What makes a cream hydrating? Humectant, emollient, occlusive, and what each ingredient does.

Moisturising ingredients work through three distinct mechanisms: a humectant binds water, an emollient smooths the skin's surface, and an occlusive seals the skin so water can't evaporate. A humectant (water-binding substance, for example glycerin, hyaluronic acid, urea) draws water molecules to itself from the air or from the skin's deeper layers. An emollient (surface-smoothing agent, for example certain ester oils) fills in the gaps in the stratum corneum, which is what makes skin feel soft to the touch. An occlusive (water-loss barrier, the best known being petrolatum) forms a physical layer on the skin's surface that slows water evaporation.

In a 2025 study (Rubio-Santoyo and colleagues), petrolatum and extra virgin olive oil were compared on the forearms of 54 healthy adults, measured one hour after a single application. Both measurably improved skin barrier function, and reduced redness and skin temperature. Petrolatum also reduced transepidermal water loss (TEWL, the amount of water evaporating through the skin per unit time), while olive oil was more supportive of skin cell renewal.1 This is the result of a single, one-hour application, not sustained use.

Our own reading (not a specific study's result): if your skin feels tight and flaky, it may mainly be missing an occlusive layer; if your skin feels dull and inelastic, a combination of humectant followed by sealing may help more.

Skin's own barrier layer consists of a fourth element beyond this trio: barrier lipids (ceramide, cholesterol, fatty acid), which fill the space between the stratum corneum's cells, like mortar between bricks. Skin's own stratum corneum lipids are present at roughly 50% ceramide, 25% cholesterol and 15% fatty acid by mass (roughly a 1:1:1 ratio by molecule count).14

In a 2018, 4-week clinical study, 50 participants (children and adults, with mild-to-moderate atopic dermatitis, meaning eczema, or other itchy, dry skin) used a ceramide-containing cream, and measurable improvement was found in water loss (TEWL), water retention and skin smoothness. Everyone received the same cream, there was no comparison group, and the study was supported by the manufacturer (Exeltis).4 The cream, however, also contained palmitamide MEA, glycyrrhetinic acid and grape seed extract, in a base of glycerin, dimethicone and petrolatum, so the effect can't be attributed solely to the ceramide, and there was no control group. A 2021 literature review assessed 12 of 41 papers by stricter criteria, and concluded that the ceramide-containing formulations in that set of 12 can improve dry skin and barrier function, measured mainly on atopic dermatitis skin, without a specific percentage; the authors themselves also stress that a large-sample, double-blind (where neither the participant nor the assessor knows who received which cream) comparative study is still missing.5

We didn't find a human clinical study on this question, either confirming or disproving it. Hyaluronic acid is a humectant: it binds water. A long-running forum claim holds that in dry air, if there isn't enough humidity from outside, a humectant could draw water out of the skin's deeper layers instead, and so hyaluronic acid would paradoxically dry the skin out. We searched specifically for this mechanism, and found no human study either confirming or disproving it.

The closest data on the mechanism is a 2024 lab study, done on isolated skin lipids rather than live skin, that compared two other humectants, glycerin and urocanic acid: urocanic acid increased water uptake and lipid film fluidity at high relative humidity, but also increased water loss at the same time, while the authors state that glycerin likely strengthened lipid structure cohesion without increasing water uptake.6 This doesn't speak directly to hyaluronic acid, it only shows that humectants don't all behave the same way. It neither proves nor disproves a drying effect for hyaluronic acid.

Our own reading, derived from the mechanism, without a source: it's worth sealing a humectant with a richer, occlusive layer, especially in a dry environment, this is the principle behind combining humectant with occlusive, discussed in the previous section. An important limit: the sealing effect also depends on the formula, it isn't automatic (see the Buraczewska study below, under the tolerance question, where some moisturiser formulas actually increased water loss over the long term). What you can take from this: we didn't find a study that would prove or disprove the drying claim, but if you seal hyaluronic acid with a cream in a dry environment, this uncertainty likely matters less, though this hasn't been measured for hyaluronic acid itself.

Hyaluronic acid's molecular weight (low or high) is also a frequently asked question: which penetrates better, which stays on the surface. We cover this question, along with the studies behind it, in detail in a separate article.

Dry skin and dehydrated skin are two different things: one is a skin type, the other a temporary state, and oily skin can be dehydrated at the same time too. In this article, we use "dry skin" in the cosmetic sense: the sebaceous glands produce little sebum, this is a persistent, skin-type-dependent trait (the medical concept of xerosis, meaning pathological skin dryness, is broader: it also includes a lack of the skin's own water-binding substances, the natural moisturising factor (NMF), and lipids). Dehydration means the skin's outer layer has too little water, a temporary state that depends on climate, washing habits and age, and can occur on any skin type, oily included. Conflating the two is one of the most common misconceptions: people with oily skin often skip moisturiser, even though their skin may be dehydrated at the same time.

We didn't find a primary clinical study specifically measuring the effect of skipping moisturiser on sebum production in oily skin, so we don't give a specific figure for this. The conceptual distinction (skin type versus temporary state) is a definitional matter, not a measurement result.

Our own reading (not a specific study's result): oiliness and water deficiency are two separate axes, so if your skin is oily but feels tight or looks dull, skipping moisturiser likely won't fix the oiliness, because moisturiser isn't what's causing it.

Urea is a documented component of skin's own natural moisturising factor (NMF), and a meta-analysis (an analysis pooling several studies' results) found that urea-containing creams are more effective than a base cream, though measured in a specific population. A 2021 review states that urea is a hygroscopic (water-binding) molecule, present in the epidermis as part of the natural moisturising factor, and several clinical studies showed improvement with urea-containing formulations on dry, flaking skin (atopic dermatitis, dry skin, psoriasis).8 A 2026 systematic review and meta-analysis, on dry diabetic foot skin (a specific population, not average facial or body skin), found that urea-containing creams were more effective than a base cream after four weeks. The authors also note that, since urea has been studied the most, the stronger evidence behind it likely reflects greater research attention, rather than proving urea is better than other humectants; based on the data, they therefore don't recommend urea over other humectants.9 In a 2026, 60-day study, 47 people with pathologically dry (xerotic) lower-leg skin used a lotion containing 10 percent urea, which also contained petrolatum, paraffin and panthenol. Hydration improved by 55.3 percent, elasticity by 55 percent. There was no comparison group, and the manufacturer funded the study, so the figure applies to the whole formula, not urea's effect alone.10

Panthenol and niacinamide appeared in the recent studies we found combined with other active ingredients, so their standalone effect on barrier function couldn't always be isolated. There is, however, data from a 2000 study (mainly cell-culture-based, with a portion done on live skin) that standalone, topically applied niacinamide also reduced water loss on dry skin.7 In a 2026 randomised study, on the post-laser skin of 88 women, a cream containing niacinamide, Pro-Xylane and panthenol reduced water loss and increased hydration more, over the first 14 days, than a base cream without these actives (glycerin and petrolatum-based); this study was funded by the manufacturer (L'Oréal / La Roche-Posay).11 In another, 28-day study, on the pathologically dry (xerotic) skin of 35 people, a niacinamide+panthenol+dipotassium glycyrrhizate formula reduced water loss by 11.3 percent by day 7 and by 13.5 percent by day 28; there was no control cream containing only the base here.12 These figures are attributable to the whole formula, not to niacinamide or panthenol individually.

We don't have recent, direct human study data on squalane (the hydrogenated, more stable cosmetic form of squalene, a substance also present in skin's own sebum), so we're not giving a figure for it; our own reading is that as an emollient, its role may be in smoothing the skin's surface. What you can take from this: of what's covered here, urea has the most and strongest evidence, though measured in a specific population and because it's been studied the most; according to the authors, this doesn't mean it's better than other humectants. Panthenol and niacinamide are useful additions, often studied in combination.

Moisturising is the second step of the minimum routine: after cleansing, before sunscreen. Moisturiser alone doesn't protect against UV radiation; we cover sunscreen numbers and types in the article on sunscreen types, and UV's effect on skin in the article on UV radiation. For the full minimum routine, and why exactly these three steps (cleansing, moisturising, sun protection), see the article on the minimum skincare routine.

Microneedling (the Dermastamp mechanism) works at a different layer: the needle gives a mechanical stimulus to the skin's deeper layers, it doesn't deliver moisturising ingredients. After microneedling, skin can be temporarily more sensitive and drier, and a simple, occlusive-humectant combination moisturiser can help then; we cover post-microneedling skincare in more detail in the Dermastamp guide.

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