Do You Know How Hair Dye Works?

It’s estimated that 75% of American women dye their hair; it’s a gigantic industry that ranges from supermarket aisles to high-end salons.

Whether you’re just adding a few natural highlights or dramatically bleaching your brown hair to blonde, it’s undeniable that hair dyeing is a central part of American life. But when you sit down in the stylist’s chair or open that box of dye at home, what are you really doing to your hair?

In order to understand how hair coloring works, it’s important to know about the structure of the hair itself. Each hair has a root beneath the skin, and a shaft which protrudes above. Hair is dead tissue made out of a protein called keratin; it gets its pigmentation from a substance called melanin. Each hair is shaped like a tiny little tube, with three layers: the medula in the center, then the cortex in the middle which holds pigment, and finally the outer layer called the cuticle. The cuticle must be opened before any color can be applied to the strands. In order to facilitate this process, almost all hair dyes contain two ingredients:

hydrogen peroxide (also called the developer) and ammonia. The hydrogen peroxide is an oxidizing agent, which “lifts” the natural pigment from the hair; ammonia opens the hair shaft and exposes the cuticle, so that the dye will bond with it. The coloring agents in hair dye are made of molecules that vary in size; this creates hair dyes which penetrate into the hair in different ways, and fade at different rates. There are four common levels of hair dye, relating to the length of time the color will stay: permanent, demi-permanent, semi-permanent, and temporary.

Temporary hair dye sits on top of your natural hair color; the dye molecules are large and do not penetrate the cuticle layer, so they wash out with a single shampoo. There is no ammonia or hydrogen peroxide in these dyes; they’re often used for costume parties and in theater makeup kits. The next level, semi-permanent, has molecules that partially penetrate the hair. They contain very low levels of developer; it will alter the underlying pigment of natural hair, but not entirely, and will wash out in 4-5 shampoos.

Demi-permanent dye is excellent for people who are sensitive to some of the chemicals in other dyes, or for those who have fragile or damaged hair; it has no ammonia, and a lower level of developer. Without ammonia, the demi-permanent dye does not remove hair’s natural pigments and does not lighten it. Demi-permanent dyes are good at covering gray hair, and the final color is more natural-looking than permanent color.

Permanent color has dye molecules that penetrate the cortex and expand in size, making them impossible to wash out. The combination of hydrogen peroxide and ammonia causes the natural hair to lighten; bleach may also be required for dark hair, if the end goal is a light hue – a brunette becoming a blonde, for instance. Permanent dyes will remain in the hair until they grow out over time. Make sure you know you want that color for a while – it’s not coming out!