1) Para-phenylenediamine is a strong sensitizer.

A sensitizer is something that your body is naturally allergic to, or may become allergic to.  Bee venom is a strong sensitizer. Poison Ivy is a strong sensitizer.  You may react to Poison Ivy or Bee Stings with intense swelling, pain, itching, or a full blown reaction the first time you come in contact with them.  The more often you touch Poison Ivy or get stung by bees, the more likely you are to become sensitized and have a serious reaction.  Extreme sensitivity is life threatening. Some people are born sensitive to PPD.  Others become sensitive to PPD.  Hairdressers, who handle PPD hair dyes every day, frequently become so sensitive to PPD that they must wear gloves when handling hair dye or may have to quit their occupation to save their health.

See http://www.hennapage.com/henna/ppd/ppdburngallery.html  to see what these PPD "Black Henna" sensitivity reactions look like.

2) If you become sensitized to PPD, that allergy will never go away and you may become cross-sensitized to many related things. 

That means other similar things may set off an allergic reaction and make you sick. Hair dye, sun block, cosmetics, even black clothing, may cause an outbreak of itching and hives.  See: http://www.truetest.com/templates/20.html  for more information on PPD sensitization
 

3) "PPD, para-phenylendiamine, is not just a sensitizer, it has also been shown to be carcinogenic and can harm your health in other ways.

Para-phenylenediamine is a strong sensitizer, carcinogen, and  transdermal toxin (it penetrates your skin and then travels through your blood stream and causes damage). It is associated with bladder cancer, asthma, and many other heath problems. See http://www.hennapage.com/henna/ppd/ppdmed.html  for a list of medical journal articles on just how dangerous PPD can be!

4)  Para-Phenylenediamine causes a "delayed hypersensitivity reaction".

Poison Ivy causes a delayed hypersensitivity reaction.  When you touch poison ivy, nothing happens for the first few days.  Then the itching and blistering starts.  Bee stings cause an immediate reaction, with itching and swelling within minutes. A para-phenylenediamine reaction starts 3 to 10 days after the PPD "Black Henna" or black hair dye is applied. 

Because of this delay, many PPD "Black Henna" artists in vacation areas and beaches don't know they have hurt people, becuase the people have gone home when the itching and blistering start.  Few people go back to confront the artist.  Therefore, the artist can go on injuring people, ignorant of the suffering they're causing.  If the reaction was immediate, like a bee sting, no one would use para-phenylenediamine black dye to make body art.