Evidence of absence is not absence of evidence

More on the ongoing fluoridation story:

Firstly, there is a very good statement from the PM’s chief science adviser, Peter Gluckman.  As he says, the scientific issues are entirely settled: at the concentrations used in treating water, fluoride reduces tooth decay and does not cause any harm.  At one time there was scientific uncertainty about adverse health effects; this is just not the case any more.  There is still a question of whether you want to treat the whole population in this way.  We add iodine to salt and folate to bread, but these prevent more serious illnesses than fluoridation does, and there are fewer people with an irrational fear of iodine or folate.

Second, the Herald has a Digipoll on fluoridation

The poll showed 48 per cent of New Zealanders supported the addition of fluoride – double the 25 per cent of those who opposed its use. A further 24 per cent believed the issue should be left to local councillors to decide.

Unfortunately, the poll tried to use a single question to address two unrelated issues: do you want fluoride in your water?, and should the decision be made nationally or locally?  As a consequence, it’s hard to interpret the results.  The ratio of for:against is about the same as in the Hamilton referendum that started fluoridation there in 2006, but if you assume all the people who want the issue decided locally  are really against fluoridation, the opinion would be nearly 50:50.  It obviously isn’t reasonable to assume everyone in favour of local decision-making is against fluoridation — I’m on record as a counterexample — but there’s no way to know how these folks would split.

The Herald story goes on to quote an antifluoride lobbyist

Ms Byrne said the group had science to back its claims that fluoride was toxic and harmful when added to water and without applying it directly to teeth offered none of the benefits health authorities claimed.

However, that is hotly disputed within the science community.

It’s not disputed within the scientific community, it’s disputed by the scientific community. The science, as Sir Peter observes, is settled.