When I was a lad the most widespread use of Epsom Salts (Magnesium Sulphate Heptahydrate) was as a laxative (taken orally) and I believe it is still use for that purpose nowadays. Incidentally, the phrase “a dose of salts” (as in “it went through me like a dose of salts”) is a reference to this property.
It was recently suggested to me, however, that a warm bath with Epsom Salts may be of therapeutic value for arthritis. Although I have had fewer problems with my arthritic knees since I started on a regular course of injections, I did experience a few issues during the recent cold spell so when I saw Epsom Salts on sale in a local supermarket I thought I’d give them a go.
My main method of ablution these days is a shower – mine is of walk-in variety – but I do also have a bathtub so I’ve been experimenting with regular hot baths. Bathing in warm water definitely reduces arthritic discomfort, but the question is whether adding Epsom Salts (or any other kind of salt, e.g. Dead Sea salt) actually makes a positive difference. One can ask the same question about natural hot springs (such as in Bath) which have long been believed to have curative properties, but whether this is just because they are warm or because of the minerals presents is a question to which I do not know the answer. Most of the medical studies I’ve seen on this rely on tiny samples and are very unconvincing.
With specific reference to Epsom Salts, I’ve seen quite a few websites claiming that the absorption through the skin of magnesium ions has a beneficial effect. Others, however, argue that there’s no scientific evidence of any significant absorption via a salt bath. I don’t know how many ions would have to be absorbed to have an effect so I’ll have to remain agnostic on this possibility.
But adding Epsom Salts to bathwater definitely has some effects. For one thing, your skin (e.g. on your fingers) doesn’t wrinkle as in a normal bath, presumably due to some sort of osmotic process in the outer layers. Moreover, the addition of magnesium sulphate to water does change several bulk properties: the specific gravity increases significantly, as does the thermal conductivity, while the heat capacity goes down. The flow of heat away from inflamed areas may be a factor, and it may alter blood circulation. I’m not a medical expert but I would imagine that this is more likely to help with muscle pain than arthritis. Notice that the packaging only mentions “muscle aches and pains”.
I doubt if the specific gravity effect is important in a hot bath at home. It will make a person more buoyant in the water, but my bathtub isn’t big enough to float freely in. This does remind me, however, of the time when I was a graduate student and in response to a suggestion made by a friend I visited a place called the Floatarium, which offered a go in an isolation tank for £15 an hour. That was quite a lot of money in the 1980s! The tank was in a dark and quiet room, and the water was a strong solution of Epsom Salts (so the subject can float easily in it), and held approximately at body temperature, the idea being to deprive the brain of sensory input. The things I remember most about this experience were: (i) almost immediately losing track of the passage of time; and (ii) losing perception of where my hands and feet were. The latter was quite disconcerting – I felt my feet floating off into the distance. Overall it was quite an interesting experience, but not one that I was anxious to repeat (especially not at £15 a go).
Anyway the results of my experimental baths are, as you would probably expect, inconclusive. I have taken baths with Epsom Salts as well as with more commonly avalaible commercial bath salts; the latter contain other ingredients (such aromatic oils), while Epsom salts are odourless. I soaked for around 15-20 minutes (as recommended), but could see no difference over and above a standard bath.
That’s what they call a null result.
In the Dark 
