How Does Peroxide Work to Bleach Hair?

Young woman with bleached blonde hair and face piercings, close-up
Be careful with your bleached hair. (Image: Medioimages/Photodisc/Photodisc/Getty Images)

Women have been changing the color of their hair since ancient times, using plants and berries to make dye. In the 1700s, fashionable women used powder to make their hair look lighter. Whether or not blondes have more fun, so-called “bottle blondes” certainly have their work cut out for them -- stripping their dark hair of color, getting it to a light and lovely shade, keeping it that way and preventing damage.

What's In a Hair

Hair starts in the follicle inside the skin and grows into a shaft -- the visible part above the scalp. The shaft consists of keratin, a protein that isn't alive. The inner layer of a hair is the medulla; covering that is the cortex; the outer layer is the cuticle. The cuticle consists of scales that overlap, like roof shingles. The medulla and the cortex contain melanin, the pigment that gives skin, eyes and hair its color. Dark hair contains more melanin, or color pigment, than light hair.

How Bleaching Works

Years ago, before commercial hair-coloring kits were available, women used combinations of hydrogen peroxide and ammonia to lighten their hair, often producing an unattractive orange color and resulting in breakage and damage. Although gentle chemicals have replaced ammonia, modern hair-coloring techniques still use hydrogen peroxide to soften the hair’s cuticle, enter the hair shaft and dissolve the molecules of color. The peroxide reacts with the melanin, lightening the hair through a process called oxidation that leaves the melanin without color. This oxidation is the process that can damage the hair.

Remove and Add

Hydrogen peroxide alone is not usually satisfactory as a hair-coloring agent. Permanently changing the color of hair is generally a two-step process, first removing the color with bleach and then adding a new shade. After hydrogen peroxide has removed the natural hair color, a dyeing agent injects the new color into the hair’s cortex, where it bonds to produce a lasting color. Different types of hair and natural hair color respond differently to hair coloring products. Most hair can be lightened up to two shades lighter than natural. Coarse hair may take longer to lighten than fine hair. As the hair grows out, the original color shows at the roots.

Bleaching Is Permanent

Since bleaching is the permanent removal of hair color, bleach can’t be removed from the hair. The bleaching agent permanently removes the color molecules from the hair shaft. If the user is unsatisfied with the result of bleached hair, she can find a coloring product near her original color and use it to go back as much as possible to her original color, bearing in mind that the multiple process of bleaching and then adding color contributes to damaging the hair. Hair that has been processed by bleach and dye requires frequent conditioning and gentle handling.

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