Fluoride

2. Where are fluorides found?

• 2.1 How are fluorides released into the environment?
• 2.2 Where in the environment can fluorides be found?
• 2.3 How much fluoride is there in environment?
• 2.4 How much fluoride can be found in living organisms?

2.1 How are fluorides released into the environment?

Naturally, fluorides are released into the environment through the weathering of rocks and through atmospheric emissions from volcanoes and seawater.

Human activities releasing fluorides into the environment are mainly the mining and processing of phosphate rock and its use as agricultural fertilizer, as well as the manufacture of aluminium. Other fluoride sources include the combustion of coal (containing fluoride impurities) and other manufacturing processes (steel, copper, nickel, glass, brick, ceramic, glues and adhesives). In addition, the use of fluoride-containing pesticides in agriculture and fluoride in drinking water supplies also contribute to the release of fluorides to the environment. More...

2.2 Where in the environment can fluorides be found?

In air, fluorides can be present as gases or particulates. They can be transported by wind over large distances before depositing on the earth’s surface or dissolving in water. In general, fluoride compounds do not remain in the troposphere for long periods nor do they move up to the stratosphere. Only sulfur hexafluoride (SF₆) can stay in the atmosphere from 500 to several thousand years.

In water, when inorganic fluoride compounds dissolve they split up into ions although the speed at which they dissolve depends on the type of compound and on factors such as the acidity of the water. The transport and transformation of fluorides is influenced by pH, water hardness and the presence of materials such as clay, which exchange ions. In water with a neutral pH and low fluoride concentrations, fluoride is predominantly present in the form of fluoride ions (F^-). As they travel through the water cycle fluorides usually combine with aluminium.

In soils, fluoride is predominantly combined with aluminium or calcium. When the soil is slightly acidic, fluoride tends to adsorb more strongly to soil particles. Fluorides are very immobile in soil and are not easily leached from it.

In living organisms, the quantity of fluoride taken-up depends on the route of exposure, on how well the particular fluorides are absorbed by the body and on how quickly they are both taken up and excreted. Fluoride released from soluble fluorides tend to bioaccumulate in some aquatic and terrestrial plants or animals. However, there is no known information on the biomagnification of fluoride, i.e. fluoride build-up in aquatic or terrestrial food-chains. More...

2.3 How much fluoride is there in environment?

In surface waters, such as rivers, fluoride levels depend on the proximity to human or natural emission sources; they generally range from 0.01 to 0.3 mg/litre. In seawater fluoride concentrations are higher, i.e. 1.2 to 1.5 mg/litre. In areas where the natural rock is rich in fluoride or where there is geothermal or volcanic activity, very high fluoride levels, up to 50 mg/litre, may be found in groundwater or hot springs.

In air, fluoride emitted both naturally and from human activities in gaseous and particulate forms generally deposits relatively near its emission source. In areas without nearby emission sources, the mean concentrations of fluoride in ambient air are generally less than 0.1µg/m³. Even near emission sources, the levels of airborne fluoride usually do not exceed 2–3 µg/m³.

In most soils, fluoride is present at concentrations ranging from 20 to 1000 µg/g. This figure can reach several thousand µg/g in mineral soils with natural phosphate or fluoride deposits. More...

2.4 How much fluoride can be found in living organisms?

Aquatic organisms take up fluorides directly from water or to a lesser extent via food, and tend to accumulate fluoride in their exoskeleton or bone tissue. Mean fluoride concentrations of more than 2000 mg/kg have been measured in the exoskeleton of krill while mean bone fluoride concentrations in aquatic mammals, such as seals and whales, ranged from 135 to 18 600 mg/kg dry weight.

Fluoride levels in terrestrial animals and plants are higher near natural and human fluoride emission sources. Lichens, which have been used extensively as biomonitors for fluorides, generally contain less than 1 mg fluoride/kg (background level), but at a distance of 2 to 3 km from fluoride emission sources mean concentrations range from 150 to 250 mg/kg.

Most of the fluoride in the soil is insoluble and, therefore, less available to plants. But the fluoride that is present in soil solution is taken up through the root and accumulates in leaves.

Fluoride accumulates in the bone tissue of terrestrial vertebrates, depending on factors such as diet and the proximity of fluoride emission sources. For example, mean fluoride concentrations of 7000 to 8000 mg/kg have been measured in the bones of small mammals living near an aluminium smelter. More...