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Almost a year ago, we wrote about calcium fluoride and its benefits as a micro-mineral or lack thereof. Well, recently, one reader had a bunch of related questions and comments that we thought were super interesting. So we’ve decided it would be a good idea to do a rapid-fire question-and-answer session. This week’s column will answer all the questions sent in by our reader. For the sake of anonymity, we won’t name them. Hopefully, they get to take a look at these answers. We’d love feedback!

I just read your calcium fluoride article and have a question: It's not calcium fluoride that is added to water. It's either sodium fluoride or hydrofluorosilicic acid. I'm wondering if calcium fluoride is the naturally present one in some areas but it is not artificially added to water ... ?

Your first statement is correct! As our article states, fluoride is usually added to water in some form other than calcium fluoride. We mentioned sodium fluoride specifically, but you’re right, hydrofluorosilicic acid can also be added. These two compounds are soluble and are much better at providing dissolved fluoride than calcium fluoride. Calcium fluoride is more commonly found as an earth mineral and isn't usually found in water, whether added by people or from the earth. Most of the time it's buried in the ground, and it naturally forms in geological time – like, millions of years.

So can you explain how water separates those ions and how our body deals with this compounds?

Ion separation has to do with the individual ion's relative attraction to water instead of the other ion it’s first bound to. If the attraction to water is bigger, then the compound dissolves. If it isn't, the compound doesn’t. Some molecules dissolve without splitting into ions – like glucose – but it’s the same idea. Each molecule of glucose is more attracted to water than other glucose molecules, so it dissolves as well. It’s all about attraction.

Relative attraction brings in a host of different properties. Some compounds are soluble in one liquid and others aren’t, which is strictly based on how attracted they are to one another. Solubility isn’t static though – it can be adjusted by either pH, pressure and temperature, or other compounds that react with what we’re trying to dissolve to form more soluble products. However, in general, calcium fluoride isn't very soluble in water. So, if you introduce a source of calcium and a source of fluoride that are both otherwise soluble, making calcium and fluoride ions in water in the process, you'll precipitate calcium fluoride. Literally – calcium fluoride will fall out of your solution. Well, up to a point – some calcium and fluoride will still be dissolved, but just a tiny amount.

Sodium fluoride, I am guessing, is just sodium and fluoride. So how about specifically hydrofluorosilicic acid?

Hydrofluorosilicic acid, chemically H2SiF6, seems like a more complicated compound than sodium fluoride, and it is, but there’s a twist. It isn't found in water the way you might think, and it undergoes more transformations than sodium fluoride when added to water. The first is that it dissociates into a SiF62- ion and two positively charged hydrogen ions (2H+), sometimes called protons. This is similar to sodium fluoride – we have ions splitting – but the transformation doesn't end there. The SiF62- ion is chemically unstable in the water being pumped to our homes, so a chemical reaction happens. The SiF62- ion rapidly dissociates into silicon and fluoride ions. The silicon ion then binds with oxygen found in water molecules to form silicon dioxide, leaving behind protons and fluoride ions. Silicon dioxide isn't super soluble (it's basically sand), so it drops out of the water, and you're left with what is essentially a very small quantity of fluoride ion and protons from H2SiF6. This process happens so quickly that sometimes the whole reaction is lumped together as such.

The only difference from sodium fluoride is that instead of sodium, you've got protons, but both sodium fluoride and hydrofluorosilicic acid add free fluoride ions to water. Protons are definitely different than sodium. They’re what makes water acidic, whereas sodium makes it salty. But if you don’t add very many of them, not a whole lot changes. The concentrations of fluoride being added are basically the same as with sodium fluoride.

Once in the body, what can fluoride bind with?

Fluoride doesn’t just bind with ionic calcium, it also binds with magnesium ions and other materials like those found in bone cells, enzymes responsible for making bone and teeth. It doesn’t always make calcium fluoride or calcium magnesium, because the calcium and magnesium found in our bodies aren’t always free ions. When fluoride binds with teeth, for instance, it doesn’t make calcium fluoride but another mineral known as fluorapatite. When it does that, the reaction doesn’t result in a loss of calcium from the mineral. A similar reaction can happen with bone, since bone also contains some of the same minerals that teeth do.

There’s also an important factor behind the binding that isn’t just related to the source: concentration. Concentration matters, always. Physiologists know this too. If you take in too much fluoride, it can precipitate the calcium and magnesium ions in your blood and can cause structural changes to bones and teeth, among other problems. This is actually what makes hydrofluoric acid so dangerous. At high enough concentrations the fluoride ion migrates through your skin and removes free calcium and magnesium, which can cause cardiac arrest. So, yeah – lots of fluoride would be a huge deal, but we aren't adding nearly enough for it to be a concern.

When someone is pregnant and not meeting their daily calcium requirement, will fluoride be pulled with the calcium toward the fetus?

Fluoride would react with a bunch of different sources of calcium it can get access to, and what source it gets first also matters a ton. If it binds to teeth for instance, it's not taken up in the same way as it would be if it's injected or ingested. How is the fluoride being put in the body and at what concentration? Using toothpaste and brushing has different bioavailability than drinking water with fluoride. Not only does concentration matter, but application! So, perhaps this could be a problem, but fluoride might also bind to teeth first, or bones or free calcium; it depends upon how it’s applied.

However, if a person isn’t getting enough calcium, they’ve got other problems to worry about than if fluoride will do some damage. Hypocalcemia, the condition where a person has low blood calcium, can be life threatening on its own. This isn’t good for the baby either.

Once hydrofluorosilicic acid is broken up in ions and in the body, can the fluoride ion bind with an aluminum ion and form fluoroaluminum complexes?

Keeping in line with what we said earlier, we’re really concerned about free fluoride ions since that’s the big culprit for both water additives. However, it’s still a legitimate concern. Could free fluoride found in the bloodstream bind with aluminum? Yes, but those complexes are mostly soluble, since many of the complexes are ions, so it won't precipitate like calcium fluoride would. Would these complexes be harmful anyway? Perhaps, but again, concentrations matter. How much fluoride and aluminum are you ingesting at any given time? That certainly depends. The reported lethal dose of aluminum fluoride is much higher than the amount of fluoride or aluminum we'd normally ingest at any given time, so I'd be surprised if it was harmful at its current concentrations.

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