Hair dye allergy is one of the commonest forms of allergic contact dermatitis. Most contact dermatitis clinics report a rate of between 3 and 7 % positive to the standard screening agent, p-phenylenediamine (ppd) [1, 2], and although there have been few studies of the prevalence of hair dye contact allergy in the normal population, this figure may approximate in some countries to 1 % or more [3]. Hair dye allergy is commoner in Asia and also in Southern Europe compared to Northern Europe as darker hair dye contains more allergen [2]. In the normal population prevalence of hair dye allergy is commoner in females [3] except in countries where dyeing of male facial hair is common. The incidence of hair dye allergy increases with age, especially over 40 years.

PPD belongs to the family of chemicals known as aromatic amines which are used to dye hair. They have been in use for over 100 years [1]. Hair dye is used by a significant number of all populations to both hide grey hair and, in the younger age group, for fashion.

Soon after their commercial introduction over 100 years ago, it was recognised that hair dye chemicals have the potential to cause allergic reactions. Whilst most hair dye preparations have other potential allergens, such as preservatives and fragrances, aromatic amines such as PPD are usually the dominant allergens in such reparations. The actual allergen is PPD or oxidised products of PPD [4]. PPD and other aromatic amines are amongst the most potent haptens in commercial use.

16.2 Clinical Presentation

As with most cosmetic allergens, dermatitis reactions occur through elicitation reactions, i.e. usually within 24–48 h. In subjects who are very sensitive, the reaction can appear within 2 h and be very oedematous and can be mistaken as angioedema. Reactions are commoner on the head and neck off the scalp rather than on the scalp itself (see Chap. 2).

The most commonly affected sites are:

Forehead

Ears and neck

Periorbital area

However, any area of the head and neck can be affected.

The reaction is usually erythematous, but there can be a significant amount of oedema and exudation. Facial oedema can be so severe that it is mistaken for angioedema (pseudoangioedema).

Reactions often occur after a change in the hair dyeing routine, e.g. a darker shade has been used or the dye has stayed on for longer. Predictive ‘skin tests’ used by hairdresser do not always detect allergy to hair dye and at present are not routinely recommended as the application of hair to the skin gives a risk of sensitisation.

Unusual forms of presentation include:

Lichenoid rash

Pigmented dermatitis

Erythema multiforme (both local and distal sites)

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