We recently spoke about how the misalignment of the cuticle is one of the dominant causes of dry hair. Stress to the hair follicle is what causes misalignment of hair cuticles. I have grouped together the classic 6 types of stress for our discussion. Today I will focus on ultraviolet stress (sunlight) and why and how it can damage hair. However, before we can discuss ultraviolet stress to the hair, we need to briefly talk about the chemistry of hair.
Chemistry of Hair
There are 3 predominant types of bonding in the hair structure: Hydrogen Bonding-gives the hair elasticity; Salt Bonding-gives the hair structure and some elasticity, and lastly Disulfide Bonding –which gives the hair the most structure. We will focus on the disulfide bond because it is primarily responsible for hair structure and the most susceptible to damage by photo-oxidation. So what exactly is a disulfide bond and why is it the most important bond for your hair?
The hair fiber is made up of 85% alpha keratin protein. Alpha keratin protein provides structure to hair. Thus it is commonly referred to as a structural protein. Keratin protein and any other protein for that matter are comprised of long chains of amino acids linked together with peptide bonds. (By now, you’re probably thinking what does this have to do with disulfide bonds? Everything! So be patient and please keep reading). There are 20 very unique amino acids associated with protein in living matter and they are the building blocks of life (i.e. your hair and skin, etc.) The predominate amino acid in keratin protein is cysteine. Cysteine amino acid is unique because it contains a sulfur side group (thiol). When a keratin protein chain aligns itself with another thiol group, it forms a disulfide bond. This disulfide bond configuration of two cysteine amino acids is referred to as cystine. It is the cystine structure that gives hair its structure. Now, back to UVA/UVB as a hair stressor.
Sunlight is comprised of visible radiation, ultraviolet radiation type A (UVA) and ultraviolet radiation type B (UVB). Radiation is energy and we can thank the depletion of the ozone layer for its increased intensity at the earth’s surface currently. UV radiation penetrates the hair’s cuticle and cortex. The intensity of the radiation is more concentrated on the cuticle surface than on the cortex; thus damage to the cuticle is disproportionally higher. With that said, a damaged cuticle surface exposes the cortex to further damage in the future.
Radiation associated with sunlight attacks keratin protein, lipids (cellular membrane complex) and melanin. As I stated earlier, keratin protein is responsible for the hair structure and the lipids act as glue to hold the cuticle layers in place. Melanin gives hair its natural color. When hair is over exposed to visual and ultraviolet radiation, it results in hair that is bleached, brittle and stiff. Therefore, it is more susceptible to breakage.
Find out more about UV Rays in part 3.