A toxic or allergic reaction may occur when certain chemicals are applied to the skin and subsequently exposed to the sun. This is called photocontact dermatitis.
What causes photocontact dermatitis?
Photocontact dermatitis most often arises from interaction between UV radiation and one or more of the products listed below:
- Some sunscreens, e.g. oxybenzone or cinnamates and others
- Coal tar products
- Fragrances, e.g. musk
- Insecticides and disinfectants
These products contain drugs or chemicals that are photosensitising agents (see drug-induced photosensitivity).
The reaction can be phototoxic and/or photoallergic.
- Phototoxic reactions result from direct damage to tissue caused by light activation of the photosensitising agent
- Photoallergic reactions are a cell mediated immune response in which the antigen is the light-activated photosensitising agent.
Another cause of photocontact dermatitis is from the interaction of UV radiation and photosensitising compounds found in various plants. This type of dermatitis is called phytophotodermatitis. The most common plant family to cause phytophotodermatitis is the Umbellliferae family. Other plant families that cause phytophotodermatitis are Rutaceae, Moraceae and Leguninosa. The main photosensitising substances found in these plants are called furocoumarins and consist of psoralens and 5-methoxypsoralens, 8-methoxypsoralens, angelicin, bergaptol and xanthotal.
|Plant family||Plant/vegetable/fruit with furocoumarins|
What are the clinical features of photocontact dermatitis?
The clinical features of photocontact dermatitis vary according to the photosensitising agent involved and the type of reaction it causes in the skin.
- A phototoxic reaction generally appears similar to a bad sunburn.
- A photoallergic reaction generally presents as an eczema (also called dermatitis) confined to the areas in contact with the responsible chemical and exposed to the sun.
Phytophotodermatitis is a result of a phototoxic reaction and has characteristic clinical features. These include:
- Blisters and brown streaks occurring from touching certain plants followed by sun exposure. These patterns result from brushing against a plant's stems or leaves when outdoors or from lime juice squeezed over the hand or down the forearm.
- Blisters or swellings may occur minutes to hours after exposure to the plant and light, but more usually erupt about 24 hours after exposure, peaking at 48-72 hours.
- Skin lesions may leave behind dark marking on the skin (postinflammatory hyperpigmentation).
What is the treatment for photocontact dermatitis?
The main goal of treatment is to identify the photosensitising agent and if possible to avoid touching it. Photodermatitis is a self-limited problem that resolves spontaneously once the offending agent is removed or avoided.
If allergic to a sunscreen agent, choose one without the responsible chemical or select a low irritant formula that relies on metal oxides such as zinc oxide or titanium dioxide.