How is the skin better hydrated by vapor as opposed to liquid water? Wrapping yourself in a damp sheet is more effective at treating the dryness of eczema than taking a bath. — CW
When your skin is immersed in pure water, the only molecules that ever collide with its surface are water molecules. That might seem to be the ideal situation for keeping skin moist, however such immersion can have other unintended consequences. First, any water soluble atoms, molecules, and ions that can move to the surface of your skin will dissolve away in the surrounding water and you’ll never see them again. Second, any water soluble atoms, molecules, and ions that can’t move to the surface of your skin will draw water into your skin by way of osmosis—the pure water will flow into your skin cells in an attempt to dilute the dissolved particles inside those cells. After a relatively short time, the cells of your skin will contain many more water molecules than before and your skin will look all wrinkly. This flow of water soluble materials out of your skin and water into your skin may not be so wonderful for your eczema.
When you wrap yourself in a wet cloth, you are ensuring that the relative humidity near the surface of your skin will be close to 100%. Air molecules will still be present around your skin but now there will be essentially no net transfer of water between your skin and the surrounding air—water molecules will leave your skin for the air at roughly the same rate as water molecules return to your skin from the air. In effect, you are stopping evaporation from your skin and very little else. Stopping evaporation from your skin will also cause it to accumulate moisture, but this time the new moisture will come from within your body. Water molecules that would have left your skin had it been surrounded by dry air are now staying in your skin, where they add to the moisture in your skin. Overall, you skin will contain more water but it will not have lost as many water-soluble chemicals and it will not have water driven into it by osmotic pressure. It may be this more gentle moisturizing effect that makes wrapping yourself in a damp sheet more pleasant for your eczema than immersing yourself in water.