ABSTRACT
INTRODUCTION The physician Santorio Sanctorius (1561-1636) who lived more than 500 years ago in Italy is considered to be the ‘rst person to work systematically on the phenomenon called perspiratio insensibilis, the invisible loss of water from the human body. Since then, it was widely agreed that water gets lost via the respiratory system and via sweating, but alternative passages of water through the skin were less clear (1). In 1911 Loewy and Wechselmann proposed for the ‘rst time that besides sweating there is the possibility of passive water diffusion through the skin (2). Why and how this water penetrates and leaves the skin was another issue of debate for the following 40 years (3,4). Irrespective of these discussions, the American dermatologist Stephen Rothman introduced the term transepidermal water loss (TEWL) in 1955, characterizing continuous water loss through the epidermis excluding sweat secretion (5). Today TEWL plays an important role in characterizing the skin barrier function in physiological and pathological conditions. This chapter gives a brief overview about the structural and functional determinants of TEWL, how it can be measured, and what this parameter means.
