Why the distinction matters
Moisturiser is a marketing category, not a mechanism. Under that word sit at least three physically distinct functions that do different things, work in different conditions and fail in different ways.
Once you can separate them, several confusing experiences stop being confusing: why one product works indoors and not on a beach, why a light lotion feels lovely and does nothing overnight, why a heavy ointment is unbearable in the day and transformative before bed, and why a hyaluronic serum applied to a face in a February gale can leave it worse.
Occlusives: slow the evaporation
Mechanism. An occlusive forms a hydrophobic film on the surface that water cannot easily cross. It does not enter the structure and it does not repair anything. It reduces the rate at which water leaves.
What that achieves. With loss reduced, the water arriving from below accumulates in the outer layer instead of escaping, so the layer's water content rises. A layer at reasonable water content is flexible, sheds normally and rebuilds better.
Where it excels. Cold dry air, wind, overnight, and any situation with a large vapour pressure gradient. This is the class that outdoor people need most and use least, because occlusives are greasy and greasy is unfashionable.
Where it fails. Under a wetsuit, where evaporation is already prevented. On skin that has not been rinsed, where it seals salt and sand against the layer. And on hands during a working day, where it is removed within the hour.
Humectants: attract and hold water
Mechanism. A humectant is a water attracting molecule. Applied to the skin, it holds water in and near the outer layer.
The critical detail. Where does the water come from. In humid air, a humectant can draw water from the atmosphere. In dry air, it cannot, so the water it holds comes from the layers beneath. If nothing prevents evaporation from the surface, that water is then lost.
What follows. A humectant applied alone, to skin in cold dry air or wind, can leave the outer layer drier than before. This is not a fringe theoretical concern; it is the standard mechanism, it is why humectant products are conventionally paired with an occlusive over them, and it is exactly the situation of a person who applies a light hydrating serum before walking to the beach in January.
Where it excels. Under an occlusive, in reasonable conditions, on skin that has water available. It is also the class most closely related to what the skin makes for itself inside the corneocytes, which is why it feels natural and works well when the conditions are right.
| Occlusive | Humectant | Emollient | |
|---|---|---|---|
| Mechanism | Hydrophobic film slows evaporation | Attracts and holds water | Fills gaps between shedding cells |
| Best conditions | Cold, dry, windy, overnight | Humid air, or under an occlusive | Rough, flaking, friction loaded skin |
| Fails when | Applied under a wetsuit or over salt | Used alone in dry air | Used alone against a large gradient |
| Feels like | Greasy, persistent | Light, absorbs fast | Smooth, softening |
| Outdoor relevance | The class you need most and use least | Needs pairing to be safe outdoors | Useful where kit rubs |
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.
Emollients: fill and smooth
Mechanism. An emollient is a lipid material that spreads over the surface and fills the spaces between partly detached cells in a rough, flaking layer. It makes the surface smoother mechanically.
What that achieves. Immediate improvement in feel and in how the skin behaves under contact. Reduced friction, which for anyone dealing with seams and collars is a functional benefit rather than a cosmetic one. And some reduction in loss, because filling the gaps also lengthens the path out.
Where it excels. Rough, flaking skin, and any site where friction is part of the problem.
Note on usage. In British clinical usage the word emollient is often used broadly to cover the whole category of leave on preparations for dry skin, including products that are largely occlusive. We use it in the narrower, mechanistic sense here, and flag the ambiguity because you will meet both.
Everything is a mixture
Almost every real product contains members of more than one class. A typical cream has an oil phase that is partly occlusive and partly emollient, a water phase with humectants in it, and an emulsifier holding the two together.
What varies is the proportion, and the proportion is what determines behaviour. Heavier, greasier, oil rich preparations lean occlusive. Light lotions and gels lean humectant. Ointments are the occlusive extreme and are unpleasant to wear and extremely effective overnight.
You can estimate where a product sits without knowing anything about the formulation, by how it behaves. If it disappears in seconds and leaves no film, it is not doing much occluding. If it is still detectable on your skin an hour later, it is.
Choosing by condition rather than by product
The useful question is never which moisturiser. It is what is the dominant mechanism today.
Cold dry wind, going out: occlusive, applied before.
Overnight, dry layer: occlusive, generously, on damp skin.
Rough and flaking with friction from kit: emollient.
Indoors, reasonable humidity, skin needs water: humectant with something over it.
Under a wetsuit: not an occlusive on the whole body, because evaporation is already prevented; a low friction interface at the specific points that rub.
Hands, during a working day: whatever you will actually reapply, which is usually lighter than optimal, plus a heavy application at night which is where the work is done.