If you find yourself heading to the county fair, be sure to doublecheck exactly what type of ink the vendor may be using on your child’s skin to draw that fine-looking temporary tattoo.

Many artists use the natural, reddish-brown henna, a dye made from a bushy flowering tree native to Asia and Africa. The traditionally formulated henna has always been weak, often fading in just a few days.

More recently, however, hawkers of the popular tattoos wanted to increase the longevity of the artwork and started adding a compound known as P-P-D, which dries the ink more quickly and makes the design bolder and more durable.

Now, the American Academy of Dermatology has issued a warning on black henna, noting that severe reactions have been caused by P-P-D, including itching, swelling and blistering. Although the reaction itself is not life-threatening, the scarring and darkening of the skin could be permanent.

The New England Journal of Medicine recently reported the case of a nineteen-year-old woman in Kuwait who adorned her skin in elaborate designs using the black henna treated with P-P-D. One week later the drawings turned to swollen red blisters.

While P-P-D is present in some health-care items, it’s generally not applied directly to the skin, and its concentration is ten times greater in henna than in other products.

For tattooing, it’s best to stay clear of P-P-D and go with natural henna.

Just because a tattoo is temporary doesn’t mean it can’t create some long-lasting problems.