Henna stain allergy is a growing concern due to the recent popularity of temporary tattoos. This form of body painting has a long tradition in India, Fiji and Morocco. It is known as mehndi, and traditionally has been used as a decorative painting on the hands and feet for social rituals.

In mehndi, tattoos are applied using coloring called henna, which is made from the leaves of an Asiatic and North African shrub called Lawsonia inermis (also known as the Egyptian privet or the Jamaica mignonette). The leaves are ground into powder and mixed with oil, lemon and other ingredients to form a paste.

Unlike permanent tattoos, ink is not injected under the skin in mehndi. Instead, the pattern is applied to the skin surface using a moistened wad of cotton, fine brushes or a plastic-bottle applicator. The paste is left on the skin for two to 12 hours while the pigment binds to the skin. The tattoo usually remains visible until the outer layer of skin exfoliates. Depending on the thickness of the skin, this takes anywhere from a week to six weeks or more.

Henna produces a brown, orange-brown or reddish-brown tint, but ingredients such as coffee and beet juice sometimes are added to henna to produce darker colors. Essential oils, such as eucalyptus and clove, may also be added to scent the henna. The additive p-phenylenediamine (PPD) is used to produce “blue henna” or “black henna.”

Some ingredients added to henna may trigger allergic reactions. This is particularly true of PPD. Most people with henna allergies develop contact dermatitis a few days after henna has been applied to their skin. There have also been reports of patients who immediately react to the henna paste after it is applied, displaying symptoms such as urticaria, sneezing and asthmatic reactions.

The U.S. Food and Drug Administration (FDA) has approved henna for use in hair dyes, but not for direct application to skin.