Henna tattoos are widely available and usually harmless. But certain kinds can cause a powerful allergic reaction.
Henna is a vegetable dye that can be brown, red or green, and it wears off in a matter of days. But to produce a darker color, some tattoo artists add a chemical called para-phenylenediamine, or PPD. The Food and Drug Administration says the only legal use for PPD is as a hair dye.
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