You’ll find fluoride in tea, beer and fish, which might sound like a balanced diet to you. This week Alan Johnson announced a major new push for putting it in the drinking water, with some very grand promises, and in the face of serious opposition.
General Ripper first developed his theories about environmental poisoning and bodily fluids when he experienced impotence, fatigue, and a pervasive sense of emptiness during the physical act of love. He instantly identified the cause: a communist plot to pollute our precious bodily fluids with fluoride.
Bill Etherington MP calls it a “poison“. Campaigners say Nazis used it to subdue people in concentration camps. According to the Guardian’s own (sadly departed) alternative health columnist, fluoride is “in the same league as lead and arsenic.”
The reality is that anybody making any confident statement about fluoride – positive or negative – is speaking way beyond the evidence. In 1999 the Department of Health commissioned the Centre for Reviews and Dissemination at York University to do a systematic review of the evidence on the benefits of fluoridation on dental health and to look for evidence of harm. Little new work has been done since.
They found 3200 research papers, mostly of very poor quality (full references on badscience.net as ever). The ones which met the minimum quality threshold suggested that there was vaguely, possibly, around a 15% increase in the number of children without dental caries in areas with fluoridated water, but the studies generally couldn’t exclude other explanations for the variance. Of course, the big idea with fluoride in water is that it can reduce social inequalities in dental health, because everyone drinks it: but there isn’t much evidence on that either, the work is even poorer quality, and the results are inconsistent.
So when the British Dental Association says there is “overwhelming evidence” that adding fluoride to water helps fight against tooth decay, they’re with General Ripper. And when Alan Johnson says: “Fluoridation is an effective and relatively easy way to help address health inequalities, giving children from poorer backgrounds a dental health boost that can last a lifetime,” he’s really just pushing an admirably old fashioned line that complex social problems can be addressed with £50m worth of atoms. The people behind the York review have had to spend a fair amount of time pointing out that people are misrepresenting their work.
But since I’m in the mood for some scaremongering, let’s not forget the potential harms. Fluoridation will give around one in eight people mottled teeth (“fluorosis”). And there’s something else to worry about, if you like worrying. An observational study from Taiwan found a high incidence of bladder cancer in women from areas where the natural fluoride content in water was high. It might easily have been a chance finding – the study in question measured lots of variables, and if you measure enough things, then some of them are bound to come out positive, just by chance. But it could be real.
The problem here is one of small effect sizes. You don’t need a careful designed study to show that falling out of a plane will probably kill you, but fluoride and bladder cancer would be a pig to research: because the effect size is small, the exposure is spread over half a century, and the outcome – bladder cancer – takes a lifetime to reveal itself. Welcome to the finer details behind “more research is needed”.
And the fascinating thing about public health is, with population effects, the numbers can start to get very scary, very quickly: in the UK, for example, just a tiny 10% increase in risk would give you 1000 extra new cases of bladder cancer every year. Fear. Actually, I enjoyed that. Maybe I should move to the Mail.
The most readable overview on the poor quality of the data is this article by Cheng, Chalmers and Sheldon. Prof Sir Iain Chalmers founded the Cochrane library, for anyone who seeks to doubt his badassness.
The Taiwan study is here (the authors attribute the increase in risk to a problem of multiple comparisons):
And if you want a particularly good example of someone pompously overstating the evidence, try this doozy from the Sunday mirror.