TEWL，全名 Transepidermal Water Loss（經皮水分散失），是皮膚水分從角質層蒸發到外界環境的過程。它每時每刻都在發生，你感覺不到，但它是敏感肌惡化的核心機制之一。
健康皮膚的 TEWL 值很低——屏障完整，水分流失速率被控制在一個穩定範圍。但當屏障受損，TEWL 急劇上升：水分加速蒸發，皮膚變得乾燥、緊繃，神經纖維對刺激的閾值降低，任何輕微的刺激都會引發泛紅和刺痛。
造成高 TEWL 的因素非常多：低濕度環境（冬季暖氣、長時間在冷氣房）、過度清潔破壞脂質層、使用剝脫性產品（AHA、BHA 高濃度頻繁使用）、甚至用力擦臉的毛巾摩擦都會提高 TEWL。
對抗高 TEWL 的策略，是雙重的：補水（Humectants — 玻尿酸、甘油吸引水分進皮膚）和封水（Occlusives — 乳木果油、矽靈、礦物油在表面形成薄膜，阻止水分蒸發）。只做前者，沒有封水，等於往一個破了洞的桶子裝水。
了解 TEWL，你就能理解為什麼保養的「最後一步」——乳霜——往往是最重要的一步。
TEWL — transepidermal water loss — is the continuous evaporation of water from the stratum corneum into the surrounding environment. It happens every moment. You cannot feel it. And it is one of the central mechanisms behind sensitive skin deterioration.
In healthy skin, TEWL is low. An intact barrier controls moisture loss within a stable range. When the barrier is compromised, TEWL accelerates: skin becomes dry and tight, the threshold for sensory nerve fibers lowers, and even mild stimuli produce redness and stinging.
The triggers for elevated TEWL are numerous: low-humidity environments (heated winter air, prolonged air conditioning), cleansing that strips the lipid layer, frequent high-concentration exfoliant use, and even vigorous towel friction on the face.
The strategy against high TEWL is two-part: draw water in with humectants (hyaluronic acid, glycerin), and then seal it in with occlusives (shea butter, silicones, mineral oil) that form a film on the surface to prevent evaporation. Doing only the first step, without occlusion, is filling a bucket with a hole in it.
Understanding TEWL clarifies why the last step of a routine — the moisturizer — is often the most important.