This question appears in the title of a presentation at the US Symposium of the International Society for Bioengineering and the Skin in Orlando, October 2004 (see Skin Research & Technology, 10(4), p15 (Abstracts), 2004). It concerned in-vitro membrane integrity testing with Franz cells, but the question is equally valid for in-vivo TEWL.

Our answer is yes. TEWL is the transport of condensed water through the skin barrier and a good skin barrier will let through less water than a poor skin barrier.

The difficulty arises because TEWL instruments do not measure TEWL directly. In fact, TEWL instruments are evaporimeters and they measure the flux of water vapour coming from the skin surface. You can interpret this measured water vapour flux as TEWL, if two conditions are satisfied, as follows:

TEWL and nothing but TEWL.

TEWL may not be the only source of water vapour flux. Other possible contributions include perspiration, evaporation of free surface water and leaks in the apparatus. The measured vapour flux can be related to the skin barrier only when these non-TEWL sources have been eliminated.

All must evaporate.

If you've satisfied that (1) above is OK, then TEWL will be your only source of water vapour. But remember that TEWL is the migration of condensed water through the epidermis, whereas TEWL instruments measure vapour flux from the surface. All the water arriving at the skin surface must evaporate for the vapour flux to be equal to TEWL. If the humidity in the measurement chamber is too high, then this won't happen.