About a year ago, the city of Portland, Oregon, was in the news because of its water supply—and not because a teenager decided to relieve himself into a reservoir. Instead, the issue was fluoridation, the addition of trace amounts of fluorine to municipal drinking water. Fluoridation is widespread in the US, as copious evidence indicates it improves oral hygiene.

That evidence prompted the Portland City Council to approve fluoridation—only to see voters reject that plan by a wide margin. While some of the opposition focused on the finances of the deal for the fluoridation process, concerns about the safety of fluoridation also played a major role in organizing the opposition.

Further Reading Portland, Oregon rejects drinking water fluoridation by wide margin

It turns out that a similar drama had been playing out in New Zealand, where the city of Hamilton reversed course on water fluoridation several times over the past two years. Now, in response to the kerfuffle, some New Zealand researchers (combined with a ringer from Duke) have looked into one of the supposed health threats posed by fluoridation: it stunts the mental development of children. Their new report finds no evidence of this, however. In fact, children who grew up with fluoridated water had slightly higher IQs than their peers, though the difference wasn't statistically significant.

The fears about fluorine's impact on intellectual development appear to derive from a meta-analysis of natural exposure to fluorine in China, where levels in the water can reach several times those generated by intentional fluoridation. Although the study found a link between high fluorine and reduced IQ, the relevance of the study to the levels used in the US and elsewhere wasn't clear.

That, however, hasn't stopped anti-fluoridation activists from latching on to the paper as an indication that fluorine represents a threat. Not surprisingly, the paper makes appearances at the Fluorine Free Hamilton website. That group helped convince the city council of Hamilton to stop fluoridating its waters. After a public outcry, a referendum showed that this measure had the support of only 30 percent of Hamilton's citizens. After a national court ruled in favor of fluoridation in a different city (suggesting opposition may be widespread in New Zealand), the city council restored fluoridation.

In the mean time, however, public health experts were motivated to look into the health impact of fluoridation. Conveniently, they had a study population on hand: the Dunedin Multidisciplinary Health and Development Study has been tracking more than 1,000 children born in that city on New Zealand's South Island since their births in the early 1970s. Conveniently, fluoridation was not uniform in Dunedin, creating experimental and control populations. The cohort also had its IQ tested at ages from seven to 13 years old, and the study tracked whether the children had used fluoride-containing toothpaste or tablets.

Overall, nothing made any difference to IQ. "No statistically significant difference in IQ existed between participants who had or had not resided in areas with [fluoridation], used fluoride toothpaste, or used fluoride tablets, both before and after adjusting for potential confounding variables," the authors conclude. In fact, the children in areas with fluoridation had slightly higher IQs (although, again, this difference was not statistically significant).

Given that New Zealand uses fluoride levels that are similar to those in the US and Australia, the study appears to indicate that the high levels seen in some water in China are not relevant to controlled administration of fluoride in water. The authors also raise the prospect that any effects on IQ were the result of something other than fluorine, given that the Chinese studies didn't always take confounding variables into account.

Will this end public resistance to fluoridation? Certainly not; people who are convinced of something aren't often swayed by evidence that runs contrary to their beliefs. But it will provide some assistance to public health authorities who have to deal with anxious legislators when this issue comes up.

American Journal of Public Health, 2014. DOI: 10.2105/AJPH.2013.301857 (About DOIs).