According to physical investigations on human hair, a correlation
exists between the amount of IHL and the moisture content in
the hair [26]. A reduction in the amount of lipids by solvent
extraction [27] and oxidative damage during chlorine bleaching of
wool [28] or during permanent waving of hair [29] favor the
diffusion of foreign materials into the fiber [30]. Photochemical
changes can occur in IHL as well as in hair protein and hair pigment
[31]. Hoting [32] have shown that the CMC lipids of hair fibers are
degraded more by visible light but also by UVA and by UVB light,helping to explain the weakening of the CMC and the multiple-step
fractures observed in hair exposed to light radiation.
Irradiation with sunlight degrades the IL in blond hair more than
that in black hair. UVB and UVA destroy approximately 25% of the
cholesterol in blond hair. Even visible (VIS) light and global radiation
degrades 60% and more of the initial cholesterol content. In contrast,
the cholesterol fraction from black hair is not significantly altered by
UVB, UVA and VIS light; in contrast with blond hair, global radiation
leads only to a lower extent (24%) of photo-oxidation of cholesterol
in black hair [32]. The photochemical degradation of free fatty acid
(FFA) occurs in blond and black hair by the influence of UVB and UVA
to comparable degrees. UVB irradiation reduces the FFA amount by
approximately 40%; and UVA irradiation, by approximately 20%.
Differences as a function of the type of pigmentation can be detected
for hair irradiated with VIS light. The FFA fraction from blond hair
was reduced by only 23%. With the exception of the FFA fraction
from black hair, IR irradiation does not show a significant
degradation of lipids. Global irradiation causes in blond hair a
degradation of fatty acids of 33%; and in black hair, of 42% [32]. The
lower photo-oxidative degradation in black hair suggests that
eumelanin is responsible for this effect. It protects the IHL mainly
from the photochemical influence of the visible range of sunlight.
The protective function of melanin against photodegradation also
applies to the UVB and UVA ranges.