Para-Phenylenediamine (PPD), also known as paraphenylenediamine, p-phenylenediamine or 1, 4 diaminobenzene, is an aromatic amine with many industrial and cosmetic applications. The chemical is a common ingredient in permanent hair dye products, as well as dyes for fabrics, fur and dark makeup. PPD is also used in printing and photocopying inks, photo and lithograph developing chemicals, rubber products, Kevlar and oil, gasoline and grease products.

PPD is a preferred chemical due to its low toxicity levels and its ability to withstand high temperatures and retain its stability. It is good for hair dyes because it produces a natural color which doesn’t fade as readily with washing and drying. PPD itself is colorless — it gains its color once it’s exposed to oxygen.

The Centers for Disease Control (CDC) cites PPD as a contact allergen, and should not be directly applied to the skin. When used in hair dyes, it may cause mild contact dermatitis on the forehead, eyelids or ears — wherever it comes in contact with the skin. Reactions usually only occur while the dye is oxidizing, versus once it has already oxidized, as is the case with dyed fabrics and furs. People who regularly work with PPD can develop allergies to it, and must take certain precautions to avoid coming in contact with it. Although the most common absorption of PPD into the body is through the skin, it can cause allergic reactions when it is inhaled, or absorbed by the eyes or ingested.

One of the most dangerous applications of PPD is when it is added to henna, a natural dye. When used for temporary tattoos, henna laced with PPD is known as “Black Henna.” Although this is not an approved use for PPD in the United States, some tattoo artists will illegally add the chemical to henna for darker temporary tattoos that dry faster than pure henna tattoos. Because the dye is applied while the PPD is in its oxidation process, its potential as an allergen is increased. Black Henna tattoos often result in a skin reaction similar to a chemical burn, which in turn results in a scar where the skin was tattooed.

When added to henna, the concentration of PPD is often much higher than what is approved for use in hair dyes, resulting in a minor to major allergic reaction. Once a person who has been exposed to PPD has an allergic reaction, they may suffer a lifelong sensitivity to the chemical.