A moisturiser does three separate jobs, and most people only want two

Most explanations of how moisturisers work stop at the point where it starts to matter. This one carries on.

Three mechanisms, one jar

Occlusives form a hydrophobic film across the surface that slows the rate at which water evaporates from the layers beneath. Humectants bind water molecules and hold them within the outer layer, drawing them either from deeper tissue or from humid air. Emollients fill the gaps between loosely stacked surface cells, which is what makes skin feel smooth and look less rough.

Almost every moisturiser combines the three, and the ratio between them is the main thing separating a gel from a balm. Reading a product as a ratio rather than as a brand makes it much easier to predict how it will behave on you.

The occlusive family

Petrolatum is the benchmark, reducing water loss dramatically and being about as unlikely to sensitise as any ingredient in cosmetics. Mineral oil, dimethicone, lanolin, shea butter and various waxes all occlude to different degrees with very different textures.

Silicones give a light, silky occlusion that sits well under makeup, which is why they dominate primers and lightweight creams. Heavier occlusives can feel intolerable on oily skin and can trap sweat, which is a comfort issue more than a pore one. The concern about petrolatum clogging pores has not held up well, though individual experience varies enough to be worth testing.

The humectant family

Glycerin is cheap, effective and appears in an enormous share of well-formulated moisturisers, usually within the first few ingredients. Hyaluronic acid holds a great deal of water for its weight and gives a characteristic light, plumped feel immediately after application. Urea acts as a humectant at lower concentrations and as a keratolytic that softens thickened skin at much higher ones, which is why body creams for rough skin use it.

After four to six weeks, panthenol, sodium PCA, betaine and amino acids all contribute, and several are components of skin's own natural moisturising factor. Humectants alone in dry air are a partial solution, and the seal above them is what completes the job.

The emollient family

Emollients are largely lipids and esters that sit between surface cells and smooth the light-scattering roughness that reads as dullness. Squalane, jojoba esters, caprylic triglyceride and various plant oils fall here, and they differ mainly in spread, weight and residue. Some emollients are also mildly occlusive, so the categories overlap and the boundaries are practical rather than strict.

Skin that feels rough despite being hydrated is usually short of emollients rather than short of water.

This is the class most responsible for whether a product feels expensive, which is why it drives so much of the pricing.

Matching texture to conditions

Gels and lotions are mostly water with light emollients and modest occlusion, which suits humid climates and oily skin. Creams increase the oil phase and the occlusive load, and balms and ointments push it further with waxes and butters.

Under makeup, the correct texture changes with season, climate, indoor heating and even the flight you took this week. Layering a light lotion under a heavier cream at night is a reasonable way to keep one product year-round and adapt the other. There is no benefit to a heavy product on skin that does not need it, beyond the sensation of having applied something.

Formulation decides how an ingredient behaves, and the percentage on the front tells you less than it implies.

What a moisturiser cannot do

It cannot change the structure of skin permanently, and any effect on fine lines is a temporary result of surface hydration and light scattering. It cannot fix a barrier while the thing damaging that barrier continues, which is usually a cleanser or an over-used active.

It does not need actives to be worth using, and a plain moisturiser used consistently outperforms a complicated one used occasionally. Claims about firming, lifting or rebuilding go well beyond what a topical emulsion has been shown to do. Skin that stays uncomfortable despite a sensible moisturiser is worth showing to a doctor rather than treating with a more expensive jar.

The takeaway

Identify whether a moisturiser is mostly sealing, mostly attracting or mostly smoothing, and buy for the job you actually have.

Sunscreen, consistency and time outperform nearly everything sold as a breakthrough.