Humectants, emollients, occlusives: the three jobs of a moisturizer

“Moisturizer” is a word invented by marketers, and dermatology reviews admit there’s still no agreed scientific definition. What the products actually do is clearer: they perform up to three distinct jobs, done by three kinds of ingredients. Once you can spot them on a label, choosing a moisturizer stops being guesswork.

Why skin needs help holding water

Your skin’s outer layer, the stratum corneum, is a wall of flattened dead cells (the “bricks”) set in a matrix of fats called lipids (the “mortar”). Inside the bricks sits a built-in humectant called natural moisturizing factor, which holds water.

That water matters more than it sounds. It keeps the layer flexible, and the enzymes that shed old cells only work properly when the layer is moist. When the water content drops too low, cells stop shedding cleanly, pile up on the surface and you see flakes. Lose lipids from the mortar at the same time and water escapes even faster.

A moisturizer’s job is to interrupt that cycle. It can do it three ways.

Job one: humectants pull water in

A humectant is an ingredient that attracts water and holds on to it. Glycerin is the classic example: cheap, effective and in almost everything. Others include urea, hyaluronic acid, panthenol, sorbitol, lactic acid and propylene glycol.

Humectants draw water into the stratum corneum from two places: the humid air around you and the living, wetter layers of skin underneath. That second route has a catch. In very dry air, water pulled up from below can just keep going and evaporate off the surface. Dermatology reviews are clear that humectants work best when paired with an occlusive to keep that water where it belongs. In a Canadian winter, with furnace-dried indoor air, that pairing matters.

Some humectants do extra work. Urea in higher concentrations and lactic acid also soften rough, scaly skin, and one review notes that lactic acid can stimulate the skin’s own ceramide production.

Job two: emollients fill the gaps

Emollients are mostly lipids and lipid-like ingredients that slip into the spaces between flaking skin cells. They smooth the surface, make it feel soft and supple, and add a bit of partial occlusion along the way.

Examples include fatty acids and fatty alcohols (stearic, linoleic and oleic acids), cholesterol, squalene, and ceramides or their synthetic look-alikes (“pseudoceramides”). Ceramides deserve a special mention: they’re the main lipid in your skin’s own mortar, and a review of the evidence describes topical mixtures of ceramides, cholesterol and free fatty acids, in roughly the proportions found in healthy skin, as able to speed up barrier repair after damage. Unlike petrolatum, these “physiological” lipids can travel into the stratum corneum rather than sitting on top.

You’ll notice the words “emollient” and “moisturizer” used interchangeably, especially in British and Canadian eczema materials. That’s fine in everyday use. Technically, an emollient is one type of ingredient inside a moisturizer.

Job three: occlusives seal the surface

An occlusive is an oily ingredient that forms a water-repelling film on the skin so that less water evaporates. It’s the most direct way to lower transepidermal water loss, the steady trickle of water leaving the skin.

Petrolatum (petroleum jelly) is the reference standard. At a concentration of just 5 percent it can reduce water loss through the skin by more than 98 percent. By comparison, lanolin, mineral oil and silicones such as dimethicone cut it by roughly 20 to 30 percent. Beeswax, zinc oxide and various plant oils and butters are occlusive too.

Occlusives work best applied to damp skin, because then they trap water that’s already there. Applied to bone-dry skin, they mostly stop the situation from getting worse.

The downsides are cosmetic and practical. Heavy occlusives feel greasy, can trap sweat in skin folds, and mineral oil and petrolatum can occasionally cause folliculitis (inflamed hair follicles) if rubbed vigorously into hairy areas. Lanolin is a known cause of allergic contact dermatitis in some people.

Why the best moisturizers combine all three

A humectant without an occlusive can lose its water to dry air. An occlusive without a humectant seals in whatever’s there, but adds nothing. Emollients alone smooth the surface without addressing water loss much.

Put them together and each covers the others’ weaknesses: the humectant brings water in, the emollient repairs the mortar, and the occlusive keeps the water from leaving. One review describes glycerin combined with occlusives as relieving dryness “synergistically”, meaning the pair does more than either alone.

This is why a good moisturizer label usually shows all three families: water and glycerin near the top, some fatty acids, cholesterol or ceramides in the middle, and petrolatum, dimethicone or a plant butter somewhere in the list.

Lotion, cream or ointment?

The other thing that separates moisturizers is the vehicle, which is mostly a question of how much oil versus water it contains:

• Lotions are oil-in-water emulsions: mostly water, a little oil. Light, quick to absorb, good for hairy areas, mild dryness and daytime use.
• Creams hold more oil, roughly a half-and-half mix in many formulas. A good middle ground for most dry skin.
• Ointments are mostly oil, often around 80 percent, with little water. Greasy, but the most effective for very dry, thick or scaly skin and in low-humidity conditions. Not ideal in skin folds, where they trap moisture.
• Gels hold their liquid in a thin polymer mesh and feel light and non-oily, useful when you want no greasy film at all.

Dermatology bodies generally recommend creams or ointments over lotions for genuinely dry skin, simply because they hold more water in.

Picking by skin type and season

A few practical rules from dermatology sources:

• Dry skin: skip lotions; choose a cream, or an ointment for the driest patches such as heels, hands and elbows. Look for ceramides, glycerin, urea or petrolatum.
• Oily or acne-prone skin: a lightweight lotion or gel, or moisturizer only on the dry parts of your face.
• Sensitive or eczema-prone skin: fragrance-free (not “unscented”, which can still contain masking fragrance), and simple formulas with fewer ingredients. Browsing fragrance-free moisturizers is a reasonable place to start.
• After 50: skin holds less water with age, so many people need to move from a lotion to a cream.
• Winter: step up a level. If you use a lotion in July, you may want a cream in January and an ointment on your hands. Indoor humidity in a heated home is often very low, and occlusives earn their keep.

Whatever you choose, timing matters as much as the product. Apply it right after bathing or hand-washing, while the skin is still damp, and reapply through the day: one review found only about half of a moisturizer still on the skin after eight hours. Very dry skin can need several applications a day.

If your skin stays dry, cracked or itchy after two weeks of this, see your doctor or pharmacist, because persistent dryness can be a sign of eczema, psoriasis or another condition that needs more than a moisturizer.

Three jobs, three ingredient families. Humectants such as glycerin pull water in; emollients such as ceramides and fatty acids fill the gaps in your skin’s mortar; occlusives such as petrolatum seal the surface so the water stays. A good moisturizer does all three, and the heavier the formula, the more occlusive it is. Match the weight to your skin and the season, put it on damp skin, and keep at it.