While it is normal for the scalp to burn slightly, tingle or itch during a hair color application, intense burning may indicate that a chemical burn is occurring or that there is an allergy to one of the hair color ingredients. In most cases, normal irritation is caused from hair color's two main ingredients: hydrogen peroxide and ammonia. According to Annmarie Gianni Skin Care, intense irritation may indicate a phenylenediamine allergy.
The chemicals in hair color work to break down the protein bonds in hair, raising the hair cuticle and allowing color pigment to pass through into the hair shaft. Since color can break down hair, it can also start to break down skin, resulting in a chemical burn. Most chemical burns happen when the color is left on too long or when developer that is too strong for scalp application is placed on the scalp.
To prevent an allergic reaction, most colorists perform a patch test 24 to 48 hours before any color application to rule out sensitivity and allergies, according to Annmarie Gianni Skin Care. If an allergy is present, the color cannot be placed on the skin or scalp. However, it is still possible to color hair using a highlighting cap or foil application, both of which do not allow color to touch the scalp.