P-Phenylenediamine

p-Phenylenediamine (PPD), also called 1,4-diaminobenzene or 1,4-phenylenediamine is an aromatic amine used as a component of engineering polymers and composites, aramid fibers, hair dyes, rubber chemicals, textile dyes and pigments. PPD is selected because of its low toxicity, high temperature stability, high strength, and chemical and electrical resistance.

Uses
This product is added to real henna to create so called "Black Henna" which in many cases causes allergic reaction, and in a precentage of people leaves a scar. PPD should never be applied directly to the skin in its pure form or mixed with anything else. 

This 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 products sold as henna have PPD added, particularly "black henna". Using body art quality (BAQ) pure henna, or indigo is the only way to avoid PPD in hair dye.

Contact allergen
The CDC lists p-phenylenediamine as being a contact allergen. The NIOSH Pocket Guide to Chemical Hazards lists exposure routes as being through inhalation, skin absorption, ingestion, and skin and/or eye contact; symptoms of exposure include throat irritation (pharynx and larynx), bronchial asthma, and sensitization dermatitis. Sensitization is a lifelong issue, which may lead to active sensitization to products including, but not limited to:
 * black clothing
 * printer ink
 * facsimile ink
 * hair dye
 * fur dye
 * leather dye
 * photographic products

One maker of this product states explicitly that it should not be used directly on the skin, however, they admit that other manufacturers of PPD may or may not warn the same.

Misconception
Some 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.