Well-hydrated skin looks and feels smoother, softer, and more supple. The skin holds that water in its outermost layer, behind a barrier that continuously resists its escape. How much water the skin retains, and how a product helps it do so, depends on several distinct mechanisms, each performing a different job. This article explains what skin hydration and transepidermal water loss (TEWL) are, how they are measured, and the mechanisms moisturisers use to increase skin hydration and reduce water loss.

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What skin hydration and water loss are, and how they are measured

 

The outermost layer of skin, the stratum corneum, is a thin sheet of flattened cells set in a matrix of lipids. Its cells hold water partly by means of the natural moisturising factor (NMF) — a mixture of small, water-soluble compounds formed inside them largely from the breakdown of a protein called filaggrin (Hoste, E., et al., 2011). Around the cells, ordered layers of lipid form the barrier that water must cross to leave, and these lamellar lipid membranes strongly inhibit its outward movement (Feingold, K.R., 2007).

 

Water nonetheless diffuses continuously from the moist tissue beneath, up through the stratum corneum, to evaporate at the surface — a process called transepidermal water loss. Both quantities can be measured instrumentally: surface hydration from the electrical properties of the stratum corneum, and water loss with an evaporimeter held against the skin (Berardesca, E., et al., 2018). Well-hydrated, intact skin appears smooth and feels supple; when water is lost faster than it is replaced, skin can feel tight and look rough, flaky and dull.

 

What Aelon IQ analyses

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Humectants

 

Humectants are water-loving ingredients (glycerin, hyaluronic acid, urea, and various glycols) that attract water and bind it within the stratum corneum. Glycerin, the most studied, measurably improves stratum-corneum hydration, barrier function and the skin's mechanical properties (Fluhr, J.W., et al., 2008), and, in clinical testing, a topical 0.1% hyaluronic acid formulation significantly raised skin hydration (Pavicic, T., et al., 2011). 

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Occlusives

 

Occlusives are hydrophobic ingredients (petrolatum, mineral oil, waxes, and silicones) that form a film over the skin and slow the evaporation of water from its surface, reducing TEWL. Petrolatum is the most studied: rather than merely resting on top, it permeates between the cells of the stratum corneum, and contrary to the intuition that sealing the skin might hold back its recovery, it actually speeds the barrier's recovery after disruption (Ghadially, R., et al., 1992). A sound occlusive layer is what keeps attracted or existing water from escaping. 

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Emollients

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Emollients (esters, fatty alcohols, plant-derived oils) settle into the spaces between the surface cells, filling the microscopic gaps left as cells loosen and shed. This smooths and softens the surface and improves skin flexibility (Rajkumar, J., et al., 2023). Emollients are water insoluble and do not form an occlusive film (Lee, C., et al., 2019) and therefore differ from occlusives in how far they limit water loss: their primary role is to condition the surface.

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Structural barrier lipids

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The lipids between the cells are what make the barrier a barrier, so a fourth approach is to supply those lipids themselves, restoring the matrix that limits water loss at its source. This is a more demanding strategy than sealing the surface or drawing in water, with requirements of its own, and it is the subject of a separate, more detailed article on rebuilding the skin's barrier lipids — see Barrier Lipid Replenishment .

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How the mechanisms work together

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Aelon IQ analyses each formulation for the presence, diversity, and total load of humectants, emollients & occlusives, and structural barrier replenishment lipids to derive the formulation's overall moisturisation factor. These four mechanisms address different parts of the same problem, and no single one covers all of it. A humectant attracts and binds water, but binding water is not the same as keeping it. That is the job of occlusives, which reduce transepidermal water loss by forming a barrier on top of the skin, and of barrier lipids, which reduce it by rebuilding the skin's own lipid matrix from within. Emollients, in turn, address how the surface looks and feels. This is why a product built around one mechanism, for example, a hydrating serum consisting largely of humectants, addresses only part of the picture. Lasting hydration depends on attracting water and holding it there, which is why these mechanisms work best in combination.

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References

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Berardesca, E., Loden, M., Serup, J., Masson, P. and Rodrigues, L.M. (2018) 'The revised EEMCO guidance for the in vivo measurement of water in the skin', Skin Research and Technology, 24(3), pp. 351–358.

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​Feingold, K.R. (2007) 'Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis', Journal of Lipid Research, 48(12), pp. 2531–2546.

 

Fluhr, J.W., Darlenski, R. and Surber, C. (2008) 'Glycerol and the skin: holistic approach to its origin and functions', British Journal of Dermatology, 159(1), pp. 23–34.

 

Ghadially, R., Halkier-Sørensen, L. and Elias, P.M. (1992) 'Effects of petrolatum on stratum corneum structure and function', Journal of the American Academy of Dermatology, 26, pp. 387–396.

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Hoste, E., Kemperman, P., Devos, M., Denecker, G., Kezic, S., Yau, N., Gilbert, B., Lippens, S., De Groote, P., Roelandt, R., Van Damme, P., Gevaert, K., Presland, R.B., Takahara, H., Puppels, G., Caspers, P., Vandenabeele, P., Declercq, W. (2011) 'Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.' J Invest Dermatol.,131(11), pp. 2233–2241. 

 

Lee, C., Bajor, J., Moaddel, T., Subramanian, V., Lee, J.-M., Marrero, D., Rocha, S. and Tharp, M.D. (2019) 'Principles of moisturizer product design', Journal of Drugs in Dermatology, 18(1 Suppl), pp. s89–s95.

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Pavicic, T., Gauglitz, G.G., Lersch, P., Schwach-Abdellaoui, K., Malle, B., Korting, H.C. and Farwick, M. (2011) 'Efficacy of cream-based novel formulations of hyaluronic acid of different molecular weights in anti-wrinkle treatment', Journal of Drugs in Dermatology, 10(9), pp. 990–1000.

 

Rajkumar, J., Chandan, N., Lio, P. and Shi, V. (2023) 'The skin barrier and moisturization: function, disruption, and mechanisms of repair', Skin Pharmacology and Physiology, 36(4), pp. 174–185.​