TEWL is measured by a gadget called the ServoMed Evaporimeter (no, you can't have one, it's very expensive!) We lose the greatest amount of water through our palms, soles of our feet, backs of our hands, and forehead. The mechanisms of TEWL are not well known, but there are been many studies on it for differences in sex, race, and age. Interestingly enough, these studies have been inconclusive. The only real determinants of TEWL are damaged to skin, skin hydration, and atmospheric effects.
When the climate is dry, TEWL increases. When we have too little water in our skin - we want 10% to 20% or so - TEWL increases. When there's damage to our skin - sunburn, burns in general, wounds - TEWL increases. In short, anything that assaults or insults our skin increases TEWL.
I love the idea of insulting the skin. This is one of those phrases that comes up a lot in my cosmetic chemistry textbooks, as well as the word "perturbs". It seems to imply that our skin is really and truly alive with a personality that is easily offended. I wonder how many of my facial skin problems are the result of me yelling into the mirror "You are not as blemish free as I would like!" or "Why are you so sensitive?"
So what can we do to reduce TEWL? We can stay away from things that damage our skin, like too much sun or wind exposure, extremes in temperatures, or really scratchy sweaters. We can live in a humid climate or have a humidifier in the house (40% to 60% humidity is ideal). And we can make lovely creations that will trap water into our skin or add more when needed.
Again, using a mild cleanser with re-fatteners in it will help a lot, as well as using lotions and creams. Thick moisturizing creams (those with nice butters or other occlusive ingredients) will reduce TEWL due to the occlusion. As the film on our skin disappears (it takes a few hours), the TEWL will increase, so re-applying our lotions is a good thing.
Light, moisturizing lotions (those with high water contents) will actually increase TEWL as measured by that the ServoMed Evaporimeter, but there's a reason for this - the evaporation of the water from the lotion itself increases the measurement and the evaporation of water from our skin...because we now have enough water in our skin, so the rest evaporates! You can make a light, moisturizing lotion more occlusive by using things like allantoin or dimethicone - these will form a barrier that will stop the evaporation, if you don't want to use butters! And aloe vera or other polysaccharides like hydrolyzed protein will offer a light moisturizing gel layer that can reduce water loss slightly.
Or you can use lovely oils containing gamma-linoleic acid (GLA) like borage oil. This is a very interesting study about the use of 24% GLA containing borage oil used neat on the skin of babies with dermatitis. The authors conclude that GLA can be useful in maintaining the water content of our skin!
If you're really interested in this topic, there is an entire wiki devoted to it!
Join me tomorrow for fun with sebum (not literally, of course!)