Both biochemical and biophysical changes occur in the outer layer of skin (i.e., stratum corneum or SC) in response to variations in indoor temperature and relative humidity (RH). For example, early work by Blank (1952) revealed that as RH approached 60%, skin became “soft and pliable’. More recent research conducted by Vyumvuhore et al. (2013) using non-invasive Raman spectroscopy has shown that both the organization of lipids and protein structure in the SC is optimized at a RH of approximately 60%.
Based on these studies the optimum or target hydration level (i.e., water content) of untreated stratum corneum is 0.10 g water/mL SC.
As indoor humidities decrease below 60% RH, the associated drying stresses produce stiffer skin and can also lead to the formation of dry, flaky skin as well as dandruff.
The primary skin-care options in indoor environments with sub 60% relative humidities consist of
- the application of moisturizing lotions, creams, and shampoos as well as
- the operation of a room humidifier.
Most moisturizers produce a thin film of an occlusive substance that inhibits or resists the diffusion of water through the skin into ambient air--thus compensating for reduced ambient humidity. In contrast, a humidifier simply adds water vapor to indoor air, which directly re-hydrates skin.
Note that an indoor relative humidity of 60% should be considered an upper-bound humidity goal because beyond this level the risk of indoor mold formation increases and air becomes less comfortable.