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Epsom salts, osmosis, and sucking

 •  • by Paul Ingraham
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Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

I have given the skeptical side-eye to thousands of ideas in the last twenty years of writing about pain, rehab, and therapy. One of the oldest — among the first I ever tackled — is the belief that Epsom salts suck toxins out of your body “by osmosis.”

Nope! I didn’t know much back then, but I knew that was wrong on many levels.

Photo of a classy antique porcelain clawfoot tub with deep navy blue lacquer in a bathroom with matching blue walls and furniture.

It isn’t intuitively obvious to anyone what kinds of substances in a bath can or cannot “soak” through the skin.

I don’t usually start a piece by saying “trust me, this isn’t wrong,” but… trust me, this isn’t wrong.

I wrote about the Epsom-osmosis thing early and somewhat clumsily, and I was challenged over and over again, sometimes by people who seemed like they should probably know more about it than I did. I did have some of the nuances wrong back then. And so I fact-checked myself to death. My osmosis rant has been carefully checked and clarified ad nauseam over many, many years, because it is so absurdly contentious.

True story: For several years in the 2000s, this little sub-topic was one of my top hate-mail generators.

And I’ll get a dozen outraged responses to this post from people doubling down on their own misinterpretation of osmosis, causing me to marvel anew at how wrongness is not powered by ignorance, per se, but by the illusion of knowledge, by the brain being so full of misinformation that there’s no more room for anything else. I have seen this demonstrated many times with this topic, and I am sure it will happen again now — over a point that is a no-brainer for the average scientifically literate person! That is just the world we live in now.

Why does osmosis even come up with Epsom?

Osmosis seems to be the average person’s notion of how Epsom salts detoxify. I guess they visualize toxins being sucked out of the body through the skin. But even if those nasty toxins are in there and need out-sucking (and that is whole ‘nother topic), that is simply not how osmosis works.

Many people get osmosis bass-ackwards: they believe it refers to the movement of things floating in water across a membrane, but that is wrong by definition. It’s actually the water itself that moves. Osmosis refers to the movement of water only across thin membranes, towards the side that is “thicker” with dissolved particles.

You can demonstrate how osmosis works quite clearly by soaking a potato in salty water. The water is clearly “sucked” osmotically out of the potato’s cells: it loses its plumpness and goes limp. Poor little potato! It’s the water that moves around.

And so, by definition, Epsom salts baths cannot suck the toxins out of anyone or anything (or suck magnesium ions into anyone or anything). If Epsom salts do get into the body, or pull anything out of it, it’s definitely not by osmosis.

Osmosis is the diffusion of water through a semipermeable membrane (red line). If a membrane is permeable to water, water will equalize its own concentration by diffusing to the side of lower water concentration.

Oh, and another another thing: we’re waterproof!

Osmosis also can’t work through the skin, because skin is almost perfectly waterproof. If it weren’t, you would dehydrate like an earthworm on a sunny sidewalk. No semi-permeable membrane, no osmosis.

The top layer of the skin, the stratum corneum, consists of dead, dry cells stuffed with a kind of embalming substance, keratin, a fibrous protein. Water can’t go through or around the keratin, thanks to a microscopic “uniquely structured fatty layer” between them (which no one knew existed until 2012). The presence and arrangement of lipids betwixt the keratin molecules results in “exceedingly low permeability.”

There are routes past the stratum corneum, “shunt pathways” such as hair follicles and sweat glands, and some water molecules end up there, in contact with relatively naked cells — but these are waterproof in their own right.

Plus we have glands that coat the skin in waterproofing oils! So there’s that too. When those oils wash off, the dead skin cells can soak up a little water and swell a bit, like soaked beans. But the water doesn’t get through.

What is waterproof is osmosis-proof by definition. The skin is an effective barrier to diffusion of water molecules and therefore of osmosis. This is not to say that nothing gets past the skin, just not very much… and definitely not water.

This post is an excerpt from one of the strangest articles on, my mighty Epsom salts screed. The full article is about 15x longer than this excerpt, and thoroughly explores the idea of Epsom-facilitated detoxification, a bunch of surprisingly weird absorption science, magnesium supplementation for pain, and more. For twenty years it has been one of the only skeptical sources of information about Epsom salts bathing … and the most detailed by far.

And it all started because one day when I was really sore, soaking in an epsom salt bath, and I thought, “Does this really do anything?”