Measuring the Water Content of Hair

Continuing a previous exploration1 of measuring hair-related attributes and related concerns, another commonly heard worry is that hair will “dry out” and accordingly, consumers may avoid treatments, environmental conditions and even ingredients that are perceived to induce this state. When this condition is recognized by the consumer, products that moisturize or hydrate hair are demanded in an attempt to “restore hair’s natural moisture balance.” However, to the hair care scientist, these statements are puzzling. For a start, it is well-recognized that the water content of hair is overwhelmingly dictated by the relative humidity (RH) of the environment—but while consumers profess a desire for “maximum moisturization,” the high humidity conditions that induce this state represent the very definition of a bad hair day. Moreover, technically, as hair becomes more damaged it has a higher affinity for water. The number of water adsorption sites is increased within the fiber as hydrophobic cross-linking disulfide bonds are converted to hydrophilic cysteic acid moieties. Also, the outermost, water-repellent surface of hair is more likely to be removed (the lipid “f” layer). Therefore, theoretically, the consumer term dry, damaged hair would seem contradictory.

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