Photoallergic Dermatitis

How photoallergic dermatitis happens

Two things are needed: a photosensitizer (a chemical on or in the skin) and UV light, usually UVA, which passes through clouds and window glass. UV energy alters the chemical so it binds to skin proteins. The immune system then recognizes this new combination as foreign and mounts a delayed (type IV) allergic reaction, the same type behind poison ivy rashes.

As with other allergies, you must first become sensitized. The first exposure may cause nothing. Later exposures to the same chemical plus sunlight trigger the rash, even with small amounts of either.

Photoallergic vs phototoxic reactions

Most light-related chemical reactions are phototoxic, not allergic. The difference matters because it changes what you avoid.

Medicines taken by mouth more often cause phototoxic reactions. These are covered in drug-induced photosensitivity. Other sun-triggered rashes are covered under photodermatitis.

Common triggers

Applied to the skin (most cases)

• Sunscreen UV filters: octocrylene, benzophenones (including oxybenzone and benzophenone-3), avobenzone (butyl methoxydibenzoylmethane) and older PABA derivatives. Sunscreen chemicals are now the leading cause in many countries.
• Topical NSAIDs: ketoprofen gel is a frequent and strong photosensitizer and can cross-react with octocrylene and some fragrances. Reactions can recur for months if the gel residue is on clothing.
• Fragrances: musk ambrette (now restricted), 6-methylcoumarin and some essential oils.
• Antimicrobials: older antibacterial agents in soaps (such as halogenated salicylanilides and fenticlor), chlorhexidine rarely.
• Plant extracts: in cosmetics, including lichens and some compositae (daisy family) plants.
• Others: promethazine cream, some hair dyes, and certain sunscreens in cosmetics and lip balms.

Taken by mouth (less common)

Some medicines cause true photoallergic reactions, including hydrochlorothiazide, quinine and quinidine, griseofulvin, sulfonamides and some NSAIDs. See the drug-induced page for a fuller list.

Symptoms

• intense itching
• red, scaly, eczema-like patches; small blisters in acute cases
• mostly on the face, the V of the neck, back of the hands and forearms
• often sparing shaded areas: under the chin, behind the ears, upper eyelids, skin folds
• may spread to covered skin in more severe reactions
• with chronic exposure, thickened, leathery skin

Diagnosis

Your dermatologist will look at the rash pattern and ask about sunscreens, cosmetics, gels, medicines and hobbies. The key test is photopatch testing:

1. Two identical sets of possible allergens are taped to your back.
2. After 1 to 2 days, one set is exposed to a measured dose of UVA light; the other stays covered.
3. The skin is read again over the next few days.

A reaction only on the irradiated side means photoallergy. A reaction on both sides means ordinary allergic contact dermatitis. Blood tests for lupus and porphyria may be done if the pattern suggests them.

Treatment

• Stop the trigger: check all products, including partner's or family's creams, and wash clothes that may hold gel residue.
• Topical corticosteroids: a medium-strength steroid cream or ointment on the body and a milder one on the face, used for 1 to 2 weeks.
• Oral steroids: a short course for severe or widespread reactions, prescribed by a doctor.
• Cool compresses and bland moisturizers for itching and dryness.
• Oral antihistamines for itch at night (follow the label dose); they do not treat the rash itself.
• Sun protection while healing: shade, clothing and wide-brimmed hats.

Prevention

• Once you know your trigger, read ingredient labels. The chemical may appear in sunscreens, moisturizers with SPF, makeup and lip balms.
• Mineral sunscreens with zinc oxide or titanium dioxide rarely cause photoallergy and are a good choice for people sensitive to chemical filters.
• Avoid ketoprofen gel if you react to octocrylene, and the reverse.
• Use protective clothing and avoid peak UV hours (10 a.m. to 4 p.m.).
• Remember UVA passes through window glass in cars and homes.

Outlook

Most photoallergic reactions clear within 1 to 2 weeks after the trigger is removed. Sensitization is usually lifelong, so the reaction will recur on re-exposure. A small number of people develop persistent light reactivity, in which the skin keeps reacting to sunlight even after the chemical is avoided. This needs specialist care.

Frequently asked questions

What is photoallergic dermatitis?

It is an allergic, eczema-like skin rash that develops only when a sensitizing chemical on the skin, such as a sunscreen filter or ketoprofen gel, is exposed to UV light. It usually appears 1 to 3 days after sun exposure and is very itchy.

Can you be allergic to sunscreen?

Yes. Some people develop allergic contact dermatitis or photoallergic dermatitis to chemical sunscreen filters, especially octocrylene and benzophenones such as oxybenzone. Fragrances and preservatives in sunscreens can also cause reactions. Mineral sunscreens containing zinc oxide or titanium dioxide are much less likely to cause allergy.

What is the difference between phototoxic and photoallergic reactions?

A phototoxic reaction can happen to anyone with enough chemical and light. It looks like a bad sunburn within hours. A photoallergic reaction affects only sensitized people, is driven by the immune system, appears 1 to 3 days later as an itchy eczema-like rash, and can spread beyond exposed skin.

How long does a photoallergic reaction last?

With the trigger removed and steroid cream used, most rashes improve within days and clear within 1 to 2 weeks. If the trigger is still present, for example ketoprofen residue on clothes, the rash keeps returning. Rarely, persistent light sensitivity lasts months or longer.

How is a sun allergy rash diagnosed?

A dermatologist examines the pattern of the rash and your product and medicine history. Photopatch testing confirms photoallergy and identifies the chemical. Other sun-related rashes, such as polymorphous light eruption, solar urticaria and lupus, are considered and sometimes tested for.