THURSDAY, July 31 (HealthDay News) -- Black henna tattoos contain a chemical called para-phenylenediamine (PPD) that can cause serious skin reactions, the American Academy of Dermatology (AAD) warns.

The popular black henna tattoos are sold everywhere from summer carnivals and open-air malls to vacation spots and cruise ships.

PPD, commonly used for black hair dye, is added to natural henna in order to increase the intensity and longevity of the temporary tattoo. The U.S. Food and Drug Administration prohibits the direct application of PPD to the skin because of known health risks. However, lack of regulation of the tattoo industry means people are getting black henna tattoos and putting themselves at risk for serious skin problems.

"Perhaps the most alarming issue we are seeing with black henna tattoos is the increase in the number of children -- even children as young as four -- who are getting them and experiencing skin reactions," Dr. Sharon E. Jacob, assistant clinical professor of pediatrics and medicine (dermatology) at the University of California, San Diego, said in an AAD news release.

She was to present her findings at the AAD's summer meeting in Chicago this week.

"Kids make up a significant portion of the population that receives temporary tattoos, because parents mistakenly think they are safe, since they are not permanent and are available at so many popular venues catering to families. In fact, nothing could be further from the truth," Jacob said.

To date, there have been hundreds of reports of black tattoos causing allergic contact dermatitis, with reactions ranging from mild eczema to blistering and permanent scarring, she noted. Signs of an allergic reaction include redness and itching, bumps, swelling and blisters. Topical steroids can stop the allergic reaction.

Jacob added that some people can become sensitized to PPD from just one exposure and develop a lifelong sensitivity to PPD, along with an allergy that can cause a cross reaction to other compounds, including certain medications.

"Each exposure to PPD re-challenges the immune system, so each time you get a black henna tattoo or use a hair dye that contains PPD, there is an increased risk of having a reaction," Jacob said. "Many people are sensitized to PPD, but don't have a reaction to it. However, each time you are exposed to black henna, you increase your risk of developing a lifelong allergy to it."