Toxic Carnival: Fun with Fluoride
Note: this was written for Sciencegeist’s Toxic Carnival. Enjoy, and be sure to check out the other contributions! (Sciencegeist is providing a daily roundup of links, or you can find them via the #ToxicCarnival hashtag on Twitter.)
Hydrogen fluoride, or hydrofluoric acid, is nasty stuff. Because it is small and has no charge, it slips easily through fatty cell membranes and penetrates deep into the body. Once there, the fluoride ion it contains grabs onto vital calcium ions, interferes with proteins that help nerves transmit signals, and helps create other chemical species which damage cells.
Needless to say, hydrofluoric acid (HF for short) is very, very toxic. It’s considered a contact poison, meaning you can be poisoned just by getting enough of it on your skin. It is also just about the only chemical I can think of for which the antidote is stored right next to where it’s used in the lab.
Hydrofluoric acid has a lot of industrial uses, ranging from making teflon to etching glass and the silicon wafers used in computer chips. But several related chemicals, also containing fluoride ions, can be found much closer to home.
Look at your toothpaste. The active ingredient is probably either sodium fluoride, tin (stannous) fluoride, or sodium monofluorophosphate. Or better yet, go fill up a glass of water at your kitchen sink. Depending on where you live, and particularly if you’re in the US, there’s a good chance that your tap water contains some sort of added fluoride compound.
These fluoride compounds aren’t nearly as toxic as hydrofluoric acid. But they can, in small quantities, convert into HF in your stomach, and from there, make their way to the rest of your body. If this is the case, and if fluoride is so toxic once it gets to our cells, then why do we drink fluoride-containing water? Why do we put fluoride compounds in our mouths every time we brush our teeth?
When is fluoride helpful, and when is it toxic?
The fact that fluoride is in toothpaste should give you a big clue. Tooth enamel is made of a mineral called hydroxyapatite. As the bacteria in our mouths digest trapped bits of food, they release acids. These acids easily erode hydroxyapatite, and this erosion leads to tooth damage and decay.
Fluoride, however, can kick out hydroxyl groups (paired oxygen and hydrogen atoms) in the hydroxyapatite crystal and settle into their place. This process forms fluoroapatite, which is much more difficult for acids to attack. Fluoride also helps get calcium to deposit and rebuild damaged enamel.
In small doses, then, like the amounts we’re exposed to from toothpaste or from controlled public water supplies, fluoride helps strengthen teeth and prevent cavities.
But as you might expect, too much fluoride isn’t particularly good for you. Excess fluoride can, for example, prevent certain proteins from breaking down and getting out of the way of the apatite crystals in developing tooth enamel. This creates small pores which show up as white spots or streaks on your teeth, a condition called dental fluorosis. Children who take in a lot of extra fluoride, either from supplements, fluoride-containing foods, or water that naturally contains more fluoride, are particularly susceptible.
Fluorosis is mostly a cosmetic problem, and doesn’t really have any health effects except in very severe cases. This doesn’t sound too bad. But if you do a quick Google search, you’ll come across several websites claiming that fluoride also affects bone density and strength, and may trigger some forms of bone cancer. You might even see that too much fluoride has tentatively been linked to decreased IQ.
This stuff sounds scary. But is it true? Most of these websites are from groups that strongly oppose fluoridating public water supplies, and they have a clear anti-fluoride bias. What do you find if you instead look to the scientific literature, where the sources are (we hope) less biased? Do they agree, or not?
And the scientists say…
When we dig into the scientific literature, things become kind of complicated. For example, the studies linking fluoride exposure to bone cancer are contradictory and far from definitive. One of the frequently cited studies on the anti-fluoridation sites is this study suggesting that exposure to higher levels of fluoride through drinking water (bottled, from the tap, or from a well) and fluoride supplements increased bone cancer rates in young boys. Another group notes, however, that this link doesn’t show up in a second, related dataset (discussed in more detail here). If you do a quick PubMed search, you quickly find some studies which suggest links between fluoride and bone cancer (like this one) while others find that there is no link at all (like this one and this one).
Because it’s hard to actually control fluoride exposure in studies on humans, researchers also use studies on rats in which they give different rats different amounts of fluoride, and see whether or not more fluoride causes health problems. Some of these studies do show that health problems like bone cancer arise when there are high levels of fluoride in the rats’ drinking water.
But the caveat here is that the link only shows up when the rats are exposed to fluoride levels much, much higher than those found in public water supplies. For example, here is a summary of one such study. Note that the mid- and high-dose rats who developed bone cancer (osteosarcoma) were exposed to >100 parts per million (ppm) of sodium fluoride, corresponding to 45 ppm of bare fluoride. Most public water supplies are fluoridated to only about one ppm fluoride, so this is almost a factor of 50 difference. Thus these results probably aren’t directly useful in assessing the risks of our everyday fluoride exposure.
The studies linking fluoride exposure to decreased IQ are similarly tricky to interpret. Most of these studies are epidemiological, looking at differences between similar groups of people living in areas with more or less natural fluoride. See, for example, this paper, this paper, and this paper, where the fluoride levels range from about 0.5 to 3 ppm. However, very few of these studies talk about how well they control for other things like differences in the amount of arsenic and lead in the water (which probably means that they didn’t control for it), so without that information it’s hard to know how much of the effect is really due to fluoride.
And as if things weren’t already complicated enough, fluoride and other water contaminants may interact with each other in your body. A small amount of fluoride in the drinking water might have one effect if there’s no lead in the water, and a different effect if there is lead in the water. So far the only controlled studies on this topic have been done in rats, again with artificially high fluoride concentrations. So, right now it is hard to know how this effect will hold up in future studies, or how it will apply to humans.
So, what’s the bottom line?
All in all, it seems like there might be links between excessive fluoride exposure and some of these health problems , but these links are far from definite. Many of the studies aren’t particularly rigorious, they study fluoride at unrealistic concentrations, or they fail to account for other factors which could explain the same results. If there is a cause-and-effect link between fluoride and these health problems, we need a lot more research to prove it.
I can understand, however, why people might be scared of fluoride, especially if it is in their water and they can’t control their exposure. But I think it’s important to keep in mind that if you do a meta-analysis of all of the papers that have been published on fluoride, the weight of the evidence shows that at normal levels fluoride is safe and that the only proven effect of over-exposure at realistic doses is fluorosis (similar reports here and here). Dental fluorosis is more of an aesthetic issue than a significant health problem, so while it might be nice to avoid it, it’s certainly not going to kill you.
Fluoride is thus one of those chemicals which sounds scary, but in order to talk about whether or not it’s toxic, you need to know how much you’re talking about. I think this makes it a perfect topic for a post for the Toxic Carnival. I also think it’s an interesting choice because a lot of the information that’s out there on fluoride uses very chemophobic language, which isn’t really a good representation of what the scientific literature says.
Anyway, the bottom line on fluoride seems to be this: at low doses, it protects your teeth. A little higher, and it might start to make your teeth look weird, especially if you’re a kid and your teeth are still growing; anything else is unproven. And at extreme exposures – especially those originating from hydrofluoric acid burns – it can be fatal. But these exposures are way, way beyond what any of us should normally encounter, and as long as we don’t do anything stupid, fluoride should be able to do its job of preventing cavities while we go about our happy, healthy lives.
Added 5/23/2012 – See comments for notes on the 2006 NRC study, another important meta-analysis of the fluoride literature (which is a little more cautious than the ones cited above).
Added 5/24/2012 – One of my friends pointed out that dose is important, but the mechanism of delivery is important, too. Fluoride in drinking water and fluoride in toothpaste, for example, are probably not equivalent from a toxicological standpoint, because they reach different parts of the body. If future studies do prove that fluoridated drinking water has significant adverse health effects (i.e. that it’s “toxic”), that doesn’t necessarily mean that the fluoride in toothpaste is bad too.
So, in the second-to-last paragraph it would be more accurate to say that in order to talk about whether or not something is toxic you need to know not only how much you’re talking about but also how it is delivered. This should be (but rarely is) an important aspect of discussions about “toxic” chemicals. In any case, I think this is a really good point, and I’m sorry that I didn’t manage to address it when I wrote this post!
It looks like someone else wrote a nice post on HF for the Toxic Carnival. Check it out, especially if you’re interested in knowing more about this toxic (yet useful!) chemical!
I’ve already linked several scientific studies and other sources in the text of this post, but I also found the following two articles interesting and informative. Unfortunately, they’re both behind paywalls, but if you’re on a university network you might be able to get access to them.
Second Thoughts about Fluoride (Scientific American, 2007) – this is a nice, accessible overview of the science and controversy over water fluoridation. It does focus on studies of the potential adverse effects of fluoride, so it comes across as being slightly anti-fluoridation, but the author is also careful to note which studies are inconclusive and where further research is needed, so I felt it was reasonably balanced.
Molecular mechanisms of fluoride toxicity (Barbier, et al., Chemico-Biological Interactions 2010) - this one is heavy on the biochemistry, but if you’re interested in specific mechanisms by which fluoride messes with cellular function, this is a nice review.
Last but not least, my thanks to Lee Bishop over at Science Minus Details for feedback on the first draft of this post. You rock, Lee!