How many people use hair dye?
According to a Danish study, around 18% of men and 75% of women dye their hair. This means that large parts of the population are exposed to chemicals found in hair dyes. Many of the ingredients have been shown to be strong or extreme sensitisers.
What is hair dye made of?
There are two main categories of hair dye based on their chemical composition;
- oxidative (permanent hair dye)
- additive (semi -permanent / temporary hair dye).
Oxidative / permanent hair dyes
For permanent hair colouring, substances are added to open the cuticle so the dye can penetrate the hair shaft. In this way the colour changes remain while the hair grows.
Permanent hair colour contains three main components of aromatic amines (dyes), a coupling agent and an oxidant, and the process can be divided into three steps:
- Oxidation of the dye
- Coupling reaction between the oxidised dye and coupling agent
- Further oxidation of the product from Step 2, where the final colour appears
When hair dye is added, ammonia opens the cuticle so that the dye can penetrate. The reaction between the dye and coupling agent allows the dye to become larger and cannot easily be washed out of the hair.
Commonly used dyes are p-phenylenediamine (PPD) and p-toluendiamine (PTD). These substances are categorised respectively as extreme and strong sensitisers. Common coupling agents are resorcinol and naphthols.
Temporary/ semi-permanent hair dyes
The substances used in products for temporary hair colouring only partially enter the cuticle or they attach to the outside and will eventually be washed out. Although these products are not oxidising, they still contain very small amounts of oxidants.
Examples of temporary products include rinses and shampoos, hair gel, sprays and similar products that contain hair dye.
What undesirable effects can hair dyes cause?
Hair dyes can cause a variety of undesirable effects but allergy is the most frequently reported.
Contact dermatitis is an allergic reaction. Symptoms often include eczema, redness, blistering, and itching of scalp, face and neck. These symptoms occur one to two days after colouring. The user may also experience swelling, especially in the forehead and around the eyes. The allergic reaction can last from about one week to several months. Cases of anaphylaxis have been reported.
The EU Scientific Committee has evaluated 46 of the substances that can be used in hair dyes. Of these, 27 substances were defined as sensitisers. A further study showed that 10 of these 27 substances were extreme, 13 strong and 4 moderate sensitisers. When a large percentage of the population dyes their hair regularly, some users develop allergies to one or more of these substances.
In several cases of hair dye allergy, consumers had previously had black henna tattoos (temporary tattoo). Henna is a plant extract that does not last long so other dyes are mixed in to enhance the colour's properties. One of the substances often added is PPD. The mixture is then called black henna. If you want to dye your hair, avoid temporary tattoos in the form of black henna.
The symptoms of contact urticaria include itching and localised areas of swelling and redness (wheal-and-flare) where the cosmetic product was applied, but also generalised urticaria (swelling anywhere on the body) and anaphylaxis. Symptoms may occur a few minutes after exposure to the product, and usually abate relatively quickly (within a few hours).
Preservatives and substances added to enhance hair colour are the ingredients that are most likely to cause contact urticaria but PPD can also give such a response.
Numerous studies have examined whether there is an association between dyeing and development of cancer, but few of these studies show such a relationship. The risk of developing cancer appears to be highest among those who began dying their hair before 1980. In the early 1980s, many of the hair dye manufacturers removed ingredients that could be carcinogenic.
How to avoid undesirable effects
The safest (and only) way to avoid these undesirable effects is not to dye your hair. If you decide to dye your hair:
- Avoid temporary tattoos with black henna (see above)
- Avoid dyeing your hair if you have had a rash (redness, itching, wounds) after previous dyeing of the hair, eyelashes and eyebrows, or from black henna tattoos
If you have experienced a rash after having dyed hair, eyelashes or eyebrows, or after application of black henna, consult a doctor and get an allergy evaluation before using hair dyes again.
About this page
Last scientific review: October 2019