The only way to really know if you're allergic, unfortunately, is to actually have a reaction to PPD, says Zeichner. If you bleach your hair or use other light-colored dyes, you don't have to worry about PPD— according to the Baylor College of Medicine, since those formulas usually don't contain it.
If you have a hair-dye allergy, the typical route is to use henna or an all-natural hair color. With the help of ammonia, PPD reacts with the air and deposits pigment inside the hair shaft at the cortex — a process that creates more long-lasting results than plant-based or semi-permanent dyes that only colour the outside of the hair. Unfortunately, the cosmetic effects of PPD can come at a price for many people.