My friend’s son bleached his hair last week, so I decided to study the science behind it. Before we start, we need to know the various names for different parts of hair:
When hair is bleached, melanin within the cortex is oxidized by hydrogen peroxide. The process of bleaching hair is known to be damaging, and I found a cool picture taken with scanning electron microscope that shows what happens to your hair follicles.
What the hair follicle looks like before and after bleaching:
Here is a cross-section of the hair follicle before and after bleaching:
A close up view of melanin after bleaching:
When hair is bleached, hydrogen peroxide first solubilizes melanin, then decomposes the melanin. Upon further bleaching, the molecular frames of the melanin granules are decomposed.
Effect of hydrogen peroxide on melanin solubilization:
There are two types of melanin in hair, and the balance between the two determines your hair color. Eumelanin imparts a dark brown or black color to hair, while pheomelanin gives hair a golden blond, ginger, or red color. The yellow tint that often remains when hair is bleached is due to keratin, a protein found in hair.
Hydrogen peroxide alone will not bleach hair. In order for hydrogen peroxide to access melanin in the cortex, it must get past the outer coat, or cuticle. Using ammonia, the pH is increased, causing the hair follicle to swell. This swelling “breaks open” the cuticle, allowing hydrogen peroxide to penetrate the cortex.
Effect of pH on hair swelling:
While the cuticle must be opened for bleaching to occur, this is damaging to the hair follicle. This is why conditioners are used after bleaching hair, which help close up the cuticle.
It seems that hydrogen peroxide also breaks down keratin, which is found in the cortex. Keratin contains disulfide bonds, which are broken by hydrogen peroxide. The breaking of these bonds are (I think) what causes the bad odor when you bleach your hair, as sulfur is released.
I have never bleached my hair, and am kind of glad now that I have not. Of course, I have dyed my hair darker, which is also bad for it. I went through a phase a few years ago where I REALLY wanted purple hair. When I realized that I would have to first bleach it, I decided I didn’t want it THAT much. 🙂
P.S. I just judged at the NE Regional Science and Engineering Fair and had a great time! I was especially excited to meet a girl named Emma whose project was about the effects of dye on hair. Great job Emma!