I cringe every time a friend of mine posts an article like “25 uses for hydrogen peroxide” on Facebook. Usually these are the same friends who rant about the evils of ethylparaben, propylene glycol, or the like. Well, I’ve spent the last 5 years learning what cells do with hydrogen peroxide…and what it does to cells.
Executive summary: Honestly, people, hydrogen peroxide is much worse news than propylene glycol. I would not brush with it, gargle with it, or even put it on cuts. Yes, it kills germs. But why? Because it can do serious damage to ANY cell. Including yours.
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Now here is the science behind my statement. Your cells always have a small amount of hydrogen peroxide in them, for two reasons. The first is that cells actually use hydrogen peroxide to “digest” fats into smaller particles. They do this because… drumroll… hydrogen peroxide is good at breaking down fats. Did you know that your cell membranes are also made of fats? Yes, in high concentration hydrogen peroxide will break your cell membranes. It does that to germs, too, which is why it is such a good disinfectant. Obviously, your cells don’t want to kill themselves, so they use hydrogen peroxide in a very specialized part of the cell (an organelle known as a peroxisome) so it can’t impact the rest of the cell. And all around the peroxisome, there are specialized enzymes–catalase and peroxidase–designed to break down hydrogen peroxide and keep it from getting out to the rest of the cell.
The second reason that cells have hydrogen peroxide revolves around what we chemists call ‘the four electron reduction of oxygen to water’–what biologists call respiration. As oxygen is reduced (gains electrons) it passes through four intermediates known collectively as reactive oxygen species (ROS). These intermediates are known as superoxide radical, hydrogen peroxide, and hydroxyl radical. (For plants, the reverse reaction, photosynthesis, is even more strongly implicated in ROS production. Having done a lot of work with photosynthetic critters, I understand the photosynthetic ROS production pathways a lot better than I do than those linked to respiration.)
Normally there are enough electrons around that the reaction goes all the way to water, but there are enough “leaks” in the system that (usually) superoxide can get out of the pipeline and into the cell. This is why there are plenty of enzymes around the mitochondria and chloroplasts–superoxide dismutase to break down superoxide (and turn it into hydrogen peroxide) and catalase and peroxidase to break down the hydrogen peroxide. The damage is contained.
It should be noted that of the three ROS produced by respiration, superoxide is highly reactive and can do lots of damage; hydrogen peroxide is the least reactive; and hydroxyl radical is the most reactive and toxic. Superoxide and hydroxyl radical, especially the latter, are the most common “free radicals” implicated in cancer, as both can irreparably damage DNA.
So what about hydrogen peroxide? Well, even though hydrogen peroxide is less reactive than the other two ROS, (1) it is still capable of doing damage; and (2) if it meets up with any metals (and you can find those in the cell in small concentrations, as they serve as the basis for many enzymes) it will react to form the highly reactive and damaging hydroxyl radical. That’s why the cell is so intent on keeping hydrogen peroxide low–not just because it can do damage itself, but also because it can turn into a highly toxic radical species.
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Let’s go back to the uses for hydrogen peroxide you frequently see proposed on the net, like using it as a mouthwash. There are several problems with this. First of all, hydrogen peroxide can diffuse through the cell membrane. If you were to gargle with a reasonably concentrated solution of superoxide, it would damage every cell membrane it came in contact with, but that’s it. Superoxide is a charged species and therefore doesn’t go through the cell membranes very well. But hydrogen peroxide, which is both electrically neutral and similar in size to water can easily pass through the aquaporins designed to let water through the cell membrane. So it gets inside your cells.
But so what? Aren’t there enzymes designed to break it down? Well, yes there are, both inside and outside the cell. However, kinetics must be taken into account. CONCENTRATION MATTERS. The more hydrogen peroxide there is, the longer it will take for the enzymes to break it down. The hydrogen peroxide in your cells is usually present in nanomolar concentrations; the hydrogen peroxide you buy at the drugstore is just a bit under 1M in concentration. So you are hitting your cells with roughly one billion times the amount of hydrogen peroxide they’re used to dealing with!
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In short–hydrogen peroxide can damage your cells in the same way it can damage germs. I would not use it topically and I particularly would not consume it in any fashion. It’s ok to use it for a disinfectant on non-living items, but be sure to rinse well with purified water to get rid of any residues. (There’s a reason hospitals use alcohol as a disinfectant instead of peroxide–it evaporates quickly, leaving no residue behind.)
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Edited to add: I have been asked for my sources. The most comprehensive source for this sort of material is “Free Radicals in Biology and Medicine” by Halliwell and Gutteridge. I think they’re on 4th edition now? It’s a textbook, but it is far more understandable to ordinary folks than scientific papers. It’s become my Biology Bible for writing my dissertation.
At some point this will be revised to be even more scientific and will include many more sources, but I’m not likely to get around to that until I’m writing my dissertation intro, which will cover a lot of hydrogen peroxide cellular biology (among other things).