@136: You realize that, in high enough concentrations, water and salt are also toxic? And that arsenic may be an essential nutrient
Let's talk numbers. It all comes down to concentration and dosage. At different doses the same element can have different effects. What are the concentrations of fluoride that cause skeletal fluorosis? According to the WHO, concentrations above 1.5 g/L are a problem. The CDC recommended concentration
for drinking water is 0.7-1.2 mg/L. The EPA maximum allowable concentration is 4.0 mg/L, and the recommended maximum is 2.0 mg/L. This is almost a thousand times less fluoride per liter than that which could cause problems such as skeletal fluorosis.
Oh, and here's a fun one for tea drinkers. Did you know that black tea
contains anywhere from 0.7-6.0 mg/L fluoride? The amount depends on the form of tea and its source.