Deposition of fluorides increases the fluoride content of vegetation, and eventually soil. Fluoride is very rapidly fixed by mineral soils so even where the concentration increases substantially, there is no increase in uptake by plants. Effects on plants are caused by direct uptake of HF from the air, not by deposition on soil.There is some observational evidence that high levels of deposited fluoride may, after many years, lead to an increase in the litter/organic layers. However, the research was done near very old, poorly controlled sources so it is not clear that it will occur under current conditions. Furthermore, some of the work is of questionable quality.
The main concern is effects on animals. If the long-term concentration in the diet averages over 35-40 ppm (dry wt. basis) then herbivores may be at risk of developing the condition called fluorosis. Excess fluoride causes:
- Dental lesions in developing teeth
- Abnormalities in the bones that can lead to stiffness, lameness and inadequate food intake.
The percentage of the population affected and the severity of the symptoms increase with the concentration of fluoride in the diet and the duration of exposure. Accumulation of fluoride in bone can be reversed if the diet is changed but tooth lesions are irreversible. Search by 'Species' to see full details of fluoride deposition impacts on mammals and macro invertebrates.
In Britain, fluorosis has been primarily associated with atmospheric deposition but soil contamination from fluorspar is also known to cause problems. There is still a risk in old mining areas - see references by Andrews and Cooke (1984) and Andrews et al. (1989). Phosphate fertiliser contains from about 0.5 to 3% fluoride so continuous use leads to an increase in F in the top 5-7.5 cm of pasture soils. There is evidence that cattle and sheep grazing on such pastures ingest sufficient soil to increase the fluoride absorption to a level that is on the verge of fluorosis. It is not known if this is a potential problem for any wild mammals in Britain.
|Critical Load/ Level|
No estimate available