The Science Behind Hair Coloring

Source: Wikimedia Commons, Public Domain.
Source: Wikimedia Commons, Public Domain.

I’ll admit it- I went through period of my life when I did some serious time in the salon getting my hair dyed. My tale is not an unusual one. Growing up there aren’t many girls who are completely happy with their hair color- those of us with naturally darker hair were always envious of those fair-haired lasses; natural redheads often looked to stand out less; and you could even find the natural blonde wanting a change. For me, at first blonde highlights sufficed but then it wasn’t enough. I’ve gone completely blonde and every variation on the brown to blonde scale, also I have been a redhead for several years. Finally I came to the realization that my natural hair color wasn’t so bad after all. But what was actually going on behind the scenes as I smocked up and turned my hair over to my hairdresser? How was it a dark haired gal could leave the salon a few hours later with blonde hair? Let’s look into how hair dye does its thing.

There are several different categories of hair dye each with overlapping but distinct ingredients. There are permanent dyes, semi-permanent, demi-permanent, and temporary; as you add permanency you also add harsher chemicals and thus the less permanent the dye the safer it is for your hair. Going dark to light involves two steps- the permanent dye concoction strips your hair of its natural color and then dyes it the color of choice. The less permanent dyes are not stripping the hair as much, just adding dye to it. Thus the logical conclusion is the color-removing agents are the more toxic entities in the dye mix. Another logical assumption is if you have really dark hair anything other than permanent dyes just aren’t going to cut it if you are looking to go lighter. But if you are going light to dark you can get away with skipping the color-stripping. We are going to focus on looking at the science behind one of two types of permanent dyes- oxidizing hair dyes (the other is known as progressive permanent dye).

Dye type
Hydrogen Peroxide/Ammonia
Duration
Temporary
No
Until one wash
Semi-Permanent
No
6 to 12 washes
Demi-Permanent
May contain
24 to 26 washes
Permanent (oxidizing)
Yes
Until hair is grown out
Cross section of hair.
Cross section of hair.
Anatomy of Hair. Source: Wikimedia Commons, Wong, D.J. and Chang, H.Y., CC BY 3.0.
Anatomy of Hair. Source: Wikimedia Commons, Wong, D.J. and Chang, H.Y., CC BY 3.0.

Before we get into the specifics of what chemicals are in dye it is important to review the basic anatomy of hair since in order for dye to work the hair itself needs to be altered in several ways. Each piece of hair has two regions- the shaft and the root. The root is below the surface, anchored into the follicle and the shaft is the above the surface portion. Hair is made up largely of dead cells, with only a few live cells at the base of the root. The shaft of hair is made up of proteins, namely keratin and melanin. Most pieces of hair have three cross-sectionally layers- the outermost cuticle, the middle cortex, and the inner air-filled medulla (not every piece of hair has a medulla). The outer protective layer of hair, the cuticle, is made up of keratinocytes, the cells that make keratin. These cells are arranged like shingles on a roof and are semitransparent allowing the melanin, the color pigment protein, in the cortex to show through. Each individual keratin molecule has a lot of cysteine, the amino acid known for its sulfur content. These sulfur molecules give strength to cysteine by forming disulfide bonds, thus any protein cysteine is largely represented in will be known for its strength. It is with this strength that keratin proteins in the hair shaft form sturdy, insoluble, intermediate fibers. The type of melanin you have dictates your hair color, there are two options-eumelanin gives all shades of brown to black hair while the less common pheomelanin gives blonde and red hair.

The general ingredients in permanent hair dye mix are the dye, initially in the form of a dye intermediate or precursor; the developer, generally hydrogen peroxide; a coupler, which varies depending on the desired hair color; and an alkaline, usually ammonia. Ammonia and hydrogen peroxide are heavy hitters in the hair dying process as they both serve multiple functions. (We’ll get to hydrogen peroxide in a bit). The ammonia acts as a vehicle to deliver the precursor dye to the cortex and as an alkaline it makes the environment basic, allowing for the appropriate conditions for the necessary reactions to occur. The coupler, and often times the dye intermediate as well, varies in its composition as it is the indicator on what the color outcome will be. The three major coupler divisions are yellow-green, red, and blue. These ingredients are stored as two separate groups- the ammonia and the dye components can be made up and stored together before the actual dying but the hydrogen peroxide cannot be stored with them, it remains separate until right before the dye job is to take place.

Hair Dye Ingredients
Hair Dye Ingredients

In order for hair dye to do its thing the dye and the associated components need to make their way to the cortex so something has to change with the outer layer of hair, the cuticle. What happens is the dye mix causes the cuticle to swell and soften, allowing it to open for access to the cortex. One specific mechanism by which this is achieved is the hydrogen peroxide breaks down disulfide bonds of the keratin’s cysteines to make the melanin accessible. Opening up the cuticle allows the small precursor dye molecules to sneak into the cortex of the hair shaft. In addition the hydrogen peroxide gets into this exclusive area and can interact with keratin, melanin, and the precursor dye. The dye intermediates meet up with each other to make larger dye molecules as well as react with the hydrogen peroxide and the dye coupler. These large molecules cannot be washed out with shampoo, only growing out one’s hair will get rid of them hence the name permanent dye. Hydrogen peroxide effects are two-fold, it oxidizes both the pre-dye molecules to make full-fledged dye and melanin to rid it of color. This is particularly important when one’s starting hair color is dark and the desired color is a lighter shade. Just simply throwing dye at dark hair is not enough to lighten it, first the color needs to be minimized.

Nicki Minaj displaying her not-so-natural pink hair. Source: Wikimedia Commons, Philip Nelson, CC BY-SA 2.0.
Nicki Minaj displaying her not-so-natural pink hair. Source: Wikimedia Commons, Philip Nelson, CC BY-SA 2.0.

Once the concoction is added to hair organic chemistry takes over. Funny isn’t it? A dreaded subject even for science geeks but without organic chemistry Beyoncé wouldn’t be blonde, Emma Stone would be without her signature red locks, and Nikki Minaj and Katy Perry would have a lot of free time on their hands. The initial ingredient, the dye intermediate, is an amine- p-phenylenediamine for example. When this amine is put together with hydrogen peroxide it is oxidated to a quinone, then this quinone can now interact with the coupler. Specifically this interaction is an attack, an electrophilic aromatic substitution. This reaction product of the quinone and the coupler will also be oxidized by the hydrogen peroxide and all of this yields the final dye. The combination of the hydrogen peroxide and these dye components-the intermediate dye and the coupler- have the two-fold effect mentioned above, they strip the melanin of its pigment to lighten the hair overall and then bind to it to change the outward appearance of the hair. This is why no two people will look exactly the same when using the same hair dye, it’s the combination of how much color is taken away from the melanin and how much dye is bonded to it.

Oxidizing Hair Dye General Reactions.
Oxidizing Hair Dye General Reactions.

So there we have it- a look at what goes on behind the cuticle to convert our locks to the color we were meant to have. There are of course many different varieties- many precursor dyes and couplers, and hence many combinations of the two. There are also different options out there for an alkaline to set the tone for the reactions to occur, as well there are alternatives to hydrogen peroxide to serve as the oxidizer. In addition to the oxidizing permanent dye discussed here there is another type known as progressive permanent dye. Also there are semi-permanent, demi-permanent, and temporary dyes all acting on the hair in slightly different ways with varying results.

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