Transepidermal Water Loss
The natural, ongoing process of water evaporating from deeper skin layers through the outer barrier into the surrounding air.
Transepidermal water loss (TEWL) describes the passive movement of water from inside the body, up through the epidermis, and out into the environment. It's not sweating — it's a continuous, invisible evaporation that happens around the clock. TEWL is a normal physiological process, but the rate at which it occurs tells us a lot about how well the skin's barrier is functioning. When the outer layer (stratum corneum) is intact and well-hydrated, TEWL stays relatively low. When that barrier is compromised — through over-cleansing, harsh actives, environmental exposure, or simply dry conditions — the rate climbs, and skin can start to feel tight, rough, or reactive.
In skincare formulations, managing TEWL is one of the core goals of any moisturiser. Occlusive ingredients like squalane, shea butter, and petrolatum work by forming a thin film over the skin's surface that physically slows evaporation. Humectants such as hyaluronic acid and glycerin draw water into the upper layers, but without an occlusive layer on top, that water can actually evaporate faster. This is why well-formulated moisturisers typically combine both humectants and occlusives, sometimes alongside barrier-supporting lipids like ceramides that help reinforce the skin's own structure.
Understanding TEWL can genuinely change how you approach your routine. If your skin feels persistently dry despite using hydrating serums, the issue may not be a lack of water but a barrier that's letting it escape too quickly. Layering a gentle humectant underneath a richer cream, especially in cooler or air-conditioned environments, can make a noticeable difference. Avoiding unnecessary exfoliation and supporting your barrier with niacinamide or ceramide-based products are practical steps worth considering.
It's worth noting that TEWL is measured in laboratory settings with specialised instruments, so product claims about "reducing TEWL" should ideally reference specific testing. Elevated TEWL on its own isn't a diagnosis of anything — it's simply an indicator. If your skin barrier feels consistently compromised despite a gentle, supportive routine, it's reasonable to speak with a dermatologist or skincare professional to explore what might be going on beneath the surface.
In skincare formulations, managing TEWL is one of the core goals of any moisturiser. Occlusive ingredients like squalane, shea butter, and petrolatum work by forming a thin film over the skin's surface that physically slows evaporation. Humectants such as hyaluronic acid and glycerin draw water into the upper layers, but without an occlusive layer on top, that water can actually evaporate faster. This is why well-formulated moisturisers typically combine both humectants and occlusives, sometimes alongside barrier-supporting lipids like ceramides that help reinforce the skin's own structure.
Understanding TEWL can genuinely change how you approach your routine. If your skin feels persistently dry despite using hydrating serums, the issue may not be a lack of water but a barrier that's letting it escape too quickly. Layering a gentle humectant underneath a richer cream, especially in cooler or air-conditioned environments, can make a noticeable difference. Avoiding unnecessary exfoliation and supporting your barrier with niacinamide or ceramide-based products are practical steps worth considering.
It's worth noting that TEWL is measured in laboratory settings with specialised instruments, so product claims about "reducing TEWL" should ideally reference specific testing. Elevated TEWL on its own isn't a diagnosis of anything — it's simply an indicator. If your skin barrier feels consistently compromised despite a gentle, supportive routine, it's reasonable to speak with a dermatologist or skincare professional to explore what might be going on beneath the surface.
