What it is, precisely
Water crosses your skin from the inside to the outside all the time. It happens while you sleep, while you sit still in a warm room, and while you are standing in a car park in February. It is not sweating: sweat is an active process with glands, ducts and nervous control. This is passive diffusion of water through the outer layer of the skin, and it does not switch off.
The rate is set by two things and only two things, in the simple form of the model that is worth carrying around.
The first is the driving force: the difference in water vapour pressure between the wet interior of the body and the air immediately outside it. The second is the resistance: how hard the stratum corneum makes it for a water molecule to get out. Raise the first or lower the second and more water leaves per hour.
What raises the driving force
The driving force depends on how much water the outside air can hold and how much it is already holding, which is the difference between absolute and relative humidity. This is where most winter skin confusion lives.
Cold air holds very little water even when it is saturated. Air at freezing point that is at one hundred per cent relative humidity contains a small absolute quantity of water vapour. Warm that same air up to room temperature without adding any water and its relative humidity falls dramatically, because the warmer air could now hold a great deal more. This is exactly what a heated building does to outside air in winter.
So the driving force is high outdoors in cold weather, and it is often higher still indoors in winter, in a heated room fed by cold outside air. This is not a paradox and it is not a trick. It is the same physics that makes a heated room feel dry and makes wooden furniture crack in January.
Two other things raise the driving force. Skin temperature: warmer skin has a higher vapour pressure at its surface, so a hot shower or a heated van increases loss while you are in it. And air movement, which is the subject of the next paragraph.
The boundary layer, and why wind matters so much
Still air next to your skin becomes locally humid. Water that has just left sits there, raising the local vapour pressure and reducing the gradient. That thin film of locally humid air is a boundary layer and it is doing you a favour.
Wind strips it away and replaces it with dry air, continuously. The gradient at the skin surface is therefore held at its maximum rather than being allowed to soften. This is why an hour in still cold air and an hour in the same cold air at twenty knots are not the same exposure, and why the face and the backs of the hands, which are usually the surfaces most exposed to moving air, are usually the first to complain.
What lowers the resistance
The resistance side is the structure described in the previous article. Three things reduce it.
Removing cells from the top. Abrasion by sand, by a towel used briskly, by a wetsuit collar, by a rope, by repeated scrubbing. Each removed layer shortens the path out.
Disordering or dissolving the lipid sheets. Detergents do this by design; that is what a surfactant is for. Hot water does it because lipids are more mobile when warm. Organic solvents do it directly. Repeated wetting and drying does it by mechanical stress on the sheets.
Rinsing out what holds water inside the cells. The small water attracting molecules inside corneocytes are water soluble, which is the whole point of them, and prolonged immersion carries them away.
What raised water loss feels like
You will not perceive water loss directly. What you perceive is the consequence: a stratum corneum that has dropped below the water content at which it stays flexible.
That presents as tightness, particularly after washing and particularly as the skin dries. Then as a rough or matte feel under the fingertips. Then as fine scaling, because cells that should have detached invisibly detach in clumps. Then as stinging on application of things that previously felt like nothing, because the products can now reach nerve endings they previously could not. Then, under mechanical load, as splits.
The order is consistent enough to be useful. If you are at stinging, you are several stages past the point where a small change would have been enough.
| Raises loss by increasing the driving force | Raises loss by lowering the resistance | |
|---|---|---|
| Cold dry air | Yes, strongly | No, not directly |
| Wind | Yes, by stripping the boundary layer | Yes, by abrasion where there is sand or grit |
| Heated indoor air | Yes, and often more than the outdoors it came from | No |
| Hot shower | Yes, while skin is warm | Yes, warm water mobilises the lipid sheets |
| Detergent wash | No | Yes, directly, that is what surfactants do |
| Long immersion | No | Yes, swelling plus loss of water attracting molecules |
| Towel used briskly | No | Yes, by removing softened surface cells |
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.
Why the two sides multiply rather than add
The reason a February surf session is disproportionately hard on skin is that it raises the driving force and lowers the resistance in the same hour.
Cold dry air and wind maximise the gradient. Immersion swells the corneocytes and rinses their contents. Salt left drying on the surface holds a water attracting film that then draws water out as it dries. The wetsuit holds skin wet and warm and slightly abraded. The changing routine finishes with a hot shower and a detergent wash. Then the heated room.
No single item on that list is a disaster. Together they are the reason people who are outdoors all winter have skin problems that people who are outdoors all summer do not.
How to use the concept
The value of thinking in these terms is that it makes interventions comparable. Anything you might do falls into one of three buckets: reduce the driving force, raise the resistance, or reduce the time spent under load.
Reducing the driving force means covering exposed skin against moving air, not heating a room more than you need to, and not standing in the wind wet. Raising the resistance means not stripping the lipid sheets when you do not have to, and applying something that slows evaporation while the structure rebuilds. Reducing time under load means changing faster, drying properly and not letting salt sit.
What it also does is tell you when something is not going to work. A humectant applied to skin in very dry air with nothing over it can pull water from the layer below and let it evaporate, which is the opposite of what you wanted. A cream applied to skin that is still covered in salt is being applied over the problem. Knowing the mechanism tells you the order of operations.