Occlusive, emollient, humectant: the three jobs a moisturiser can do

The three functional classes explained by mechanism, with common balm ingredients sorted into each and an honest note on occlusivity rankings.

Almost every moisturising ingredient does one of three physically distinct things, and the words for them get used interchangeably in product copy even though the mechanisms have nothing in common. Sorting an ingredient list into occlusives, emollients and humectants tells you more about how a product will behave than any amount of front-of-pack language, and it explains immediately why balms behave the way they do.

Occlusive: raising the resistance of the surface

Water moves constantly out of the body through the skin by diffusion, a flux called transepidermal water loss. An occlusive puts a continuous hydrophobic layer over the surface, so the water has to diffuse through that layer as well as through the skin. Diffusion through a lipid film is slow, so the flux drops and the outer stratum corneum stays wetter than it otherwise would.

Three things determine how well a material does this. Continuity: a film with gaps leaks. Thickness and persistence: a film that absorbs or rubs off stops working. And the diffusion coefficient of water through the material itself, which is why petrolatum, an interlocking hydrocarbon system that also penetrates into the intercellular spaces of the upper stratum corneum, outperforms materials that only sit on top.

Note what an occlusive does not do. It does not add water, it does not repair anything, and it cannot help skin that has nothing to retain. Occlusion over genuinely dehydrated skin in dry air keeps a small amount from getting worse; occlusion over damp skin holds a real amount in. That distinction is the practical core of how moisturisers work.

Emollient: filling the gaps

Dry skin is rough because corneocytes at the surface have lifted at their edges instead of shedding cleanly. Light scatters off those raised edges, which is why dry skin looks dull and grey as well as feeling rough. Emollients are lipids fluid enough to flow into those gaps and lie down flat, restoring smoothness, flexibility and light reflection.

Emolliency is mostly a matter of spreadability and viscosity rather than chemistry. The fast-spreading esters and light oils are the best emollients per gram; heavy butters are less efficient but persist longer. Castor oil is the outlier, spreading poorly but clinging and glossing better than anything else in common use. This is also the property that determines whether a balm feels good or feels like a candle, and it is the reason two balms with identical wax content can feel completely different. The oil selection consequences run through jojoba and the rest of the carrier oil pages.

Humectant: attracting water

Humectants are hygroscopic, usually small and water-soluble: glycerin, sodium lactate, urea, propanediol, sugars, honey, and the polymer humectant sodium hyaluronate. They bind water and hold it in the outer layers where it plasticises the corneocytes and keeps enzymes such as the ones responsible for orderly shedding working properly.

Where they draw the water from depends on the humidity. In humid air, a humectant film picks up water from the atmosphere. In genuinely dry air, roughly below 40-50% relative humidity, there is not much atmospheric water to take, so a humectant layer with no occlusive above it draws water from the deeper epidermis toward the surface, where it evaporates. The skin can end up feeling tighter and drier than before. This is the single most common reason a "hydrating" product disappoints in winter, and it is why humectants and occlusives are almost always used together.

Where common balm ingredients actually sit

Most real ingredients do more than one job. The table shows the dominant function first.

Approximate occlusivity, with the caveat first

Occlusivity is usually measured as percentage reduction in transepidermal water loss over a set film weight and time, and published figures for the same material differ substantially between studies because the method varies: in vitro versus in vivo, film thickness, application weight, occlusion time, chamber humidity and instrument all move the result. Any single number you see quoted for "petrolatum reduces water loss by X percent" is a result from one protocol, not a material constant. Treat the ordering as reliable and the magnitudes as indicative.

The practical implication for formulating is that a natural balm is not simply a plant-based version of an ointment. It sits several places down this ranking, and if maximum occlusion is the goal there is no plant material that matches petrolatum. The nearest plant-adjacent material is lanolin, which is animal-derived rather than plant. What plant fats offer instead is a better sensory profile, fatty acids that resemble the skin's own surface lipids, and a set of unsaponifiables with their own effects. The trade is real in both directions, which is the honest version of the argument.

Why balms are occlusive plus emollient and nothing else

This is the structural point the rest of the site keeps returning to. A balm has no water phase, so it can carry any lipid you like and essentially no humectant. Glycerin beads out. Sodium hyaluronate never hydrates. Honey brings water and a preservation problem with it. The best you can do is a solubilised glycerin at low percentage with an emulsifier, and at that point you have started building something else.

So a balm gets two of the three levers. It seals, and it softens. That is a genuinely useful pair, and for lips, hands, heels and weather exposure it is often the right pair. But it means a balm is not a complete moisturiser in dry conditions on its own, and it works best over damp skin or over a humectant-containing layer. See anhydrous formulation for the routes and their costs.

Reading a label with these three classes

Take any moisturiser and sort the first eight ingredients into the three buckets. The shape of the result predicts the behaviour better than the marketing.

1. All occlusive, no humectant. A classic balm or ointment. Long lasting, heavy, best over damp skin or as a top layer.
2. All humectant, no occlusive. A hydrating serum or gel. Excellent in humid conditions, potentially counterproductive in dry cold air without something over it.
3. Balanced across all three. A well-built cream. This is what emulsions are for, and it is why creams remain the default face product.
4. Emollient-heavy with little of either. A face oil. Pleasant, smoothing, modest effect on water loss.

Two further classes worth knowing about sit outside this scheme. Barrier lipids (ceramides, cholesterol, free fatty acids) aim to supplement the skin's own lamellar lipids rather than to seal or soften. And keratolytics such as urea at high percentage or lactic acid work on the shedding process itself. Neither belongs in a plain balm, and neither is a substitute for the three classes above.

If you want to go further, the comedogenic ratings page deals with the other axis people worry about when choosing lipids, and does lip balm dry your lips works through the humectant-in-dry-air question in the one place people actually notice it. For most purposes the useful habit is simply to ask, of every product you own, which of the three jobs it is doing, and whether the other two are being covered by something else.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.