PPD is short for Paraphenylenediamine(PPD); a chemical substance widely used as a permanent hair dye, textile/fur dye, dark colored cosmetic dye, and temporary tattoo dye. It can be chemically engineered in three different ways, but its really one chemical. PPD is primarily used as a component of industrial engineered composites and polymers: printing inks, black rubber, oils, greases and gasoline.
It’s nasty stuff. An industrial-grade additive that should Not be on your skin—ever. In fact, the CDC even lists PPD as a contact allergen. Exposure causes throat irritation, bronchial asthma, and sensitization dermatitis. And it was voted allergen of the year in 2006 by the American Contact Dermatitis Society (um, not a positive trophy). So try to avoid like the plague.