The Invisible Dryness Under Air Conditioning — The Science of TEWL and Barrier Function

After a long stretch in an air-conditioned room at the height of summer — the inside of your cheeks feels tight, your foundation goes powdery, fine lines stand out by evening. Have you noticed this? Skin drying out in a season of high temperature and high humidity seems contradictory, yet it has a clear structural explanation. In this article, we use the metric of transepidermal water loss (TEWL) to examine, layer by layer, how the stratum corneum holds water and why air conditioning strips it away.

1. What TEWL Is — The Amount of Water Escaping Your Skin

TEWL (Transepidermal Water Loss) refers to the volume of water that travels from inside the skin, through the stratum corneum, and evaporates into the air. In skin-science research, TEWL has long been treated as one of the most fundamental indicators of barrier health. In skin with an intact barrier, TEWL stays low; when the barrier is disrupted, TEWL rises — in other words, the skin becomes one from which water escapes easily. A healthy stratum corneum is not merely a stack of dead cells; it has been reported to function like a semi-permeable membrane that precisely regulates water outflow.

What matters here is that TEWL is a different question from whether the skin surface looks moist. Even if the surface appears dewy, if water keeps leaving from within, structurally the skin is drying. Air-conditioned airflow lowers ambient humidity and widens the water-vapor gradient between the skin surface and the surrounding air. The larger this gradient, the more readily water physically migrates outward. Drying under air conditioning is less a matter of temperature than a phenomenon of dry air pulling water out.

2. How the Stratum Corneum Holds Water — Bricks and Mortar

The structure of the stratum corneum is often likened to bricks and mortar. The bricks are the corneocytes; the mortar filling the gaps is the intercellular lipid. The lead actor in this mortar is ceramide, which together with cholesterol and free fatty acids forms an orderly lamellar structure that physically blocks the pathways for water. Research shows that when this lamellar structure is disrupted, TEWL rises and external irritants penetrate more easily.

Inside each brick — each corneocyte — sits a tiny sponge that holds water. This is the NMF (Natural Moisturizing Factor). NMF is an assembly of low-molecular-weight compounds such as amino acids, sodium PCA, lactate and urea, drawing water from the air and from within the skin and retaining it inside the cell. Above the stratum corneum, the sebum film — a blend of sebum and sweat — acts as a final lid that slows evaporation. The lipid lamellae, the intracellular NMF and the surface sebum film: this three-tiered arrangement is the true nature of the skin's water-retention system.

Skin hydration is decided not by adding water, but by maintaining a structure that keeps water from escaping. No matter how many layers of toner you apply, if the mortar has crumbled, water leaves at the same rate.

3. Heparan Sulfate and the Structure of Inner Dryness

When discussing water retention, the glycans and glycosaminoglycans typified by heparan sulfate and hyaluronic acid have drawn attention in recent years. These hold many times their own weight in water and are reported to support the hydration environment from the dermis up into the epidermis. Heparan sulfate is thought to participate in the handover of water and growth factors at the cell surface, and research is advancing on its involvement not only in hydration but in cell-to-cell communication.

The state called inner dryness becomes easier to understand when read as a sign that this water-retention system has partially broken down. Surface sebum is present, yet the inside is dry — it has been suggested that skin with a disrupted barrier and elevated TEWL may raise sebum production in an attempt to compensate for the water deficit. Shine and dryness are therefore not opposites but two expressions arising from the same water-losing structure. Care that only strips sebum can worsen dryness and push TEWL still higher, a vicious cycle that the structure itself explains.

4. KAIAN's Perspective — Don't Add, Don't Let It Escape

Seen through KAIAN's philosophy of Skin Longevity — extending the functional lifespan of the skin — the essence of countering dryness under air conditioning lies not in temporarily replacing lost water, but in preserving the very structure that keeps water in. Barrier recovery is known to slow with age, and repeated exposure to environments that raise TEWL throughout the summer can, over the long term, become a factor that wears down skin function. Not curing aging but preserving function — from that vantage point, invisible dryness is a high-priority concern.

As for choosing ingredients, the first sound instinct is to replenish the mortar of the stratum corneum. Ceramide and phytosphingosine serve as material for the lamellar structure; hyaluronic acid, heparan sulfate, glycerin and betaine act as humectants that hold water; and squalane and shea butter act as emollients/occlusives that suppress surface evaporation — each plays a distinct role. To calm distressed skin, panthenol and allantoin are used alongside research reports on barrier-derived irritation. We note honestly that a product line systematically integrating this moisturizing design is currently not offered under EVOLURE.

5. Practice — A Defensive Design for the Air-Conditioning Season

Let us translate this structural understanding into daily action. Nothing difficult — simply combining an environment that does not raise TEWL with care that does not strip the mortar.

• Be mindful of indoor relative humidity. Use humidification and adjust airflow direction to keep the water-vapor gradient between skin and air small.
• Cleanse with lukewarm (not hot) water and avoid washing too frequently. Over-cleansing flushes out the mortar (intercellular lipids) and raises TEWL.
• After supplying water with toner, always seal it in with a lipid lid (emollient/occlusive). Humectants alone are easily robbed of water by dry air.
• Even if shine bothers you, don't strip too much sebum. To break the inner-dryness cycle, prioritize retaining over removing.

Because summer dryness is hard to notice, countermeasures tend to lag. Yet once you understand the principle that skin hydration is decided by a structure that keeps water in — not by the amount of water — your judgment stays steady across every season. Defend against invisible dryness through structure. That is the way of thinking KAIAN proposes for skin you can live with for a long time.