Final answer:
Evaporation from the skin surface, including sweat, causes transepidermal water loss (TEWL).
Step-by-step explanation:
Transepidermal water loss (TEWL) is primarily caused by evaporation of water from the skin surface. Sweat, which is produced by sweat glands, is one of the sources of water that contributes to TEWL. When the body temperature rises, the sweat glands secrete sweat onto the skin, which then evaporates to help cool down the body.