Summer is here, it’s time for a sunny break abroad and perhaps a temporary tattoo which is very fashionable. But beware, temporary Henna tattoos done on unsuspecting holiday makers in tourist resorts can have a disfiguring long-term and even dangerous result.  The usually harmless Henna-plant tattoo chemicals are often illegally darkened by unscrupulous vendors adding a concentrated hair dye chemical called PPD (para phenylene diamine) or “Black Henna”.  This dangerously concentrated hair dye chemical is highly sensitising when applied neat onto the skin and in 15% of people will lead to a delayed allergic reaction some days later (usually after returning home from their holiday).  Tell-tale allergy signs include irritation and reddening of the tattooed skin, later resulting in a blistering and scarring contact dermatitis which in some cases can last many weeks. The long-term consequences are that once sensitised to PPD, you will have a life-long risk of reacting to other less concentrated-PPD containing hair colourings, black rubber products, sulphonamide antibiotics, local anaesthetics and even sunscreens containing PABA (para-aminobenzoic acid). Therefore be extremely cautious about having any temporary Henna tattoos applied to your skin whilst on holiday.

Reference  http://en.wikipedia.org/wiki/Henna