This question appeared in the title of a poster presentation at
the US Symposium of the International Society for Bioengineering and the Skin in Orlando, October 2004
. It concerned in-vitro membrane integrity testing with Franz cells, but the question is equally valid
for in-vivo TEWL.
Our answer is that TEWL does indeed give a useful measure of barrier
function because 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,