Moisturisation is usually discussed as a single effect, whereas the literature distinguishes three separate mechanisms. The distinction matters because it determines which product suits which skin state, and because it allows measurement rather than impression.
TEWL — the Quantitative Measure of Barrier Function
Transepidermal water loss, abbreviated TEWL, is the quantity of water vapour diffusing from the body outward through the stratum corneum per unit time and area. It is measurable with an evaporimeter placed against the skin surface.
Elias (2005) presents the stratum corneum as an integrated defensive system in which water retention is inseparable from intercellular lipid structure. Rising TEWL is treated as a marker of barrier compromise, falling again as that structure recovers — this is the endpoint commonly used in moisturiser efficacy studies.
Three Classes, Three Mechanisms
Rawlings and Harding (2004) classify them as follows. Humectants include glycerin, hyaluronic acid, urea and polyols — they bind water molecules and retain them within the stratum corneum. This class depends on ambient humidity.
Emollients include esters, light plant oils and certain fatty alcohols — they fill the gaps between partially shed corneocytes, improving surface smoothness and pliability. This is an effect on feel and surface texture.
Occlusives include petrolatum, dimethicone and waxes — they form a hydrophobic film that slows evaporation, directly lowering TEWL. Of the three classes, this produces the clearest short-term change in TEWL.
A complete formulation typically combines all three, since each class compensates for the limits of the others: a humectant has nothing to hold water against without a film, while an occlusive alone adds no water.
Aloe Within the Humectant Class
Aloe leaf gel is roughly 98 to 99 percent water, with the dry fraction consisting largely of polysaccharides rich in free hydroxyl groups. That structure confers water-binding capacity, so within the classification above aloe extract belongs to the humectant and surface film-forming class.
It also means aloe extract cannot substitute for the lipid phase of a formulation. A product containing only humectants, used in a dry environment, may not deliver the expected result — this is a mechanistic limit, not a raw material quality issue.
Why “Deep Hydration” Is an Imprecise Claim
Water in the dermis comes from the circulation, not from a topical product. The measurable effect of a moisturiser is at the stratum corneum: retaining the water already present and slowing its loss. Phrasing that describes delivering water to deeper layers therefore does not correspond to any documented mechanism.
The above is scientific information and is not a substitute for medical diagnosis, advice or treatment. Persistent dryness accompanied by fissuring or inflammation should be assessed by a dermatologist.
