Environmental Process of Hair

 Environmental Process of Hair

All hair is exposed to the environment. Unless the hair is covered for cultural or religious reasons, there will be some impact from the source of all life the sun.

UV exposure and its impact on hair have been widely studied. There are changes to the underlying hair structure by breaking down the protein structure, but there are also more visible sings of high levels of UV exposure. Extended exposure to sunlight can bleach melanin, giving hair highlighted look, especially in lighter shades where melanin levels are lower and the bleaching effect is more obvious (i.e. medium blond and lighter). Those with medium or dark brown hair may never observe melanin bleaching. People may also notice permanent hair dyes being bleached by UV, but this may be less easy to distinguish versus washing processes that can also rapidly fade dyes. Hair is exposed to light made up of a combination of different wavelengths, from UV to visible to infrared light. The atmosphere and ozone layer will filter out some of UVC light, and window glass will filter out UVB, so lifestyle and time spent outdoors will influence the level of light exposure. In addition, the irradiance (i.e. the radiant power per unit area) will vary according to time of year and location. As an example, irradiance in summer sun is ~0.55 W/m² compared to 0.35 W/m² in winter sun.

The mechanisms of UV damage are complex, but it has been well established that all the components of hair structure are affected, including proteins, lipids, and melanin. In the protein structure the major amino acid residues that are impacted are the sulfur-containing residues cysteine and methionine and the aromatic containing residues tryptophan, tyrosine, and phenylalanine.  The aromatic amino acids strongly absorb in the UV, forming excited state species which eventually form reactive oxygen species such as singlet oxygen, hydrogen peroxide, and hydro peroxides. Figure 1.29 shows basic pathways that can occur where proteins and lipids are slowly broken down via reaction with formed reactive oxygen species in this case, singlet oxygen. Contaminants in hair such as copper ions have been shown to accelerate these damage mechanisms by creating additional reactive oxygen species. Several oxidation products in the protein structure have been identified including kynurenines, which are yellow in color and are thought to be responsible for the photo-yellowing of hair sometime observed in gray hair. Since all the hair structural components are susceptible to UV damage, many different hair properties are impacted. These include hair strength, both Dry and Wet, and also increased combing forces. Thus hair exposed to UV may look lighter at the tips but it may also look less healthy, as structural damage manifests as broken fivers and split ends.

All hair types will be impacted by UV damage, although it is documented that darker hair will experience less damage due to the protective effect of melanin, which will also strongly absorb light. In addition, it is documented that artificial dyes will offer a protective effect by acting as UV absorbers, and it is also known that these dyes are oxidized and destroyed during this process. 

 

Water is an important part of the hair care process and can have beneficial effects by removing sebum lipids and dirt / dust from hair during the washing process, in some cases water can also contain harmful metals such as copper and iron, which are absorbed by hair. As mentioned elsewhere in this chapter, it has been demonstrated that copper absorbed by hair can accelerate damage from oxidative processes such as hair coloring and UV exposure. Water hardness (i.e. Calcium and magnesium ions) can also impact hair health and product performance.  For example, in hard water, deposition of conditioning actives such as silicones is reduced and lather amount is also reduced.