The henna itself does not cause contact allergies. The powder is sourced from the leaves of the henna tree and is mixed with water to form a paste, which is used to dye the skin an orange-brown colour. In India, Morocco and Egypt, brides traditionally have their hands and feet decorated with filigree symbols using henna. But the paste has to set for several hours in order for it to remain visible for several days.

Most holiday tourists do not want to wait for hours to obtain a good colour result. So street artists often use henna powder containing the additive para-phenylenediamine (PPD), which shortens the setting time. PPD creates a black dye that sets quickly. The disadvantage is that it has a strong sensitising effect. This means that if the immune system was previously exposed to PPD and considered it a harmful substance (although there was no initial reaction), a visible allergic reaction occurs upon re-exposure (the tattoo). After a delay of 24 to 72 hours at the earliest after the dye is applied, the skin swells and becomes red and itchy. Blisters often appear, which burst and weep. Scars may remain after they have healed.

Even if there is no skin reaction, the tattoo can trigger a PPD allergy, which can have serious consequences. Once an individual has been sensitised to PPD, he or she must avoid products containing this substance in the future. This can be difficult because PPD is widely used. It is found in dark leather and textile dyes and also in printing dyes and hair dyes. Furthermore, allergies to related substances in black rubber (tyres, bicycle and tool handles) may occur, as well as cross reactions to other (azo) dyes or medications (sulphonamide, benzocaine).

It is best to steer clear of henna tattoos in order to avoid this health risk. Although the use of PPD in cosmetic skin dyes has been banned in the European Union, it is difficult to know whether regulations are properly observed in all cases. And the ban does not apply to countries outside of Europe.