PPD is short for Phenylenediamine. Learn more about what it is and what other applications it is used for e.g. Henna and make up.
Here is the information on what PPD hair dye is and how it works. This includes the science behind it and best practices for perfect application.
See the science behind what causes allergies here. There are many causes and reasons for allergic reactions and the many factors are mentioned in here.
See information related to PPD and PTD allergies here. This page shows how these allergies occur and what you can do about it.