Contact dermatitis is an inflammation of your scalp caused by a reaction to an irritant or allergen. The most publicized reaction, and the most severe, is anaphylaxis from hair colouring,however, this only occurs in a very small percentage of people. More commonly, contact dermatitis produces a milder reaction characterized by an eczema-like irritation of the scalp.
It can be brought on by sensitivity to a specific ingredient in one of your hair products, chemical straightening, hair dyes, stress, an existing or newly-acquired food intolerance, or the combination of the products you use to style your hair.
Patch-testing can often determine the cause of contact dermatitis, so the irritant or allergen can be avoided. However, this can be difficult if either is a commonly circulated ingredient.