What is TEWL?

TEWL (Transepidermal Water Loss) is the rate at which water evaporates from the skin into the air. It's how dermatologists measure skin barrier function — higher TEWL means a more compromised barrier. Occlusive moisturizers (ceramides, petrolatum) reduce TEWL; stripping cleansers, heat, and low-humidity environments increase it. The winter dryness everyone complains about is TEWL at work.