441 SUSTAINABLE HAIR
hangs from the head and moves. Increasing fiber curvature will increase interactions with
neighbors and similarly affect these two properties. In more extreme cases, manageability
might become an issue while also potentially restricting the ability to create certain styles.
All things being equal, thicker fibers will have higher mechanical properties: thicker hair
necessitates higher bending forces and higher extensional forces to cause breakage. In
short, vastly different hair array properties can be explained without requiring structural
differences.
This holds true even for extremely curly hair of African origin, which is prone to many
cosmetic issues.23 In addition to the tight curls, this hair can have a highly elliptical cross-
section, which may also twist down the length of the fiber (see Figure 15). Most obviously,
the extreme conformation leads to grooming issues and style restrictions. In this hair type,
overall array properties essentially relate to interactions involving thousands of irregularly
shaped spring-like fibers, as compared to the more uniform, rod-like strands in straight hair.
Again, it can be seen how dramatically different properties will result without structural
differences. The ellipticity and twisting might be expected to affect tactile properties, such
that individual fibers don’t feel as smooth. The combination of this high curl, the high
ellipticity and the twisting has long been suspected to produce points of high localized
stresses in fibers where flaws and cracks might form. These weaknesses may then propagate
rapidly to produce the high propensity for hair breakage. Figure 15 also seems to show an
example of this occurrence as large longitudinal cracks can be seen going into and out of
the twist.
While it is commonplace to try and pigeonhole hair, it is again worth mentioning the
extreme variability of this substrate. For example, it is commonly heard that Asian hair is
thicker than Caucasian hair Figure 16 shows this author’s results for statistical distributions
obtained from measuring the diameters of around 1,500 hair fibers for both hair types. The
Asian hair indeed possesses a significantly higher mean, but standard deviations are very
high. So, while Asian hair is in general thicker than Caucasian hair, it is still eminently
possible to find some Asians with quite thin hair and some Caucasians with quite thick
hair. Further to this point, there are also very high standard deviations in the dimensions of
hair from any given head.43 The above statement should therefore be considered a generality
and not a universal absolute. The same is true of many parameters that are frequently used
in such pigeonholing.
Figure 15. The twisting structure of highly curly hair of African origin.

442 JOURNAL OF COSMETIC SCIENCE
THE COUNTER ARGUMENT
As highlighted above, it is possible to rationalize all manner of differences in hair properties,
that might traditionally be associated with being from different “hair types” without the
need for a difference in makeup. However, upon digging deeper, some curiosities emerge.
Most obviously, it is well recognized that chemical treatments can give rise to very
different outcomes when applied to hair from different heads. For example, application of
a permanent wave to one head might produce the exact desired outcome but might also be
wholly ineffectual when applied in exactly the same manner to the next head a bleaching
treatment might give rise to substantial lightening on one head, but considerably less on
the next.
While the activity of daily-use hair-care products (e.g., shampoos, conditioners, and styling
products) relates to the hair surface, so-called chemical treatments involve ingredients that
penetrate the inner cortex and engage in chemical reactions. In the case of perms, it has
been shown that progression rates can vary considerably when testing hair procured from
different individuals.9 Two explanations may be hypothesized: either chemical reaction rates
vary, or ingredient permeation rates differ. Given that this occurrence arises in treatments
that involve very different chemistries, attention would seem to fixate on the second option.
In support, penetration rates are facilitated at an elevated pH and can also be seen when
testing damaged hair where, in both cases, hair fibers can swell to greater extents. So,
a theory arises that hair from different heads can potentially swell to different degrees,
which subsequently dictates the progression rate of the desired transformation. This author
has generated some data that would seem to agree with the above hypothesis but not yet
enough for confirmation. However, returning to the point at hand, differing degrees of
swelling seemingly suggests different internal structures.
Figure 16. Statistical distributions for fiber diameters in Caucasian and Asian hair.