435 SUSTAINABLE HAIR
WATER CONTENT OF HAIR
As previously mentioned, many hair properties are highly dependent on its water content.
Outside of purposely wetting the hair by immersion or spraying, the technical water content
is dictated by the relative humidity of the surrounding atmosphere.29,30 This relationship is
depicted by the hair-water adsorption isotherm show in Figure 14.
That is, the water content of hair is not fixed, but instead is a dynamically changing
variable that will change as the wearer moves from indoors to outdoors, room to room,
and to encounters with differing climatic conditions. When the relative humidity rises, so
does the technical water content of hair. Furthermore, as already noted, many properties
vary notably with water content and therefore are also at the mercy of climatic conditions.
A major consumer concern involves hair “drying out,” but technical evidence shows that
extreme conditions are needed to meaningfully change the shape of this isotherm. To this
end, this author’s experience in testing hair from many panelists that reported having “dry,
damaged hair,” all produced unmodified isotherms. Similarly, while many products claim
to “hydrate” or “moisturize,” they also generally have no effect on the isotherm.
Illuminating work by Davis &Stofel31,32 involved using the relative humidity to fix the
water content of hair, and then tresses were presented to panelists for evaluation. That is,
hair was equilibrated at either 15% or 80% relative humidity, which, from the isotherm
in Figure 14, is seen producing approximately 5% and 17% water content, respectively.
Despite having less than a third of the technical water content, the hair equilibrated at 15%
relative humidity was overwhelmingly ranked a feeling the “most moisturized.” This sample
was similarly ranked as being “smoother,” “less tangled,” and “less damaged” by sizable
margins. In short, despite consumer protestations, the tactile properties of hair suffer when
Figure 13. Youngs modulus of hair as a function of the relative humidity.

436 JOURNAL OF COSMETIC SCIENCE
the technical water content is raised. This likely relates to the heightened swelling and an
accompanying degree of cuticle uplift. On top of this, tensile testing experiments show
the break stress decreasing with increased water content (for reasons described previously).
As highlighted earlier, breakage in fatigue experiments shows much more sizable effects
and produces alarming results.21 In short, there is overwhelming evidence for considerable
negatives associated with increasing water content in hair.
In this same consumer study, panelists were also asked to assess the same attributes in
conditioned and nonconditioned hair wherein the conditioned hair was sizably ranked
as “most moisturized.” These products have an undisputed positive effect on consumer
perception of this property, but this is not because the technical water content is being
altered. The primary technical function of these products is to coat the hair with an
aesthetically pleasing, lubricating layer. It is, therefore, concluded that a perception of
“dryness,” and conversely “moisturization,” is a tactile property of the hair rather than
that of the strict technical definition. To this end, “moisturization” and “hydration” claims
are commonplace on hair-care products without consternation as it is understood that the
propositions relate to the consumer’s view.
These technical evaluations of water content as a function of relative humidity are
performed by a gravimetric gas adsorption approach using commercial equipment that is
today termed Dynamic Vapor Sorption (Surface Measurement Systems, PA).29
HAIR SHINE
Shine is also an attribute that consumers frequently associate with healthy hair, whereby
dulling is a negative accompanying diminished well-being. Technically, this attribute would
Figure 14. The hair-water adsorption isotherm.