Why fresh water swells skin more
The counterintuitive result first. Prolonged immersion in fresh water swells the outer layer of skin more than immersion in sea water does, and the reason is the concentration difference across the cell membranes and the corneocyte envelope.
Fresh water contains very little dissolved material. The contents of your corneocytes contain a great deal. Water moves down that gradient into the cells, and the wrinkling that follows a long bath is more pronounced than the wrinkling that follows an equivalent time in the sea.
Sea water contains dissolved salts at a concentration that reduces the gradient. Less water moves in, so the swelling is less severe. This is not an argument that sea water is gentler overall, because the salt then does something else entirely when you come out.
Why fresh water rinses more out
The same logic applies to what leaves the cells. The small water attracting molecules inside corneocytes are water soluble, and they move down their own concentration gradient into surrounding water.
Water with nothing dissolved in it is the most efficient solvent for them. So a long fresh water soak removes more of them than the equivalent time in the sea would.
This is the physical basis for a piece of advice that otherwise sounds arbitrary: a long hot bath at the end of a cold water day is not a neutral act. It is the most efficient single way to strip an outer layer that has already had a hard afternoon.
What salt water does differently
Salt water swells less and leaches less during the immersion, and then leaves a concentrated, water attracting, abrasive film on the surface afterwards. That film keeps working until it is rinsed off, and it is the specific reason that sea sessions and pool sessions produce different complaints.
The full account is in the article on salt water. The comparative point here is simple: the sea does more of its damage after you get out, and fresh water does more of it while you are in.
Treated water, and where we stop
Treated water means water to which a disinfection chemistry has been added and in which pH is controlled. Swimming pools are the obvious case and there is a whole discipline concerned with them: water balance, disinfection by product formation, ventilation of indoor pool halls, and the duties of the people who operate them.
That is not our subject, and there is a publication in this family that covers pools and the duty of care around them properly. We are going to restrict ourselves to the barrier mechanism, which is this.
Pool water is fresh water for the purposes of everything in the two sections above, so the swelling and leaching effects apply in full. The added chemistry contributes an additional interaction with the surface, and it can leave residues on the skin and in hair after the swim. The physical control that follows is the same as for the sea and for exactly the same reason: rinse promptly with fresh water, then treat the skin as you would after any long immersion.
Questions about pool chemistry, about how pools are operated, about who is responsible for what and about symptoms that people attribute to pool water are questions for that publication and for the relevant authorities, not for us.
| Fresh water | Sea water | Treated water | |
|---|---|---|---|
| Swelling of the outer cells | Greatest | Less | As fresh water |
| Leaching of internal water holders | Greatest | Less | As fresh water |
| What is left on the surface after | Nothing much | A drawing, abrasive salt film | Residues, which the pool publication covers |
| Where most of the cost falls | During the immersion | After you get out | Both |
| The control that matters most | Shorter and cooler | Rinse promptly | Rinse promptly, and ask the pool people the rest |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
Temperature does more than water type
Across all three waters, the single variable that most changes the barrier consequence is temperature, and it is the one people pay least attention to.
Warm water mobilises the intercellular lipids, so more of them are removed for the same contact time. Warm water also raises skin temperature, so evaporative loss on getting out is higher. And warm water is comfortable, so people stay in it longer, which multiplies both.
The heated indoor pool and the very hot post session shower are therefore harder on the barrier per minute than the cold sea is, which is a genuinely surprising result and one that most cold water swimmers do not expect.
Duration and repetition
The last comparative point is the one that matters most and it is the least interesting. Which water it was makes less difference than how long you were in it and how often you do it.
An hour in any of these waters, four times a week, through a winter, with a hot shower after each, produces the same underlying picture: cells depleted of what holds water inside them, lipid sheets repeatedly disordered, and a layer that never completes a repair cycle before the next immersion.
That is why the controls in this publication are almost all about what happens around the water rather than about the water itself.