TEWL – transepidermal water lossThe protective function of the skin not only comprises preventing anything from outside entering the body but also includes stopping excessive fluid from escaping from the body. Liquid emitted by the body can be either through the evaporation of water vapour or the secretion of sweat.
If the skin barrier is damaged, the amount of water emitted from the body is increased (increased TEWL). As there is no control over this loss of water, the TEWL can be interpreted as a reflection of the skin barrier’s functionality.
When the skin has been treated by cosmetic and pharmaceutical products, the skin’s secretion processes are hindered to a greater or lesser extent. Active agents that penetrate the skin result in an additional loss of pressure. Hindering the TEWL results in a swelling of the horny skin layer (the sensation that you feel after having worn rubber gloves for an extended amount of time).
As a result of diffusion of the body’s water, a pressure builds up, which can be measured as TEWL (in g/hm2). The microclimate is determined by two moisture sensors and two thermometers positioned in a measuring head (open-chamber method).
The system measures the water vapour flow density: JV= g/(m x h) (mass of water vapour (in g) per area (in m2) and time (in h)).
Each of the two sensors measures the partial pressure of the water vapour: the gradient between the two sensors is directly proportional to the rate of evaporation. The water vapour gradient can subsequently be converted into the TEWL value, taking into account the length and diameter of the open measuring cylinder.