Henna tattoos are a pretty harmless way for hippies to entertain and adorn themselves. A relatively benign dye, Lawsone, stains the skin (or hair) a ruddy brown. Like a semi-temporary tattoo. However, you're limited to pretty much just that color. Some manufacturers have added p-phenylenediamine to their stuff, to create so-called "black henna."
p-phenylenediamine is a contact sensitizer (i.e., you can develop an allergy). This wouldn't be such a big deal if it weren't so ubiquitous - hair dyes, inks, and the like all have it too. Black's a tough effect to achieve.
Chemical sensitivity (the real kind, not "MCS") can dramatically affect quality of life. Honestly, it's one of the things I'm most careful of in the lab. Carcinogens are well worth avoiding, of course - but it's a long-term thing. A few significant exposures to a typical carcinogen of some potency over a year or so (not the super-persistent stuff like asbestos) is exactly what your DNA repair mechanisms are built to handle.
Repeated exposures to a sensitizer, however (usually more than just a few, but you can get sensitized over a suprisingly short duration and number of exposures), induces an immune memory in your body that ensures that later exposures to the chemical can result in a more pronounced result every time.
Oddly, benzaldehyde (ubiquitous as bitter almond flavoring) is labeled a "sensitizer". I'm not sure how bad it actually is. However, I've always been terrified that I'd get sensitized to it and start tearing up every time I faced a piece of marzipan, so I've used an unusual amount of care for what's otherwise a relatively benign chemical).
So that's the danger of black henna - you can get sensitized to all kinds of black dyes.
Aromatic diamines are always bad in toxicology.
Acetone is also meant to be a skin sensitizer but does that doesn't seem to stop everyone squirting it everywhere. IMO Benzaldehyde is not that much of a baddie, at least you can smell it so you can wash it off.
I dyed my hair with henna for a while a few years ago. It looked pretty good after a few days--the dye oxidizes over time, which in my case changed my hair color from "my word, that man has exceptionally orange hair" to "who's that friendly-looking red-headed fellow over there?".
I got the henna from this site, which includes quite a bit of information about the dangers of PPD. For those who are looking for a natural black dye, they also carry indigo, which apparently works pretty well when used in combination with henna. You can also supposedly control the color somewhat by mixing the henna with other substances. I don't really have much experience in this regard, though.
Thanks so much for handling this in a clear manor! So many times warnings are against henna in general, as opposed to specifically henna adulterated by p-phenylenediamine. Articles like this are much appreciated by henna artists everywhere!
Here's a helpful tid-bit. You can tell if a henna artist is using safe, natural henna very simply. Ask what color the "tattoo" will be right when the paste is removed. P-phenylenediamine adulterated henna will be brown or black at first while natural henna will be orange and take a day or two mature into a deeper shade of brown. This is the same effect Adam had with his hair. Over time the oranges settle into brown due to oxidation.
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