Effect of photodamage on the outermost cuticle layer of human hair

• The morphology of hair outermost cuticle is significantly modified after irradiation with a mercury lamp.
• The irradiation process resulted in the formation of small bumps (up to 5 nm height) on all cuticle surfaces.
• Another photodegradation effect is the enlargement of pre-existing holes (holes increased around 338 % in depth).
• As a result of irradiation, the height of the steps formed between the edges of two cuticle scales also increased ≈ 66 %.
• The morphological changes were followed by an increase in the cystine oxidation level on the hair surface.

The surface of the hair is the region most exposed to solar radiation and to the environment in general. Many of the well-known damaging effects of sun exposure on hair must start or even be restricted to the most external cuticle layers. As such, this work investigates morphological, ultrastructural and chemical changes in the outermost cuticle layer of dark brown hair, using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The results showed that after 230 h of irradiation with a mercury lamp, small bumps with globular shape (heights lying in the 1–5 nm range) appeared on the cuticle surface and their size increased with increasing irradiation times. In addition, the enlargement of pre-existing holes was also observed (holes increase around 350% in depth) and the height of the steps formed between the edges of two cuticle scales increased around 65%, as a result of 500 h of irradiation. The damages in hair strands were accurately identified by analyzing exactly the same surface region before and after irradiation by AFM images. Finally, the results were discussed in terms of the chemical differences between the non-irradiated and the irradiated hair, for instance, the increased level of cystine oxidation as a consequence of photodegradation.

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