Fluoride’s definitely poisonous if you’re exposed to too much of it. A dose of less than a gram of sodium fluoride can cause nausea and vomiting; 5–10 g can be lethal. Not surprisingly, a major criticism of the public fluoridated water program is that it can provide too much fluoride.
Too much fluoride can cause dental fluorosis, a condition where the enamel surface of the teeth becomes mottled in appearance. Most dental fluorosis is very mild and doesn’t damage teeth, and it occurs only during tooth development in early childhood, so older children and adults aren’t at risk. Although it’s more common in fluoridated areas, it can occur in other areas as well.
Most fluorosis seems to be associated with kids swallowing too much fluoride toothpaste, or parents giving them fluoride supplements. Fluorosis levels have halved since the early 1990s with the wider use of special low-fluoride children’s toothpastes and recommendations that children use only very small amounts of toothpaste. But it’s an area that health authorities continue to monitor.
It’s known that high amounts of fluoride can also cause fluorosis of the bones, increasing your risk of fracture. It occurs most commonly in countries such as India and Pakistan where the natural concentration of fluoride in the water can be as high as 18 ppm.
A number of studies have looked for a relationship between exposure to fluoride at about 1 ppm and the risk of bone fracture. The results have varied, but when both of the UK expert reports looked at the results overall they concluded that there’s no proven additional risk of bone fracture associated with water fluoridation.
An Australian review in 2001 looked at 33 individual studies and concluded that fluoride to 1 ppm doesn’t have an adverse effect on bone strength, mineral density or evidence of fractures.
There have been claims that long-term exposure to levels of fluoride even as low as those found in Australian water may cause bone cancer or birth defects. The two expert reports from the UK found no association between bone cancer and fluoridation. The Australian NHMRC report considered three controlled studies and came to the same conclusion. And there’s no sound evidence at all that fluoride causes birth defects. Studies haven’t found any increase in their prevalence, even in those areas of India and Africa that have very high levels of fluoride in the water.
Is this the end of the story?
There’s a continuing need for high-quality research to track any potential harmful effects of fluoridation. But at present the evidence is very strong that it’s a safe and effective public health measure.