You’re slicing some vegetables for dinner, and the knife slips. “Ow!”
It’s just a small cut, but you’d better clean it out. So you go into the bathroom and reach for the rubbing alcohol.
Now it’s “Ow!” all over again.
Alcohol and hydrogen peroxide make sure bacteria don’t take up residence when you get a cut (though peroxide may harm more than it helps). Sometimes, though, cleaning out the cut hurts worse than the cut itself. Why do alcohol and peroxide sting so much?
Turning down the heat
Lets start with alcohol. Now mind you, what you’re about to read has to be taken with a grain of salt because rubbing alcohol is made from isopropyl alcohol, but the science I’m talking about below is based on ethanol (aka drinkin’ alcohol), which chemically is a little different. However, when I poked around to find out why isopropyl alcohol stings, I could only find information about ethanol. My assumption is that what applies to one applies to the other.
With that caveat out of the way….
Ethanol makes you feel the burn by truly making you feel like you’re burning. According to a paper published in 2002 in Nature Neuroscience, it turns on a receptor on pain-sensing nerves that alternately goes by the name VR1; other publications call it TRPV1, which is what I’ll stick with.
TRPV1 helps sensory nerves keep you safe by creating the sensation of pain. But not just any kind of pain; specifically pain associated with heat or irritant chemicals. In short, it reacts to capsaicin (which gives hot peppers their heat), acids, and temperatures over 42ºC (a little over 107ºF).
Ethanol, though, messes with TRPV1 a little, making it turn on at a lower temperature: about 34ºC, which is below the body’s normal temperature of 37.5ºC. So basically, ethanol makes TRPV1 think the body is being burned even though it’s not even close.
That stinging fizz
In addition to making your cuts fizz (which, by the way, has nothing to do with killing bacteria), hydrogen peroxide also hurts like [bleep] when you pour it on a wound, but not for the same reasons that alcohol does. In fact, no on quite knows what gives it its sting, but according to a 2008 paper in the European Journal of Neuroscience there’s some evidence that it does so at least in part by activating another receptor on nerve cells called TRPA1.
TRPA1 and TRPV1 are structurally similar, and are often found together on the same cells at the same time; indeed, they may coordinate when it comes to creating the sensation of pain. The list of things that activate TRPV1 is pretty clear, but we’re only just starting to understand how TRPA1 gets turned on. However, it looks like TRPA1 acts as a kind of generalized sentry for harmful chemicals (including, it turns out, tear gas). It may also be responsible for the pain you feel in response to cold, like if you were to stick your hand into really really really cold water.
So you could almost say that alcohol and hydrogen peroxide make your nerves run hot and cold. (wha-wha….)