Hair Care Product Claims 518 Furthermore, there are additional external stimuli that will also induce changes in hair color. Exposure to the sun’s ultraviolet radiation is well-recognized to lighten hair due to photo-oxidization of melanin. Similarly, dye molecules can be degraded by the same mechanism. Figure 18 shows color change (ΔE) for permanently dyed hair as a function of irradiation time within an accelerated weathering chamber. Similarly, Figure 19 shows that dye molecules can be thermally degraded by the extreme temperatures encountered during heat styling with straightening and curling irons. Another factor that contributes to the rate of color fade involves the condition of the hair. Chapter 10 described how damaged hair is more prone to swelling, which facilitates diffusion both into and out of the hair. Therefore, as shown in Figure 20, color fading occurs far faster in damaged hair. There is no doubt that color fading of permanently dyed hair is an issue for consumers, yet mechanisms by which conventional products may retard this occurrence are difficult to rationalize. It is sometimes suggested that hydrophobic surface deposits can provide a barrier function, but, of course, these materials themselves are removed during the washing process. It is also unreasonable to Figure 18. Color fade in permanently dyed hair as a function of UV exposure in an accelerated weathering chamber