The Science Behind Hair Coloring – Everything You Need to Know

Miller's Hair Studio

The key to long-lasting results is the chemistry

Hair coloring is a hugely popular cosmetic treatment. Whether you’re going darker, lighter or just want to add some highlights, the process relies on chemistry.

Color wheel hair theory can help you pick a flattering shade that won’t clash with your skin’s undertones (warm, cool or neutral). But the key to long-lasting results is the chemistry.


Hair color works by coating each individual hair strand with a colored product (non-permanent) or by penetrating the hair cuticle, entering the hair cortex and bonding with it (permanent). The coloring products used are known as dyes. Generally, hair dyes contain primary intermediates, which are aromatic para compounds like 1,4-diaminobenzene or 4-aminophenol. They also contain coupler compounds, which are meta-substituted derivatives of aniline. Various combinations of these compounds produce the wide range of shades available in commercial hair dyes.

Hair contains natural pigment, or melanin, which gives it its color. The melanin is housed in the cortex, a secondary layer of the hair shaft. The number of melanin pigments in each strand is genetically determined. Each strand has a specific amount of melanin, which accounts for differences in the color of each person’s hair.

When you want to change your hair color, your stylist first needs to remove the existing melanin from the hair. This process is called lightening. This can be done by using bleach or an ammonia-peroxide hair color. Once the hair is lighter, the new dye can be deposited into the hair.

Some dyes can be applied directly to the hair, while others must be mixed with a developer or oxidizing agent. The type of hair coloring you choose determines how much time it will take to achieve your desired result, as well as the results that are possible.

There are many different coloring techniques that can be used to create unique effects, such as ombre, where the color fades gradually from dark to light, or balayage, which involves hand-painting sections of the hair with color. These techniques can help you achieve a more natural look, or create a dramatic effect.

Hair dyes are designed to produce colors that are not naturally found in the human body. In addition to traditional blonde, brown and black hair colors, there are also alternative hair dyes that are designed to fluoresce under blacklights. The use of these dyes is not without risk, however. A study published in 2001 found that women who had dyed their hair regularly for 15 years or more doubled their risk of bladder cancer compared to those who didn’t. This risk is even higher for hairdressers and barbers who have been working with hair dyes for a long time.


Ammonia is a very strong base and reacts with water to form ammonium hydroxide, or NH3 + H2O. It is also an effective solvent and dissolves a range of other ionic compounds, including many salts of nitrogen (ammonium nitrate, for example), metal cyanides, and thiocyanates. It is used as a catalyst in the manufacture of some synthetic resins, such as nylon and rayon. It is also employed in the process for producing soda ash, and in the production of some rubber chemicals, such as the ammonia-soda process, and the Ostwald process, which produces nitric acid from ammonium sulfate.

In hair coloring, ammonia serves as a catalyst in a chemical reaction that forces the cuticle of each hair strand to open, allowing dye molecules to penetrate and deposit their color inside the shaft. Ammonia is mainly used in “permanent” hair colors, although there are alternatives such as Coconut MEA that essentially produce the same result without ammonia. Non-permanent colors, which are typically referred to as semi-permanent and temporary, do not use ammonia at all and rely on other mechanisms to color the hair.

Because ammonia is a strong base, if you get it on your skin or in your eyes, you will need to rinse out the substance immediately. If it gets into your throat or nose, you may have difficulty breathing. If you experience this, seek medical attention right away.

If you live near an area where ammonia has been released, you should follow emergency coordinators instructions for avoiding exposure to the chemical. This may involve moving outdoors, or if indoors, closing and locking doors and windows; turning off air conditioners, fans and heaters; and keeping pets and children out of the affected area.

Many people prefer to use ammonia-free hair dyes because they do not have the pungent odor or damage the cuticle of the hair. However, these products do not work the same way as ammonia-containing hair dyes, and may not provide as good of coverage. They usually rely on other chemicals to break down the outer layers of the cuticle, which can cause your hair to look damaged and dry, rather than colored.


In order to lighten the hair, a chemical known as hydrogen peroxide is used. This is the most common bleaching agent found in hair dyes, and it is responsible for removing pre-existing color pigments from the hair. Peroxide is a strong oxidizing agent, which means that it breaks down chemical bonds quickly. As a result, it can cause damage to the cuticle. The cuticle is the outer protective layer of the hair shaft, and it is important for maintaining the health of your hair.

The hydrogen peroxide in hair dye is typically highly diluted to make it safe for use. However, it is still a caustic chemical that can cause skin and scalp irritation if used improperly. It also can cause chemical burns if it comes into contact with your skin.

If you want to avoid the risk of damage to your hair and scalp, then look for a hair coloring product that does not contain ammonia or peroxide. There are many options available, such as semi-permanent or wash out hair colors. You can also try henna hair dye, which is a natural option that does not require any bleaching agents.

Hair dyes are formulated with various chemicals, including hydrogen peroxide, ammonia, lead acetates, and paraphenylenediamine (PPDA). They all work together to alter the structure of the hair shaft, changing its color. When you dye your hair, the dyes deposit acidic pigment molecules onto the surface of the hair, and then small pigment particles slip inside the shaft, leaving behind a new shade.

The most popular hair dyes contain either ammonia or hydrogen peroxide, both of which can be harmful if not used correctly. The Cleveland Clinic reports that a major side effect of hair dye is dermatitis, which can be caused by any caustic substance. Symptoms of dermatitis include itchy, red, and painful skin.

Benzoyl peroxide is another chemical commonly found in hair dyes. It is a milder alternative to hydrogen peroxide, but it is also dangerous if not used properly. It can cause chemical burns to the skin and scalp, resulting in painful welts and blisters.

Other Ingredients

Hair coloring is more than just dyes. There are also surfactants to help the color spread evenly, conditioners to keep your hair shiny and soft, and polymers to strengthen your hair strands and prevent them from breaking down. These ingredients make sure the color isn’t drying out your hair or stripping away your natural shine, and they also help your dye last longer.

Most hair dyes have a much nicer smell than the rotten egg-like fragrance that used to come with permanent hair color, and most can be applied easily: simply pour over your head, massage, leave it to process (some formulas call for covering with a shower cap; others don’t) and then rinse and condition. Depending on the type of hair you have, it may take more or less time to absorb the dye. For example, coarse hair takes more time to get the dye deep inside, and if you have a perm or other chemical treatment on your hair, it will also affect the timing.

The heat that you naturally generate in your scalp or from a hot styling tool or blow dryer helps the color set and last, because it opens up the cuticle, which allows the ammonia-peroxide reaction to create a new color base in your hair shafts. This is why some hair dyes recommend covering your head with a shower cap or heating the hair color in the microwave; it optimizes the processing for stronger results.

Other ingredients that help your dye stay vibrant for a longer period of time are anti-oxidants and preservatives, which protect the chemical from degradation by the sun or air. They are also the reason why you shouldn’t shampoo or wash your hair while your dye is processing; each time your hair is soaked, it pulls out some of the color molecules, which makes your shade fade faster. You should also avoid extremely hot showers or steamers, because the heat can open up your cuticle and cause your dye to fade more quickly. Other products that help your dye last include toners, which neutralize unwanted tones by using colors around the color wheel, and blacklight-reactive hair dyes, which fluoresce under blacklights like those commonly used at nightclubs.

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