How Does It Work? Hair Bleaching

An estimated one third of women over the age of 18 use some form of hair dye. As any bottle blonde knows, in order to get a lighter color of hair, one needs to use a bleaching agent. This post will explore the mechanism of hair bleaching.

We’re gonna need some bleach.

In order to learn how hair bleach works, we’ll first need to establish why some people have naturally blonde hair, whereas others have naturally dark hair.

There are two kinds of naturally occurring hair pigments: eumelanin and pheomelanin. Eumelanin is what gives hair a dark brown color. Pheomelanin is what gives red pigmentation. The molecules themselves may differ slightly, resulting in, for example, eumelanin that is brown as opposed to black. Low concentrations of both eumelanin and pheomelanin result in blonde hair.

Eumelanin and pheomelanin are long polymers. Because they are conjugated polymers, meaning they alternate between double and single carbon bonds, they are capable of absorbing light. Thus, they appear dark in color.

A quick chemistry reminder: a single bond is when two chemical elements are bound by a single pair of electrons. A double bond is when they are bound by two pairs of electrons.

Eumelanin structure

The primary bleaching ingredient in hair dye is hydrogen peroxide. It is an oxidizing agent, meaning that it is capable of removing electrons from other molecules. Melanin molecules, with their numerous double bonds, have a few pairs of electrons to spare.

Hydrogen peroxide reacts with melanin, breaking their double bonds and eliminating their ability to absorb light. Because pheomelanin is more stable than eumelanin, hair that is in the process of being bleached often takes on an orange hue.

In addition to hydrogen peroxide, commercial bleaching products will usually have persulfate salts to help accelerate the process and stabilizers to help prevent the breakdown of hydrogen peroxide.

The brassy mid-bleach color is the result of pheomelanin.


Unfortunately, lighter color is not the only effect of hydrogen peroxide on human hair. Since hair is proteinaceous, it contains many oxidizable groups. Thus, hydrogen peroxide also weakens the cell membrane complex by oxidizing thioester bonds between cuticle cells, disulfide bonds in the cortical matrix, and other areas rich in amino acids (especially cystine). This makes the hair brittle and weak. Hydrogen peroxide also can damage a lipid on the surface of the hair called 18-MEA. As a result, bleached hair will also frequently feel dry.
This is why it is so difficult to get a Marilyn Monroe-blonde color when you bleach very dark hair. The amount of time needed to fully oxidize hair melanins will result in far too much structural damage to the hair.
It’s okay. True gentlemen don’t have hair color preferences.