6. To what extent can fluoride exposure be harmful to organisms in the environment?
- 6.1 What levels of fluoride exposure are harmful to aquatic organisms?
- 6.2 What levels of fluoride exposure are harmful to microbes and plants?
- 6.3 What levels of fluoride exposure are harmful to birds and mammals?
6.1 What levels of fluoride exposure are harmful to aquatic organisms?
Fluoride can be toxic to aquatic life but some organisms are more sensitive to its effects than others. Its toxicity is very low for bacteria involved in wastewater treatment and appears to be low for algae.
Fluoride toxicity is low for an aquatic floating plant, common duckweed, with a concentration giving a 50% reduction in growth (EC50) greater than 60 mg fluoride/litre.
How easily invertebrates are affected depends on the species. The most sensitive appear to be caddisfly species with calculated “safe concentrations” varying between 0.2 mg/litre and 1.79 mg/litre.
The data on fish are quite variable but fluoride toxicity to fish appears to be less in hard water than soft water and greater at high temperatures than low temperatures. The concentration of fluoride lethal to 50% of groups of rainbow trout (LC50) exposed for 20 days in laboratory experiments carried out in soft water, ranged from 2.7 to 4.7 mg/litre. However, for a wild fish population in a specific river with hard water, safe concentrations for rainbow and brown trout were calculated to be 5.1 and 7.5 mg/l respectively. More...
6.2 What levels of fluoride exposure are harmful to microbes and plants?
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. More...
6.3 What levels of fluoride exposure are harmful to birds and mammals?
In birds, laboratory tests were carried out on European starling chicks to determine at what levels fluoride would affect their growth or be lethal. In the case of 1-day-old laboratory chicks, growth rates were significantly reduced at 13 mg fluoride/kg and half of the chicks died within the next 24h when administered a fluoride dose of 50 mg/kg body weight. That figure dropped to 17 mg/kg body weight for 16-day-old nestlings.
In mammals, the lowest concentration of fluoride in food to cause dental fluorosis was 35 mg/kg food in wild white-tailed deer. Fluorosis has also been observed in cattle and sheep, with dairy cattle having the lowest tolerance value at 30 mg/kg feed or 2.5 mg/litre drinking water.
Symptoms of fluoride toxicity include emaciation, stiffness of joints and abnormal teeth and bones. Other effects include lowered milk production and detrimental effects on reproduction. Near smelters, animal lameness has also been observed. More...