What levels of fluoride exposure are harmful to microbes and plants?
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 wate