What is the name of the chemical commonly used in permanent hair dyes that can cause allergic reactions?

The chemical commonly used in permanent hair dyes that can cause allergic reactions is paraphenylenediamine, or PPD.

PPD is an aromatic amine used as a dye precursor in many permanent oxidative hair-color products, particularly darker shades. During coloring, PPD is oxidized and reacts with other ingredients called couplers. The resulting larger colored molecules become trapped inside the hair shaft, producing a durable color that does not simply wash away like temporary dye.

PPD can cause allergic contact dermatitis, a delayed immune reaction that may produce itching, redness, burning, swelling, blisters, or sores. Sensitization can be lifelong. This is why hair-color products commonly include instructions for an allergy alert or patch test before use, although a negative test does not guarantee that a later reaction will not occur.

The other options have different roles. Hydrogen peroxide is the oxidant that helps develop color and lighten natural pigment; ammonia creates an alkaline environment and helps open the hair cuticle; resorcinol is commonly used as a coupler. PPD is also associated with “black henna” body art, where it can cause severe sensitization and make later reactions to hair dye more likely.