Should we think twice about fluoride?

If you live in the US, there’s a good chance you’re one of the approximately 209 million people drinking tap water that contains added fluoride. Local governments have been putting fluoride in city water supplies for nearly 80 years. In many ways, its addition has been a remarkable public health success: According to the Centers for Disease Control and Prevention (CDC), drinking fluoridated water reduces tooth decay by 25 percent.

But in late August, the US National Toxicology Program (NTP) published a long-awaited report declaring, with “moderate confidence,” that drinking water with elevated fluoride levels is linked to lower IQ in children.

Fluoride in our drinking water has inspired conspiracy theories for generations. Robert F. Kennedy Jr., who is also a prominent anti-vaxxer and Covid-19 skeptic, is one of the most vocal proponents of banning fluoridation to protect children from neurodevelopmental problems.

This, though, isn’t a conspiracy theory. Scientists have spent decades trying to figure out what level of fluoride strikes the best balance between oral health and healthy brain development. The NTP’s new 324-page report reviews results from over 500 experiments, lending more weight to the idea that fluoride can be connected to brain problems than, say, a single fraudulent, now-retracted study linking vaccines to autism.

Science is a frustratingly non-linear process. Facts that seemed unquestionable a decade ago can be overturned by new data at any moment. If you’ve never thought about fluoride outside of the dentist’s office, hearing that it might have neurotoxic effects could be confusing, scary, or infuriating. Why is fluoride still in our water and toothpaste, then?

It’s not because dentists and scientists are lying to us (although sometimes they do). It’s because finding the whole, unshakable truth is a never-ending process.

So, don’t throw out your toothpaste and stockpile bottled water just yet. Here’s what the science says (and doesn’t say) about fluoride and the brain.

Fluoride is a mineral found naturally in soil, water, and things we consume, like brewed tea and shellfish. It’s also in the toothpaste most of us use. Fluoride strengthens our enamel and prevents cavities. It’s about more than maintaining pearly white teeth; left untreated, tooth decay leads to severe infection, which can spread throughout the entire body.

At the turn of the 20th century, dental researchers noticed that many people living near high-fluoride water sources had splotches on their teeth, a condition called fluorosis. Once scientists began studying the effects of fluoride on dental health, they realized that, while fluoride can damage teeth and bones at high concentrations, drinking water with low levels of fluoride actually prevents tooth decay. Today, many cities add a small amount of fluoride to their tap water — about 0.7 milligrams per liter (mg/L).

But around 1.9 million people in the US live in areas where tap water naturally contains more fluoride than the government recommends, according to the report. For example, the Ogallala Aquifer, which provides water for West Texas, is infused with fluoride-rich volcanic ash, bringing its fluoride concentrations up to a whopping 5 mg/L.

The report linked fluoride levels of 1.5 mg/L — about twice the level recommended by federal health officials — with lower IQ in children. According to the NTP, more research is needed to figure out whether there are also risks associated with higher levels of fluoride for adults, or with the lower concentrations the government recommends.

While these findings have been reproduced enough times to be worth taking seriously, the science of intelligence — including the concept of IQ itself — is questionable.

Measuring brain function isn’t straightforward. After all, the brain has many functions: keeping the body alive, helping us walk and talk, guiding our learning and decision-making.

More than 70 of the 100 human studies reviewed by the NTP specifically looked at children’s IQ as a proxy for brain development — IQ tests are easier and cheaper to run than, say, brain scans or full neurological exams. Of those 72 IQ-related studies, only 19 were flagged by the NTP as having a low risk of bias, controlling for factors like socioeconomic status, which could also contribute to differences in IQ scores. Only a handful of those 19 studies were conducted in North America, and none in the US.

So, critically, none of these human studies tell us anything about how fluoride changes the brain at a biological level. Even studies in lab animals and cells did not identify how fluoride might affect learning, memory, or intelligence.

IQ scores also only capture an incomplete picture of early childhood brain development and are tangled up in a number of socioeconomic, racial, and cultural factors beyond how much fluoride someone is exposed to.

Historically, mothers were blamed for their children’s cognitive disabilities. Redirecting that blame to environmental factors like MMR vaccines — or, say, fluoridated water — can give parents a sense of control over scary-sounding diagnoses. A 2017 federal lawsuit calling on the EPA to ban water fluoridation already has the enthusiastic support of high-profile anti-vaxxers.

The trial hinges on the results of NTP’s final report, a draft of which was originally presented over a year ago. The drawn-out legal battle has been messy, with dental health advocates obstructing fluoride research and denying the NTP’s findings.

If the court decides fluoride poses health risks that outweigh its benefits, it could end water fluoridation in the US.

Amid scientific uncertainty and political shadiness, it’s hard to know what to believe.

For now, the NTP has simply determined “with moderate confidence” that there is a correlation between higher fluoride exposure and lower IQ in children — specifically, IQs 2 to 5 points lower.

But it’s hard to know how we would measure the differences in lived experiences based on a few IQ points.

If you’re concerned about your family’s fluoride exposure, you can check your tap water’s fluoride levels on the CDC’s My Water’s Fluoride page. If you happen to live somewhere with levels above around 1.5 mg/L, your tap water crosses the threshold of neurodevelopmental concern. No need to switch to bottled water, though; home water purifiers can filter fluoride out completely.

To make sure you don’t throw out the oral health benefits of fluoride with the bathwater, keep using fluoride toothpaste for those over the age of 2 (just don’t swallow it).

And if you’re still fluoride-skeptical, you can order some hydroxyapatite toothpaste — which has similar cavity-preventive benefits — from Europe or Canada, where it’s been approved and endorsed by dental associations.

This story originally appeared in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.