p-Phenylenediamine (PPD) is a low toxicity diamine used as a component of engineering polymers and composites, aramid fibers, hair dyes, rubber chemicals, textile dyes and pigments. PPD is selected for the outstanding properties it imparts, including high temperature stability, high strength, and chemical and electrical resistance.

This aromatic amine compound is used in almost every hair dye on the market, regardless of brand. The darker the colour, usually, the higher the concentrations. Even the so-called "natural" and "herbal" hair colours, while ammonia-free, contain PPD.

Some particularly atrocious misinformation twists have led to PPD being described as a black mineral from the banks of the River Nile. This gives PPD an undeserved distinction as being both natural and exotic, which it is not. Being incredibly cheap, this product is added to real henna or used alone to create black tattoo-like body art. The people who use the product in this manner (completely at odds with the companies who see the product) either promote the product as a natural and traditional additive or warn only that it is mildly allergenic and that some people shouldn't use it. Those who market the so called "black henna" and PPD as completely safe warn consumers who get a reaction to never use the stuff again. That’s because sensitization is a life long issue. The application of PPD may lead to active sensitization to black clothing, printer's ink, Fax ink, hair dye, fur dye, leather dye, and photographic products among many other products.

One notable maker of this products states explicitly that it should not be used directly on the skin, however, they admit other companies that produce may or may not warn the same.

The CDC lists p-phenylenediamine as being known as one of many contact allergens. The NIOSH Pocket Guide to Chemical Hazards lists exposure routes as through inhalation, skin absorption, ingestion, skin and/or eye contact. It lists: throat irritation (pharynx and larynx), bronchial asthma, and sensitization dermatitis as symptoms of exposure.