Allergies develop with repeated exposure to an allergen. A common allergen in most oxidative haircolor is PPD. You may or may not react to this in EcoColors. We always recommend you do an allergy test and a strand test according to the kit directions before applying any color to your entire head. Also just because you don't react one time, doesn't mean you won't react the next time. Please read EcoColors Frequently Asked Questions for instructions on how to order the FREE ALLERGY PATCH TEST.