Dermatologists are warning that harmless-looking henna tattoos could contain a harmful chemical known as paraphenylenediamine, or PPD, used to create longer-lasting black henna tattoos. PPD has been associated with a spate of major skin problems, according to the American Academy of Dermatology.

Natural henna used for temporary tattoos is made from leaves of the lawsonia inermis plant, which provides a vegetable coloring that comes in shades of brown, green, or red. Dyeing the skin with natural henna is considered harmless and only lasts for a few days. To increase the intensity of the tattoo and make it last longer, some henna tattoo artists are adding PPD into the henna mix, turning the tattoo black.

To date, there have been hundreds of reports of allergic contact dermatitis from black henna tattoos, with reactions ranging from mild eczema to blistering and even permanent scarring. The first sign of a reaction is typically redness and itching, followed by bumps, swelling and then blisters. Topical steroids can be used to stop the reaction, but scarring may occur. In addition, some people may become sensitized to PPD from just one exposure — meaning people can develop a lifelong sensitivity to PPD and an allergy can cause a cross reaction to other compounds, including certain medications.