Transepidermal water loss (TEWL) is the natural process of water evaporating from the skin’s surface. This occurs in everyone, with an average person losing about 300-400ml of water daily. TEWL measures skin integrity and its ability to retain moisture, highlighting its role in overall skin health and hydration.
The Skin’s Water Barrier
The skin’s primary defense against excessive water loss is its outermost layer, the stratum corneum, often described as a “brick-and-mortar” structure. The “bricks” are tough, flattened skin cells called corneocytes, primarily consisting of keratin. These cells are held together by a “mortar” of lipids, including ceramides, free fatty acids, and cholesterol.
This arrangement forms a protective barrier, preventing water from escaping and blocking irritants and allergens. The stratum corneum also contains natural moisturizing factors (NMFs), water-soluble compounds like amino acids, urea, and lactic acid, which absorb water to maintain hydration. A healthy, intact skin barrier, with its slightly acidic pH of 4.5-5.5, regulates moisture levels and defends against external threats.
What Increases Water Loss
Various factors can disrupt the skin barrier, leading to increased transepidermal water loss. Environmental conditions like low humidity, dry air, strong winds, and extreme temperatures (hot and cold) can pull moisture from the skin. Prolonged sun exposure also contributes to barrier impairment.
Internal factors also influence TEWL. Genetics can predispose individuals to conditions like eczema or psoriasis, which often have a weaker barrier function. As people age, skin naturally thins, and the production of lipids and natural moisturizing factors declines. Lifestyle habits such as frequent hot showers, harsh skincare products, over-cleansing, or excessive exfoliation can strip the skin of its natural oils, compromising the barrier.
Protecting Your Skin Barrier
Maintaining a healthy skin barrier and minimizing TEWL involves consistent, gentle care. Proper moisturization is important, using products with humectants, emollients, and occlusives. Humectants, like hyaluronic acid, glycerin, and urea, attract water into the skin.
Emollients, such as ceramides, fatty acids, squalane, and shea butter, fill gaps between skin cells, smoothing the skin and repairing the barrier. Occlusives, including petrolatum, beeswax, or mineral oil, form a protective layer on the skin’s surface, sealing in moisture and preventing evaporation. Applying humectants to slightly damp skin, followed by emollients, and then an occlusive, can maximize hydration.
Beyond moisturizers, gentle cleansing with sulfate-free products helps preserve the skin’s natural oils and acidic pH. Avoid harsh exfoliants or over-exfoliation, as these can strip the skin. Using a humidifier can add moisture to the air, reducing evaporative pull. Protecting skin from environmental aggressors like sun exposure with broad-spectrum SPF 30+ sunscreen and being mindful of extreme temperatures and wind also supports barrier health.