What is transepidermal water loss (TEWL)?

Transepidermal water loss (TEWL) refers specifically to the loss of water that occurs through the epidermis, primarily due to evaporation. This physiological process happens when water from within the body moves through the skin barrier and evaporates into the environment. Option B accurately captures this concept by focusing on evaporation occurring at the skin's surface, representing a vital aspect of skin physiology.

Understanding TEWL is important for skin care and dermatology, as it helps professionals recognize how moisture is lost from the skin and why maintaining proper hydration levels is essential for skin health. High levels of TEWL can indicate compromised skin barriers, leading to dryness or irritation, while a lower level is generally indicative of healthy skin.

In contrast, the other options do not accurately define TEWL. For instance, perspiration is a process controlled by the sweat glands and involves active secretion, which is distinct from the passive evaporation involved in TEWL. Water gain from environmental humidity does not relate to the loss of water but rather to the absorption of moisture by the skin. Water retention refers to the skin's ability to hold onto moisture, which is crucial for overall skin health but again does not define the loss of water through evaporation.