Henna is a dye used on the skin or hair to give a temporary change. It is made from leaves from the henna tree which is a plant in the Lythraceae family of plants. This family include lots of flowering plants as well as pomegranate and water caltrop (also sometimes called water chestnut).

The dried leaves of the plants are ground into a powder and this can be made into a paste to use as a dye. Essential oils are sometimes added to paste to improve the longevity of the stain. Some of these essential oils include tea tree and lavender, they contain monoterpenes, which can act as an irritant on the skin.

Henna is always orange when first put on the skin and then dries to a dark brown. Senna, another natural dye made from a legume plant can be mixed into the paste to make even darker shades. Senna when used alone makes red or blonder shades.

Henna on it's own is not usually associated with allergy, but black henna is henna mixed with paraphenylenediamine (PPD) and this component is the most common cause of contact allergic reaction with this type of dye.

Allergic reactions to henna on it's own has been noted to be more frequent in people who have a G6PD (Glucose-6-phosphate dehydrogenase) enzyme deficiency. This mutation is on the X gene, so is more commonly linked to men than women (as they only have one X gene compared to the two women have). This will cause more severe anaemia symptoms and contact allergic skin reactions.

Henna is often associated with allergic contact dermatitis, this is usually from the essential oils added to the henna paste mix or in the case of black henna, from the PPD.

When the henna paste is made with lemon juice rather than water, this can cause phytophotodermatitis. This is a skin condition that occurs when a person gets the juice from the fruit on the skin and doesn't wash it off. The furocoumarins in the juice of the fruit are activated by the sun and cause an angry red rash on the skin.

Other things which contain paraphenylenediamine include some hair dyes as well as eye brow and eye lash tints. If you have had an allergic reaction to black henna you will need to check ingredients on these type of products to avoid them in the future.

You can find the most up to date information on the Cross Reactivity Tool.

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