Also known as: PPDA, para-phenylenediamine, p-phenylenediamine, phenylenediamine base, 4-phenylenediamine, 1,4-Phenylenediamine, 4-Benzenediamine, 1,4-Benzenediamine, para-Diaminobenzene, p-Diaminobenzene, para-Aminoaniline, p-Aminoaniline, Orsin, Rodol, Ursol

PPD is a dye which is derived, like many other dyes, from the coal-tar extraction process. It is a black dye, and is used in black rubber, inks, greases, gasoline, and most disturbingly, dark colored hair dyes and cosmetics. As a hair dye, it was popular due to its permanence and ability to resist discoloration after many washings. However, it is rapidly becoming less popular due to common allergic reactions - in the UK, it is banned outright for cosmetics for this reason. However, in the US, PPD is still used in hair dyes, some of which claim to be henna, a traditionally organic dye.

Reactions to the dye are common enough that DuPont, a leading manufacturer of the chemical, states:

"DuPont does not recomend and will not knowingly offer or sell p-phenylenediamine (PPD) for uses involving prolonged skin contact. Such uses may involve, but are not limited to, products formulated with henna for tattoo applications or other skin coloration effects. This use of PPD in prolonged skin contact application has the potential to induce allergic skin reactions in sensitive individuals. Persons proposing to use PPD in any formulation involving any more than incidental skin contact must rely on their own medical and legal judgment without any representation on our part. They must accept full responsibility for the safety and effectiveness of their formulations."

In 1979, the FDA required all hair dyes containing a similar coal-tar derivative, MMPD, to bear a warning label after it was found the chemical caused cancer in lab animals.

First-time dermal exposure to PPD results in allergic reactions in somewhere between 4% and 7.3% of the population. However, PPD is sensitizing, meaning that repeated exposure increases the risk of reaction; according to one study, a second exposure 8 days after the first doubles the chance of reaction. This may seem like a relatively low risk, until you compare with peanut allergy rates, which are around 1%.

The most common allergic reaction to the dye is contact dermatitis. High sensitivity or repeated exposure can result in contact urticaria (hives) or anaphylaxia (body-wide reactions, including shock or death).

The only way to know if you will react to a product containing PPD is with a patch test. However, due to the sensitizing nature of PPD, performing a patch test may actually increase your chances of having an allergic reaction.

As I write this, I'm suffering through the tail end of a three-day-long allergic reaction to PPD. From experience, a contact dermatitis on the scalp is NO FUN. It means oozing sores, the worst itching scalp you can think of, and puffing up of the face. Check your ingredients and avoid this stuff, it's not worth the risk.