There is evidence that fluoride may be toxic to microbiological processes in soils at concentrations found in moderately polluted areas.
Signs of harmful effects on plants include the yellowing of leaves and slowed growth. When plants take up fluoride from soil its toxic effects depend on the ionic species of fluoride present and on the type of soil.
Many studies have assessed the effects of fluoride emissions on plants by exposing them to hydrogen fluoride (HF) gas. Leave tissue starts to die (leaf necrosis) above certain concentrations in air, e.g. 0.17 and 0.27 µg/m3 in the case of grapevines (for an exposure of 99 and 83 days). Airborne fluoride can also have an impact on the development of plant diseases, but the type and magnitude of the effects depend on the specific plant and its disease.
Evidence shows that the smaller the distance from great fluoride sources such as aluminium smelters or phosphorus plants, the higher the fluoride levels in soils and hence the degree of damage to vegetation. Mean levels of fluoride in vegetation near one of these large fluoride sources ranged from 281 mg/kg in severely damaged areas to 44 mg/kg in slightly damaged areas. Near an aluminium smelter, differences in plant community composition and structure were observed due partly to variations in fluoride tolerance, but other variables such as other atmospheric pollutants must also be taken into account when interpreting the many field studies on fluoride pollution.
