Podcast: Is Fluoride Toxic at Low Levels?

Welcome to Entanglements. In this episode, hosts Brooke Borel and Anna Rothschild ask: Is fluoride toxic at low levels? The history of the mineral is scattered with alleged health claims — and conspiracy theories. But a growing body of research on fluoride’s effect on developing brains has raised new questions and heated debates.

To dig in, our hosts invited two experts with differing opinions to share their points of view, in an effort to find some common ground. The point isn’t to both-sides an issue or to try to force agreement. Instead, the show aims to explore the nuance and subtleties that are often overlooked in heated online forums or in debate-style media. 

Their guests this week are Scott Tomar, a professor and associate dean for prevention and public health sciences at the University of Illinois Chicago College of Dentistry, and Christine Till, a professor of psychology at York University in Toronto and a licensed clinical neuropsychologist.

Below is the full transcript of the podcast, lightly edited for clarity. New episodes drop every Monday through the end of the year. You can also subscribe to Entanglements at Apple Podcasts and Spotify.


Brooke Borel: Welcome to Entanglements, the show where we dive into some of the most heated debates in science. I’m Brooke Borel.

Anna Rothschild: And I’m Anna Rothschild.

Brooke Borel: Well, Anna. There’s no question that today’s topic is particularly contentious — and it’s been that way for a long, long time.

Anna Rothschild: Yeah, contentious may even be an understatement.

Brooke Borel: That’s right. Because here’s the question of the day: Is fluoride toxic at low levels? 

Anna Rothschild: Yeah, this sounds like we’re veering into Dr. Strangelove territory.

Brooke Borel: Yeah, I know. Honestly, even a couple of years ago I personally would have rolled my eyes at this one, I will admit. There’s clear evidence that fluoride can help strengthen and protect teeth. And starting in the 1940s, some municipal water supplies started adding low levels of fluoride to drinking water, and since then it’s been praised as a public health triumph. 

Anna Rothschild: Right.

Brooke Borel: But ever since then, there have also been conspiracies and just plain false claims related to health about fluoride in our drinking water.

Anna Rothschild: Exactly. People have claimed water fluoridation was actually a Communist plot for mind control, and stuff like that. 

Brooke Borel: Right, right. But more recently, RFK Jr., Trump’s nominee for Health and Human Services secretary, has been talking about alleged dangers of fluoride. And in September, a federal judge in California told the Environmental Protection Agency that it has to respond to safety concerns over fluoridated water. It’s really unclear how all of that will play out, but that ruling could actually force policy changes for drinking water.

Anna Rothschild: Right, so, this is sort of where my knowledge on this ends. What evidence is there that fluoride is bad for us? 

Brooke Borel: Well, so, since March of this year, Undark’s contributing editor Michael Schulson has been reporting on fluoride and how it may or may not affect health. And I’ll link to a bunch of his articles in the show notes. And so there’s this modest body evidence that high levels of fluoride might be bad for developing brains. So, we’re talking brains in fetuses and young kids. 

Anna Rothschild: Hm, OK.

Brooke Borel: Yeah, and when I say high levels here, to be clear I mean levels of fluoride that are at least twice as high, or higher, than what we add to the water supply in the U.S.

Anna Rothschild: OK, well if that’s the case do we need to worry about drinking water then?

Brooke Borel: Well, maybe not everyone. But we do consume fluoride in lots of places – it’s not just from our drinking water. And there’s some famous (or infamous, depending on your perspective) work from Canada, that suggests that even the amounts pregnant people are currently consuming — not just from water, but from everywhere — that could have an impact on their kids. These Canadian researchers and their team, they found a connection between higher fluoride levels in a pregnant person’s body and lower IQ in their children — specifically, in young boys.

Anna Rothschild: Wow. OK. 

Brooke Borel: Yeah, those studies are not totally conclusive though, and it’s sparked this heated situation amongst all these scientists. A group of dentists and other researchers actually accused the research team led by those Canadian researchers of professional misconduct and tried to get two key researchers from that team fired from their respective universities.

Anna Rothschild: Yikes OK.

Brooke Borel: Yeah and one of those researchers, Christine Till, is actually one of my guests today. 

Anna Rothschild: Oh, I’m super interested to hear what she has to say.

Brooke Borel: Yes, but first, let’s hear from someone with a different point of view. 

[Music]

Brooke Borel: The question of the day is: Is fluoride toxic at low doses?

Scott Tomar: So the short answer to that is no.

Brooke Borel: This is Scott Tomar. He’s a professor and associate dean for prevention and public health sciences at the University of Illinois Chicago College of Dentistry. He’s also an epidemiologist and used to work at the Division of Oral Health at the Centers for Disease Control and Prevention. And for the most part, any epidemiological research he’s done on fluoride has focused on health and cost benefits.

Brooke Borel: Why do you think that? Could you lay out the evidence that you’re pulling from for people who aren’t following the science closely?

Scott Tomar: All the studies that have looked at potential associations between IQ or other measures of neurodevelopment and exposure to fluoride in the levels found in community water fluoridation have found no association whatsoever. There’s been a recent systematic, actually several recent systematic reviews on that. A study that just came out in Australia. So a number of well done studies in areas where the level of exposure is what you would find in community water fluoridation. And again, they found no association.

Anna Rothschild: That all sounds pretty positive. 

Brooke Borel: Well, it might not be painting the entire picture, because there are some studies that do find a potential association between fluoride and lower IQs. So the argument is whether those studies are done well and if they’re actually showing what they claim. So, some context, and this is crucial to understand: All of the studies on the risks of fluoride to human health are “observational studies” — or sometimes they’re called “epidemiological studies.”

Anna Rothschild: Right, I mean a lot of what we talk about on this show relies on studies like this. 

Brooke Borel: Yeah.

Anna Rothschild: These are studies by the way, just for a little refresher, that look backwards at the outcomes for people just living their lives in the real world. And they’re really good at showing a link, or a correlation, between a particular substance and a particular outcome. But they’re not good at showing causation, or definitive proof.

Brooke Borel: Right. It would be unethical to intentionally give a suspected risky substance to someone, yeah? So, in order to measure the effects of a mineral like fluoride out in the real world, the scientists who do these studies have to find proxies to sort of sit in for the real deal.

So, take the Australian study that Scott’s talking about. It estimates the intake of fluoride based on where the kids live. So, is the water fluoridated there? And then the researchers evaluated the brains of these kids by having their parents respond to standardized surveys that try to measure behavioral development and executive function.

Anna Rothschild: Hm.

Brooke Borel: Yeah. The researchers aren’t actually taking samples from people to know the actual level of fluoride in their bodies, or, you know, peering inside of their brains to see how they may be affected. 

Anna Rothschild: I think Scott also cited a few systematic reviews that he said found no correlation? 

Brooke Borel: Right, I followed up with him about that, and he sent me a paper that reviewed many studies, which collectively, according to those researchers, did not find an association between water fluoridation and children’s cognition. 

Anna Rothschild: OK.

Brooke Borel: However.

Anna Rothschild: Oh no.

Brooke Borel: There are other big reports that pull together lots of papers on this subject too that do find an association. One of those is a giant one from the National Toxicology Program, or NTP, which is this interagency program that’s housed in the U.S. National Institutes of Health. If you want an even more in depth look at the NTP report, Michael Schulson, Undark’s contributing editor, wrote a great piece on it.

Anna Rothschild: Awesome. I mean did both of those reports include the Canadian study you talked about before?

Brooke Borel: Yes.

Anna Rothschild: OK, that’s very confusing. What does Scott think about that Canadian research?

Brooke Borel: Yeah, I asked him about that. 

Scott Tomar: I say that they’re interesting, but, you know, hardly definitive studies. Many questions have been raised on these studies on just how valid their measures are, you know, certainly on the exposure side.

Brooke Borel: I know I’m talking a lot, bear with me. But I should explain just what these Canadian studies did, right?

Anna Rothschild: Yes, and Brooke, I love the sound of your voice. Please, please go one. 

Brooke Borel: Great, great. Hopefully everyone else does too.

Brooke Borel: Anyway. These studies used different proxies than the Australian study that we already talked about, OK? So instead of estimating how much fluoride kids were exposed to based on their location, these researchers tried to look at how much fluoride was in a pregnant woman’s body, while she was pregnant, which could have passed to her fetus. 

Anna Rothschild: Right. I mean, if you have high levels of fluoride in your body, it doesn’t necessarily mean it’s from your drinking water, though right? Fluoride comes from lots of places, as you said before.

Brooke Borel: Yeah. I mean, you could be drinking a lot of lot of water, so that makes a difference. It also is found in different foods or other products made with fluoridated water. And it also occurs naturally in some stuff. Like, black tea has a lot of fluoride in it.

Anna Rothschild: Interesting, I didn’t know that.

Brooke Borel: Yeah, it bioaccumulates I guess. Anyway, they used data from a biobank called Maternal-Infant Research on Environmental Chemicals, or MIREC.

Anna Rothschild: Oh, that’s a mouthful.

Brooke Borel: It is, it is. We’ll just call it MIREC, OK, for the rest of this episode. And they collected that data from 2008 to 2011. And so the MIREC researchers, one of the many things they did is that they collected women’s urine at multiple points during their pregnancies

Anna Rothschild: Love it.

Brooke Borel: Yes, love it. Who doesn’t love peeing in a cup? Anyway. And so the group led by researchers at York University, these Canadian researchers, later took those samples and measured the fluoride content. And so they were using fluoride in the urine as a proxy for how much fluoride might be in their bodies as a whole. 

Anna Rothschild: Right, OK, I get that. So presumably the more fluoride in your pee the more you had in your blood. And the more you might be able to pass to your child.

Brooke Borel: Right. And this is what’s called an “individualized biomarker,” which is a technical term, but it’s gonna keep coming up as we talk about these studies. 

Anna Rothschild: OK, good to know, thank you.

Brooke Borel: Anyway,  the Canadian researchers and their team did several studies with this data, but there was one that published in 2019 that we’re going to talk about a lot in this episode. In that one they looked at IQ tests that were administered to the kids a few years after they were born. And to be clear, they also controlled for a lot of variables, like socioeconomic status.

Anna Rothschild: OK, so what did they find?

Brooke Borel: So they found that higher levels of maternal fluoride, those were linked to IQs dropping by several points, at least in one subgroup, which was young boys. But Scott brought up some common criticisms for this approach.

Scott Tomar: Maternal spot urine fluoride levels really depend on what they ate or drank in the 24 hours before that. Fluoride in urine has a very short half life. And so taking, you know, even three measurements over the course of a 9 month pregnancy, isn’t really a valid measure of exposure throughout pregnancy or the type of chronic fluoride exposure that, you know, we might want to measure if we were thinking that it leads to some neurologic condition or reduced IQ.

Brooke Borel: Scott’s right here that there are better ways to measure maternal fluoride levels. For instance, through blood samples, or continuous urine measurements throughout the pregnancy. But the researchers were limited by that MIREC data. That said, I did ask a nephrologist — so that’s a kidney doctor — about how the Canadian scientists controlled for variation in the data, and that expert said that they used a decent approach. But Scott had other criticisms too — in particular, how the IQ tests were measured and why the effects weren’t seen across all the kids.

Scott Tomar: There’s actually been a number of people, certainly with far more expertise than I have in measuring IQ, as to whether — particularly in the MIREC study, the studies out of Canada — whether their measures of IQ are reliable. The way it was measured by a variety of different research assistants that measured it, in an age where it’s notoriously difficult to measure. These were three and four year olds. It takes an incredibly skilled set of professionals to do that. I don’t know if that was met in that study. If you actually read the full paper, they did not find an overall difference in IQ based on the level of fluoride exposure during pregnancy. They only found that when they did a post hoc sex specific analysis. So they found that effect in boys, not in girls.

Brooke Borel: Just a quick fact-check: What Scott’s saying here is that this research team hadn’t planned to separate their analysis by sex, and that they decided to do that after they collected the data, which is considered poor methodology. But we asked Christine Till, who is an author on those Canadian studies, about this and she said the sex specific analysis was planned from the beginning and was even included in her team’s original grant proposal. And also there are possible developmental reasons that a substance could affect boys and girls differently.

Brooke Borel: When we’re talking about observational studies like this, if there is a signal, but it’s not a definite thing, how do you think this should be communicated to the public? And at what point should it be communicated to the public when there is still a certain amount of uncertainty?

Scott Tomar: I certainly think that we as health care professionals, as well as researchers, I mean, I think we do have an obligation to share information. It’s, you know, frankly it’s better that they hear it from responsible sources. And so I think as these studies come out, yes, they should be communicated with the public. What certainly concerns me with some of these is how they have been communicated with the public. You know, often in ways that dramatically overstate or in some cases I’d say misrepresent what the studies really found.

Brooke Borel: Christine has recommended that pregnant women reduce their fluoride intake, which Scott and others don’t think is an appropriate recommendation. And for Scott, what’s at stake is the dental health of young kids who might not otherwise have access to preventative care.

Scott Tomar: Dental caries, you know, tooth decay, still remains an incredible public health problem. It’s unfortunate that it’s overlooked often by our policymakers because they and their families probably have not experienced it. But what I see coming through the doors of the dental school where I work every day, just a hundred feet from the office where I’m sitting in now, we treat over a thousand young children, primarily under 4 years of age, for a disease that we say is preventable. They’re treating them under general anesthesia with all the risks and expense that comes with that. I don’t pretend that community water fluoridation is it any way a panacea. It’s not going to solve the problem. But again, we’ve got a pretty consistent and long body of literature showing that it does reduce the problem.

[Music]

Anna Rothschild: So I do hear Scott’s point about tooth decay. I am exceedingly lucky that my parents could afford good dental care for me, and my experience with cavities is that they’re a minimal problem. But that really I think is just a testament to my privilege more than anything else. 

Brooke Borel: Yeah, totally. And same. 

Anna Rothschild: That being said, though, anytime you look at a public health measure, you have to weigh the cost and the benefit. So Scott doesn’t think water fluoridation is hurting anyone, so to him the benefits are clear. But obviously, other scientists hold a different point of view.

Brooke Borel: Right. And are you ready to hear from one of those scientists now?

Anna Rothschild: Oh I absolutely am.

[Music]

Brooke Borel: The question of the day is: Is fluoride toxic at low doses? And specifically, we’re talking about the doses that typically might be found in drinking water. 

Christine Till: So when I’m answering the question about fluoride at current levels, I’m looking as a scientist who measures fluoride in the urine of the individuals we are studying. I’m looking at the current levels, from the biomarkers that we are measuring. And the question is fluoride, you know, toxic at current levels? I would say that we do have evidence of a dose response where higher levels of fluoride in urine, especially in pregnant women measured through the course of pregnancy, are associated with lower IQ in children.

Brooke Borel: This is Christine Till. She’s a professor of psychology at York University in Toronto and a licensed clinical neuropsychologist. And she’s an author of the epidemiological studies that looked at the MIREC data. Her team included a wide range of experts, too, including epidemiologists and a dentist.

Brooke Borel: Why do you think there might be a concern regarding toxicity? I was hoping you could lay out the evidence that you’re pulling from for people who aren’t following the science closely. 

Christine Till: Yeah. So, I’ll start by saying there is a lot of evidence from the last few decades And to give you an idea, the National Toxicology Program identified studies done in humans that were relevant to the topic of neurodevelopmental effects of fluoride or thyroid related effects of fluoride. It provides extensive evidence showing a consistent association between higher fluoride exposures and diminished IQ in children.

Brooke Borel: So here, let’s pause, again, sorry, so we can talk just a bit more about the NTP report.

Anna Rothschild: Great, OK. So it sounds like it looked at a lot of different research. So yes, I think that makes sense.

Brooke Borel: They sure did. So, the authors of the NTP report pulled together all the studies they could find that looked at the effects of fluoride on brains. Most of those studies focus on kids. And then they separated these studies into those that looked at IQ and those that looked at other outcomes related to cognition or brain development. And then they also sorted the papers into high and low quality. And to be clear, the authors of the report have stressed that it is not intended to specifically make recommendations about drinking water levels.

Anna Rothschild: OK. Gotcha

Brooke Borel: So, the vast majority of both the high- and low-qualities studies found an association between IQ and fluoride. So as fluoride levels went up in the body, IQ went down. And that was true for studies that measured cognition other ways, as well. But again, and I stress: These are observational studies. So there seems to be some cohesion in the findings, but it’s just not conclusive.

Anna Rothschild: OK. And so, one more thing, when we’re talking about higher exposure here, are these higher levels of fluoride than what’s normally found in fluoridated water?

Brooke Borel: Yeah, again, a lot of these studies were carried out in places where fluoride occurs naturally in the groundwater. Some of those places have a much, much higher fluoride level than what you’d get from the tap in a U.S. city or wherever that has fluoridated water.

Anna Rothschild: So, how did the NTP report rate Christine’s research?

Brooke Borel: So the authors rated hers as high quality. And here’s what she had to say about the common criticisms of her work.

Christine Till: There’s been a lot of debate about the use of urinary fluoride concentration that we get from spot samples. So for sure, there is measurement error in this biomarker, and that is precisely why we looked at collecting measurements at each trimester and taking the average, that’s a lot better than only taking one spot sample because there will be measurement error.

But with respect to the claim that it’s not valid, I would disagree that urinary fluoride concentration, including ones from spot samples, have been used for decades as a valid biomarker of an individual’s level of fluoride exposure. It’s routinely used in epidemiological studies of fluoride and other toxic chemicals that are excreted in the urine, like arsenic and mercury and cadmium and so on. It’s a proxy for fetal exposure, but it is the best that we have.

Brooke Borel: The other pushback I have heard, or one other pushback I have heard on this kind of study is using IQ and trying to use that measurement in kids that are so young. And some of the difficulties that can arise in even testing kids that are this young in an IQ test. I was wondering if you could speak to that a little bit.

Christine Till: Yeah, so I’m a psychologist by training and I teach IQ testing. And that is a standardized measure that has been used for decades, and it is the one measure that psychology has been studying now for, you know, over a hundred years. And the reason that it is so valued by society is because it is meaningful. I can’t think of any other variable that predicts educational attainment, employment status, health status, and so many other outcomes in life that we value. In terms of measuring it in young children, there’s going to be variability, of course children respond to their environment and IQ can change over time. But again, it is the best measurement that we have at the time. I would definitely not call it an invalid measure of a child’s cognitive ability.

Brooke Borel: And one thing that we just hear so much as science journalists is correlation does not mean causation, right? But of course, those kinds of studies, that’s what they’re looking for, because they have limitations in whether or not they can say that something’s causal. Do you think there’s a causality here in terms of fluoride? And if so, why?

Christine Till: So epidemiological studies demonstrate association, not causality. I completely agree with that. However, they are an essential domain of evidence. So, if we dismissed studies that could not prove causation, we would have failed to make public health breakthroughs like the link between cigarette smoking and lung cancer. But ironically, we also wouldn’t know that water fluoridation is protective against tooth decay if we dismissed observational studies. So, they’re clearly important in informing public health issues. So in terms of causality, we really need to look at the consistency of the evidence from the observational studies, but also, looking at the animal studies to weigh in on the causality claim.

Anna Rothschild: Wait, is that true about the evidence for tooth decay?

Brooke Borel: Yeah, so actually, there has never been a gold-standard study to prove that fluoridated water protects against tooth decay. I’m talking about a randomized controlled trials here, right. 

Anna Rothschild: Oh my god, that’s crazy.

Brooke Borel: And so they’re also depending on epidemiological research to prove the benefits, just a side note. Anyway, so the first ever such study is actually currently underway at the Adams School of Dentistry at the University of North Carolina.

Anna Rothschild: And there aren’t any of these gold standard trials on the possible neurological risks of fluoride, right?

Brooke Borel: No, and ironically, I’m not sure such a study would be able to be done, right? Like, would that be able to get past the ethics boards that are required to look at these things if they truly thought it was a neurotoxin? I’m really not so sure. So I emailed with the study lead from that project at UNC. And he said they are not looking at fluoride as a risk in the study at all, just the benefits.

Brooke Borel: I’m curious, when you first started asking these research questions, were you worried about fluoride or were you just seeing a gap in the knowledge and you just thought it was worth looking at.

Christine Till: Yeah, so that has — the history of how I even got into this topic is kind of interesting. But it really started with a graduate student named Ashley Malin, who brought it to my attention over 10 years ago. And I thought fluoride was safe, pretty much like everyone else, at low levels that we’re exposed to. But after reading some of the animal studies on the topic, I realized that there was a really good rationale for studying this further. I’ve been, you know, advised by colleagues that it would ruin my career as a researcher. I was tenured at the time, so I didn’t have to fear losing my job by stepping into a controversial area.

Brooke Borel: Were those colleagues proven right or wrong, that warned that it would ruin your career?

Christine Till: I don’t have any evidence that my career has been ruined. You know it — there have been some difficult times dealing with some of the backlash that we have faced as researchers. At the same time, I also get a lot of support from other environmental health scientists that we are doing important work. So no, I don’t think my career has been ruined.

[Music]

Brooke Borel: So, Christine isn’t exaggerating when she says that her work has sparked backlash. As I mentioned, some of that backlash has come from other researchers. But what I didn’t tell you before is that includes our other guest, Scott Tomar.

Anna Rothschild: Oh my goodness. The drama, Brooke.

Brooke Borel: I know. So after Christine’s group published their first paper in 2019 in the scientific journal JAMA Pediatrics, Scott and 29 other scientists and dentists wrote a letter to the National Institute of Environmental Health Sciences in the U.S., which had funded the study. And that letter claimed that the authors had not done their work correctly, more or less, and also said that they had blocked other researchers from accessing their data and analytical methods for an impartial outside assessment.

Anna Rothschild: OK, I mean, wouldn’t the paper have gone through peer review? There should have been — 

Brooke Borel: Yeah, of course. But these researchers who signed the letter wanted to see the data for themselves. And as Christine explained it, the problem was, the data wasn’t hers to give — she herself had to go through a lengthy application process for access to the MIREC data. And so these other researchers would need to do that same thing. So there was a long dispute over all that and I know that at least one group tried to get access to that data but was ultimately turned down. And anyway, the people who wrote this particular letter wanted the U.S. funder to provide the data, which it couldn’t do — because it didn’t own the data. But there is more.

Anna Rothschild: Oh boy, OK.

Brooke Borel: Yes. So then in 2021, some of these same researchers sent a massive complaint to several other places, including the Office of Research Integrity in the U.S. about Till and her team and accusing them of research misconduct. And then another letter that is nearly identical went to York University, but it went even further, calling for Till to be fired. 

Anna Rothschild: Wow, oh that’s really intense.

Brooke Borel: Yeah, so Scott signed many letters, he did refuse to sign the one that went to her university calling for her termination. And also, to be clear: Till and her team were cleared of any accusations of misconduct and she was not fired.

Anna Rothschild: And after all that Scott and Christine were still willing to talk to each other? Did you ask about the letters?

Brooke Borel: Yes and yes. Have a listen.

Brooke Borel: So as we all know, this topic of fluoride can be pretty contentious. And I was really curious to hear a little bit from both of you about why you decided to come on the show with us today.

Christine Till: Well, I think this is exactly what we need to bring people with different views to come together to talk about why we look at the same evidence and see it differently. So I can’t think of any other forum where we have the conversation about how we interpret the science

Brooke Borel: Great. Thank you. How about you, Scott?

Scott Tomar: Yeah, I mean, I agree. You know, it’s just the nature of how science progresses. The studies come out, somebody either, you know, confirms, refutes, comes up with a different interpretation. But that’s the nature of science. That’s why we, that’s why we do it and why we publish it.

Brooke Borel: And why did you come on the show?

Scott Tomar: You know, again to have a civil discussion on what the evidence is, what it means, what its public health implications are. It’s an important topic.

Brooke Borel: I have noticed in this conversation that depending on your  perspective on fluoride and your interpretation of these papers, people tend to lean on one set of papers or another set of papers. So Scott, for example, you’ve mentioned the Australia study, and then there are also these studies out of Canada. These were all epidemiological studies, but they’re looking at different things, right? I was wondering if you could both talk a little bit about observational studies and how we determine what a good proxy is.

Christine Till: There are some key design features to determine one’s initial confidence in a study. So temporality: Is exposure measured before the outcome. The outcome measure, is it valid? So, for example, IQ, using a standardized instrument is considered a very sensitive and valid measure used, for example, by EPA for determining policies with lead exposure. Accounting for key confounders, like maternal age, child sex, socioeconomic status. So, other variables that might be associated with both exposure and the outcome are really important that they are well controlled in addition to other neurotoxic exposures that might be common to that particular area. And then how that exposure is measured. And an individualized biomarker is considered a gold standard over, um, drinking water.

And that’s because, just because water is fluoridated that tells you nothing about whether the person drinks, two cups of water or two liters of water, which will be directly associated with their internal dose. So we want to look at how that exposure was measured. And then lastly, the statistical approaches that are applied to analyze the data. Were they rigorous? Was it done properly? So these are some important variables that have to be considered when we’re determining the quality of a study. 

Brooke Borel: And Scott?

Scott Tomar: As Dr. Till already said what we typically want to see is the strength of association, consistency of association, temporality, you know, all the things that were mentioned. There’s always limitations on our measures of both the exposure and the outcome. So, you know, an individual level biomarker, while it can be superior there are limitations. For example, a single spot urine measurement is not necessarily an ideal measure of chronic exposure or even exposure over the course of a full day or a week. But again, you know, it’s the best that could be done in many studies. Same thing with reported measures, including in studies that have a pretty good residential history. But again, you know, again, it’s the assumption that they lived in that area. We don’t know how much they drank, whether they, you know, exclusively drank tap water, you know. So yeah, there’s always limitations.

Brooke Borel: OK, could we talk about the Australia study a little bit more? I’m curious because Scott, you’ve gone to that more than these other studies as, as your sort of main evidence for there not being an association. What is it about that study that you find particularly compelling?

Scott Tomar: So, you know, again, Australia has widespread use of community water fluoridation — it’s over 90 percent of the population that are on public water systems are receiving fluoridated water. And so we would think with that level of exposure in the population, if there was a negative effect, we would expect to see it quite a bit in Australia. And there are, you know, again, there are areas where they don’t have fluoridation. Smaller parts of the population. But again to the extent that  they could tease that out in their population based cohort study — in that study, they found no association between fluoride exposure, based again on community water fluoridation, and their measures of child neurodevelopment. That it was not IQ, they used a different scale, but no association.

Brooke Borel: Right. And Christine, what do you make of that Australian study?

Christine Till: I would like to look at that study again. I don’t even think it was rated amongst the high quality studies by the NTP. So I have been focusing on the higher quality studies. 

Brooke Borel: Quick side note: the Australia study wasn’t actually included in the NTP report because it published after their cut-off date. But Michael Schulson at Undark wrote to one of the NTP authors about the study, who said, and I quote: “As to study quality, we didn’t formally assess it, but I recall thinking at the time that it was one of only very few studies that we ran across that didn’t include any direct measurements of exposure or outcome from the actual children.”

Christine Till: But also just to reiterate that policymakers are not going to be well informed or persuaded by any one study, but rather by the best available evidence as a whole. So we really need to look, I mean, we could spend all day talking about the study and that study. I can say that I have been involved in many studies now looking at prenatal exposure and postnatal exposure in childhood. And it’s the prenatal exposure that I find most compelling — very strong effects, consistency across many studies and newer studies that are coming out. So I also want to emphasize that looking at critical periods of exposure is also very important when we’re evaluating the science.

Brooke Borel: So, these scientists don’t see eye to eye on the research — just like many others don’t. And I wanted to know what they each thought about the general animosity surrounding this topic.

Brooke Borel: I’m curious how each of you sort of view that and whether you think it’s blocked productive discussion in this area.

Scott Tomar: Those of us that have been involved in fluoridation for a long time —  it’s probably 30 years that I’ve been involved in fluoridation. I think part of my skepticism, having been involved in this for so many years, is that the nature of the supposed adverse health effects has changed over time. You know, when I was in graduate school, there were accusations of fluoridated water, increasing risk for bone fracture. And then we went through a period of accusations of osteosarcoma. And then in the eighties, we had accusations that had increased the susceptibility to HIV infection. I mean, it just — there’s just a long history of accusations of fluoride.

Brooke Borel: Of course, some conspiracy theories, too, like mind control and the sort of stuff that.

Scott Tomar: Oh yeah. I mean going back to the sixties with, you know, Dr. Strangelove and all that. But you know I’d say even attempts to show adverse health effects, aside from sort of the John Birch society conspiracy theory. So I think over time maybe it has made us defensive. It’s like, you know, it’s another series of studies. So I think that that’s certainly part of the landscape. That said, you know, all of us — and again I shouldn’t speak for others —  I’m a dentist, but I’m also an epidemiologist, a researcher. Of course we should constantly review the literature and adjust as necessary. So it’s not that — I don’t want to think that I’m so close minded that if the preponderance of evidence ultimately shows that there’s a consistent negative effect — yeah, we might have to rethink this. As I’ve said earlier, I don’t think we’re at that point.

Christine Till: Yeah, I’d like to comment on that too. 

Brooke Borel: Yes, of course.

Christine Till: So, you know, I’m on the other side as someone raising questions about the safety of fluoride intake. You know, it’s a lopsided level of science because we’re coming at it asking questions, legitimate scientific questions, with good research designs. And just to give you an example of how much gritting of teeth needs to be done, no pun intended, to get the work done, just to get funding, for example. In one year when I was starting out, I submitted about four grants to our Canadian funding agency. And the tone of the feedback was, you can’t look for risks of fluoride without evaluating its benefits. You would never see the opposite. If somebody was proposing a study to look at the benefits of fluoride, you would never get a dentist being asked by your grant reviewer, well, you can’t look at the benefits without looking at the risks. So, that comment illustrates the concept of “my side” bias, where people evaluate a study in a way that’s biased towards prior beliefs or attitudes about a topic.

So, we’ve been kind of fighting that “my side” bias for years, just to do the research, to publish the research. And that’s part of the scientific process. It has overall made the science stronger, in my opinion, because we are refining our methods based on all of the critiques we receive. But I have to say, as a fluoride researcher on the other side, just trying to do the research, you know, I honestly, I didn’t care what the finding was. We just wanted to do the research and see what the data showed. But we felt muzzled doing this research. And then when we published the study, there’s this fear of retaliation or scholarship harassment from particular groups and people. Which now affects other scientists who may not want to ask this question because they fear that retaliation.

Brooke Borel: So I do want to talk about a little bit about that retaliation — the potential for retaliation in this field. So, there were some letters that came out about specifically your work, Christine. And I know, Scott, you’re involved in some of those letters. I was wondering if you could talk a little bit about that.

Scott Tomar: So, the ones that I signed — there was various attempts to access the data from the MIREC study, because there, frankly were some questions that a number of had about the data analysis. We were attempting to get the data, to do independent analysis of it. For a variety of reasons that never happened. So we’ve never able to get it. The one that I would not sign, there was a letter and they had approached me and I flat out refused, was accusing the investigators, including Dr. Till of professional misconduct. Which to this day I do not think was the case. And I would not lend my name to that. So, you know, requesting data, again, I think as part of the normal scientific process. You know, accusing people of fabrication or misrepresentation or other professional misconduct —  again, I do not, I did not then and do not now feel that that is the case. And I would not lend my name to that letter.

Brooke Borel: A quick fact-check: Scott actually did sign some letters accusing Christine and her team of misconduct. There were many letters and this happened several years ago, but during our fact-check of this episode we found out that there were several with his signature. I followed up with him and he said that he misspoke here. He said, in part and I quote from an email: “I do not believe that there was fabrication of data, and I think that’s what I had in mind when I answered your question. I was mistaken when I said that I did not sign letters alleging research misconduct.” He also said that he continues to believe the paper intentionally omitted key information in its abstract and dramatically overstated its conclusion.

Brooke Borel: Christine, do you have any response to any of that? How, how was that and what, what do you have, what questions or comments do you have on that?

Christine Till: Well, what was the response? I mean, we’ve had a lot of letters sent, as some, by your guest here, and some more serious. Accusations of research misconduct is a serious threat to the principles of academic freedom. 

The day the paper was published in JAMA Pediatrics I said, you know, no one’s going to believe this. Someone really should replicate the analyses. We did have multiple statisticians. An epidemiologist working in isolation did the analyses separate from a graduate student. Then we did hire a third analyst to replicate again. But, you know, it’d be nice to have someone outside of the group. Okay. So I was the one who originally said this should — I think all fluoride research should be done by two groups, quite honestly, because it’s so polarized and nobody believes anyone. They’ll say, oh, they’re biased. 

But, back to these letters, I received an email asking for access to the data, and I replied with here you go, there is an application, it took me almost a year to get through, it’s a very lengthy and lots of forms. I went through it, I accessed the data belonging to Health Canada. It doesn’t belong to me, but if you or anybody else would like to access it, you have to go through the process. So, to be accused later on — and actually I just saw it in the National Post again, just last week — that our research group are not sharing something. That’s just completely false. But the media continues to — like, the damage has been done. 

All of these letters that have been sent widely to media, now the media is putting those links up again. And so people naturally are confused. They don’t know who to believe because they’re hearing, “Oh, this group is withholding data,” which is just not true. That is not the case. Anybody can write to me, ask me, can you run the analysis with this point missing or whatever. And we reply. So, you know, I should hope that people who are interested in the results can reach out to our group and ask a question and get an answer. That’s, you know, how I think science should work.

Brooke Borel: Scott, did anyone in these letter writing campaigns try to reach out to Christine’s group directly for any of this back and forth or did it go straight to the letters? 

Scott Tomar: I have to actually dig out the history. I don’t remember where the —  honestly I don’t remember. I think that they did originally reach out to Christine’s group. 

Brooke Borel: Christine’s shaking her head. No, you don’t remember. Yeah.

Christine Till: I know who reached out and, um, definitely, well, there wasn’t I can tell you.

Scott Tomar: I honestly, I don’t remember. I honestly don’t remember the chronology of the letters. 

Brooke Borel: And what are your feelings about these letters now, Scott? Are you glad that you wrote them or do you have any regrets? Would you do anything — would you do things the same way or differently if you had the chance to do it again?

Scott Tomar: Good question. I probably would address it in maybe a less threatening way. At the end of the day most of us on, on one side or probably in the dental world, you know we’re all health care providers, most of also epidemiologists, we’re parents, we’re grandparents, you know. So I think at the end of the day, we all want to have some assurance of what’s really going on. But I think if I were to do it again, maybe there’s a less confrontational, less threatening way to have done that.

Brooke Borel: I’m curious from both of you if it’s harder or easier to be speaking directly about this stuff rather than these sort of confines of like the traditional sort of debate in science where it will be like a letter or a commentary or an op-ed or this sort of social media at this point, going back and forth. Like, how are you finding this conversation in comparison to those more formal and — informal with social media — channels?

Christine Till: Well. I don’t engage on social media. 

Brooke Borel: Probably wise.

Christine Till: It’s not the forum for scientific discussion. This is a really complex topic. There’s a lot of nuance. We have to bring people together, the dental community and the non-dental public health community. You know, I’ve come to realize that there’s a differential weighting in what we value. Some value teeth more than IQ points. And it really comes down to this. I think overall, we’re all invested in improving children’s health. I do believe that, and it’s trying to understand, can we come up with a way that we can achieve children’s health with minimal harm and, you know, improve oral health and prevent any harm at the same time. So, I think, you know, we’re moving in that direction. I would like to hope so. Yeah.

Brooke Borel: I’m curious whether you have agreed or disagreed more than you were expecting during this conversation.

Christine Till: Well, could I ask Dr. Tomar on his views on — would you recommend that a pregnant woman reduce her fluoride intake?

Scott Tomar: No, I don’t think that I would recommend it. 

Christine Till: OK, and if she told you she drank, you know, three cups of black tea made with fluoridated water — no problem?

Scott Tomar: I would say, and again I’m not the expert on the exposure to high levels of caffeine. So there may be other reasons that a pregnant woman might want to reduce her intake of black tea. I’m not yet convinced that this is a real effect again. Again, I may change my answer down the road. But as of today, no, I don’t think so.

Christine Till: OK. So Brooke, to answer your question of whether we agree or disagree, I would say that on that point, I would say based on the science today it is time to at least acknowledge the evolving science, particularly for pregnant women. And knowing that there is zero benefit to the fetus of gestational exposure to fluoride, I think it is prudent to recommend reducing fluoride intake during pregnancy.

And yeah, maybe we’re all going to be —  maybe the science will change. But until we see that data, I will say that I think it is prudent to reduce fluoride intake from whatever source. And black tea is one that can have very high sources of fluoride —  levels of fluoride, I mean. So, that’s one area I think we would disagree on.

Brooke Borel: OK. Are there ways in which this conversation has not gone how you expected, where you did seem to have some overlap or just have things go a different way than you’re expecting?

Scott Tomar: I don’t think so. I always find that it’s —  and I really do appreciate the opportunity to be part of this because I really do think that it’s always helpful to have people that have diverse opinions or at least, you know, different viewpoints, talking face to face. If I go to a dental conference I know that there will be several sessions on fluorides, but it’s going to be mostly fluoride and dental researchers talking to fluoride and dental researchers. And probably the same thing on the environmental health and neurology groups. Maybe really is getting a time for serious researchers to be in the same conference, so we can, we can hear from each other, understand — you know, may not necessarily agree on everything, but at least understand where we’re coming from.

Christine Till: Yeah, I agree. I think it’s time for that. And, you know, we need to look at both angles for sure.

[Music]

Anna Rothschild: Wow. Brooke, that was a doozy. Oh my gosh. 

Brooke Borel: I know, right? And I didn’t even include all the data and all the studies that I could have, right? 

Anna Rothschild: Well of course, this is a podcast. You can’t fit everything in.

Brooke Borel: Yeah.This is a podcast, not an encyclopedia. 

Anna Rothschild: Yeah. I mean, what’s your takeaway, ultimately, though?

Brooke Borel: I mean, my takeaway is that this is all far from conclusive, but I think that the ways in which some researchers have dug in and tried to discredit these studies — you know, it sort of feels like they’re kind of painting this more recent work with the same brush as previous sort of claims against fluoride. And I don’t know if that’s entirely fair. Like, this is definitely not on the level of like Dr. Strange Love conspiracy theories, right? This isn’t Communist mind control claims, right? And even some of the previous health claims about fluoride, I don’t think that this is necessarily in the same sort of category as that. And part of science is looking at new data and being willing to engage with it. And it just seems like the engagement here is not happening in a way that’s fruitful to answer any of these questions.

Anna Rothschild: Right, exactly. I mean, I will say that what Scott said at the end there about, you know, how long he’s been working on this and how many of these sometimes really crazy claims he and his profession have had to sort of fight off over the years. Like, Christine has dealt with these very targeted attacks for a few years now. But Scott’s been dealing with all sorts of attacks for many years. So I do understand why he like a knee jerk reaction.

Brooke Borel: Yeah.

Anna Rothschild: And is like, you know, this is just the most recent one. 

Brooke Borel: Right.

Anna Rothschild: That doesn’t mean that it’s like a valid thing to do. It seems like, you know, Christine’s publishing work with big data sets that’s gone through peer review. It’s a very different thing than Communist mind control. But still, I get where that reaction comes from.

Brooke Borel: Yeah, and I will say, this didn’t make it into the final cut of this episode, but I did ask both of them if they did receive hate mail for their work, and they both said yes. And I suspect that he’s been doing this work for longer, and so I suspect that there’s a longer trail of that kind of reaction as well, right?

Anna Rothschild: Right. So it can feel like Christine is sort of alone here under attack by these people, but I think that it’s probably a little bit more complex than that.

Brooke Borel: It’s a far more complex ecosystem in this longstanding conversation and fight.

Anna Rothschild: Exactly. I mean, I think ultimately, like you said before, this is just one of those issues where we need more data and we all need to be open minded enough to, you know, collect more data on this.

Brooke Borel: Yeah collect it. Don’t overstate it. Collect it. Talk about it. Maybe don’t like threaten to fire people over it. I don’t know.

Anna Rothschild: Yeah.

Brooke Borel: But you know, I’m curious what our listeners think about all of this. Should we have even tackled this? Are you mad? Are you happy? What do you think?

Anna Rothschild: I know. I’m really curious, too. If you have any thoughts at all, please send us an email at [email protected].

Brooke Borel: And that’s it for this episode of Entanglements, brought to you by Undark magazine, which is published by the Knight Science Journalism Program at MIT. The show is fact checked by Undark deputy editor Jane Reza. Our production editor is Amanda Grennell, and Adriana Lacy is our audience engagement editor. For this episode, we had additional reporting by Michael Schulson. Special thanks to our editor in chief, Tom Zeller Jr. I’m Brooke Borel. 

Anna Rothschild: And I’m Anna Rothschild. Thanks for listening. See you next time.

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