What is the Dry Skin Index (DSI)?

The Dry Skin Index (or DSI for short) is a measure of the drying stresses on skin caused by changes in indoor relative humidity and temperature. Until we developed the DSI, there was no way to easily monitor or gauge the magnitude of these external drying stressors, making it difficult to prevent or manage dry skin in a proactive way.  Ideally,  the Dry Skin Index can be used along with the UV Index, which represents the magnitude of ultraviolent radiation exposure from the sun, to guide year-round skin care.  

Background

An index involving dry skin has to be based on both humidity and temperature since both of these parameters are positively correlated with skin hydration (Cravello and Ferri, 2008). Moreover, Levi et al. (2010) showed that the mechanical drying stress on the external layer of skin (i.e., stratum corneum) is directly related to the chemical potential of water in air, which is a measure of the potential of water molecules to react or move, and is calculated from ambient temperature and relative humidity.

Chemical potential has units of kilojoules per mole, which are not particularly "user friendly" for an index, so we developed a proprietary scale based on chemical potential that ranges from 0 to 10.  This index or scale essentially covers the range of drying stresses produced by the temperature and relative humidity levels typically encountered in the indoor environment. The resulting Dry Skin Index is therefore based on a thermodynamic property of water that influences skin hydration as well as its related biochemical and biomechanical properties.

Similar to the UV Index, the Dry Skin Index is broken down into five categories for convenience in interpreting the magnitude of humidity and temperature skin stressors:

 

The Dry Skin Index increases as indoor air becomes dryer and cooler.

The adjacent graph shows how the Dry Skin Index changes with indoor relative humidity and two target temperatures. 

Relationship between indoor relative humidity and the Dry Skin Index for two temperatures.

Relationship between indoor relative humidity and the Dry Skin Index for two temperatures.

It is important to note that the DSI is a measure of the external drying stresses on untreated skin (i.e., stratum corneum), and does not necessarily predict the actual response of skin to such stresses due to inter-individual differences in skin properties. The same is true for the UV sunlight Index, that is, for a given or fixed UV exposure level, the potential for skin damage will vary with the darkness of people's skin.

Summary

The Dry Skin Index provides an easy-to-use metric for evaluating the magnitude of drying stresses on untreated skin as a function of indoor relative humidity and temperature.

References

Cravello, Barbara, and Ada Ferri. 2008. “Relationships between Skin Properties and Environmental Parameters.” Skin Research and Technology 14: 180–86. 

Levi, K., R. J. Weber, J. Q. Do, and R. H. Dauskardt. 2010. “Drying Stress and Damage Processes in Human Stratum Corneum.” International Journal of Cosmetic Science 32: 276–93.