Which ingredient in hair color is known to cause allergic reactions?

Para-phenylenediamine (PPD) is the ingredient in hair color that is most commonly associated with allergic reactions. It is a chemical compound often used in permanent hair dyes due to its ability to provide rich and long-lasting color. PPD can cause skin sensitization, leading to allergic reactions in some individuals, which can manifest as itching, redness, or swelling at the application site.

Unlike PPD, ammonia and hydrogen peroxide are mainly agents that facilitate the hair dyeing process. Ammonia raises the hair cuticle to allow color penetration, while hydrogen peroxide acts as an oxidizing agent to develop color. Although some people may have sensitivities to these chemicals, they are not typically the primary cause of allergic reactions in hair coloring products. Silicone, on the other hand, is used primarily for its conditioning properties and does not cause allergic reactions related to hair dyeing. Therefore, PPD stands out as the key ingredient that poses a risk for allergies in hair color formulations.