canada, fluoride levels in drinking-water of <0.05?.2 mg/litre (non-fluoridated) and 0.6?.1 mg/litre (fluoridated) have been reported in municipal waters; in drinking-water prepared from well-water, levels up to 3.3 mg/litre have been reported. In the USA, 0.2% of the population is exposed to more than 2.0 mg/litre (3). In the Netherlands, year-round averages for all drinking-water plants are below 0.2 mg/litre (2). In some African countries where the soil is rich in fluoride-containing minerals, levels in drinking-water are relatively high (e.g., 8 mg/litre in the United Republic of Tanzania) (3).