To test a certain property in a hair sample, scientists subject a portion of that sample to a given insult to assess before and after effects, then compare results. In doing so, they assume the hair is “undamaged” before treatment, and that the relative amplitude of the change in the specified property precisely reflects the damage caused by the treatment. Put another way, it is assumed the hair sample has no history. Furthermore, although there is a common notion of what healthy hair is, there is not a clear, objective definition for it that allows one to debate, by contrast, what damaged hair is.
This essay reviews the structure of hair and introduces the concept of hidden hair damage, which at first is either not immediately detectable or occurs at unchanging and minimal levels, but then unfolds and amplifies the effects of a subsequent damaging operation. As a consequence, the same treatment may lead to different end results depending on whether the reference, i.e., “un-damaged” hair, holds such hidden damage. This concept of hidden damage underlines the importance of using more than one test method for understanding and evaluating the degree of damage in hair.
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