Dermatitis Due to Sun Exposure

What dermatitis due to sun exposure means

Dermatitis means inflamed skin. When ultraviolet light is the trigger, doctors call it photodermatitis or a photosensitivity reaction. People often say "sun allergy", though only some types involve the immune system.

The key clue is the distribution: the rash appears on skin that saw the sun, such as the face, the V of the neck, the backs of the hands and forearms, and the lower legs. It typically spares areas covered by clothing, the skin under the chin, behind the ears and the upper eyelids. This sets it apart from an ordinary sunburn, which is simple UV damage that anyone can get with enough exposure.

Types of sun-related dermatitis

Polymorphous light eruption (PMLE)

This is by far the most common form, affecting up to 1 in 10 people in temperate countries. It is an immune reaction to UV light. It typically starts in spring or on the first days of a sunny vacation and improves as summer goes on, a process called "hardening". It is not dangerous and leaves no scars.

Phototoxic reactions

A chemical in the skin absorbs UV light and damages skin cells directly. No allergy is needed, so anyone exposed to enough of the chemical and sunlight can react. Common triggers:

• Medicines: doxycycline and other tetracyclines, fluoroquinolone antibiotics, thiazide diuretics, amiodarone, some NSAIDs such as naproxen, voriconazole and isotretinoin. See drug-induced photosensitivity for a fuller list.
• Plants (phytophotodermatitis): juice from limes, lemons, celery, parsnip, figs and giant hogweed. Classic examples are streaky blisters on the hands after making drinks with limes outdoors, which leave brown marks for months.
• Coal tar and some perfumes containing bergamot oil.

Photoallergic reactions

Here the immune system becomes sensitized to a chemical that changes when hit by UV light. It behaves like contact dermatitis, with itchy eczema that appears a day or more after exposure. Sunscreen filters (especially older chemical filters such as benzophenones), fragrances and topical NSAID gels such as ketoprofen are common culprits.

Sun sensitivity as a sign of another disease

• Lupus: a red rash on the cheeks and nose, or scaly round patches, after sun exposure can be a sign of lupus erythematosus. Joint pain, tiredness and mouth ulcers add to the suspicion.
• Porphyria: erythropoietic protoporphyria causes burning pain within minutes of sunlight, often from childhood. Porphyria cutanea tarda causes fragile, blistering skin on the backs of the hands.
• Pellagra: niacin deficiency causes a dark, scaly rash on sun-exposed skin, especially around the neck.
• Dermatomyositis and rosacea can also flare in sunlight.

Symptoms

• Itching, burning or stinging on exposed skin
• Small red bumps, larger raised plaques, blisters or hives depending on the type
• Swelling, especially of the face and hands
• Scaling and thickening in long-standing cases
• Brown or gray staining after phototoxic reactions

Fever, joint pain or feeling unwell with a sun rash is not typical of simple sun allergy and should be checked by a doctor.

How it is diagnosed

A dermatologist will ask when the rash appears in relation to sun exposure, how long it lasts, whether glass (car or window) protects you, what medicines and skin products you use, and whether anyone in your family has similar problems. Window glass blocks most UVB but lets UVA through, which helps identify the trigger.

Tests are used when the picture is unclear:

• Phototesting: small areas of skin are exposed to measured doses of UVA, UVB and visible light to reproduce the rash.
• Photopatch testing: suspected chemicals are applied to the back, then one set is exposed to UVA to confirm a photoallergy.
• Blood tests: antinuclear antibodies (ANA) and related tests for lupus, and porphyrin levels for porphyria.
• Skin biopsy in persistent or unusual cases.

Treatment

Treating a flare

• Get out of the sun and cover up until the rash settles.
• Cool compresses and fragrance-free moisturizer for burning and itching.
• Steroid creams: over-the-counter hydrocortisone for mild cases. A doctor may prescribe a stronger steroid for a few days for severe itching.
• Oral antihistamines help itch and are the main treatment for solar urticaria.
• Short courses of oral steroids for severe PMLE, prescribed by a doctor.
• Stop the trigger: if a medicine is the cause, ask your prescriber whether it can be changed. Never stop a needed medicine on your own.

Prevention for recurring sun allergy

• Use a broad-spectrum sunscreen labeled SPF 30 or higher with good UVA protection, applied generously and reapplied every 2 hours. Mineral sunscreens (zinc oxide, titanium dioxide) rarely cause photoallergy.
• Wear tightly woven or UPF-rated clothing, a wide-brimmed hat and sunglasses.
• Avoid the strongest sun, roughly 10 a.m. to 4 p.m.
• Build up exposure gradually in spring, which helps PMLE.
• For severe PMLE, dermatologists can offer desensitization phototherapy (controlled UV treatments) in early spring to harden the skin before summer.
• Remember that UVA passes through car windows. Window film can help people with UVA sensitivity.
• Wash hands after handling limes and other citrus outdoors.

Doctors sometimes prescribe hydroxychloroquine for severe PMLE or lupus-related sun sensitivity.

Outlook

Most sun-related dermatitis is a nuisance rather than a danger. PMLE tends to come back every year but often becomes milder over time. Phototoxic and photoallergic reactions stop once the trigger is removed, though dark marks can last months. Conditions such as lupus, porphyria and chronic actinic dermatitis need long-term specialist care. Strict sun protection also lowers the risk of skin cancer.

Frequently asked questions

What does a sun allergy rash look like?

It usually appears as itchy small red bumps, blisters or raised patches on skin exposed to the sun, such as the chest, arms and backs of the hands. It often shows up hours to 2 days after sun exposure and spares skin that was covered by clothing.

How long does dermatitis from the sun last?

Polymorphous light eruption usually clears within 1 to 2 weeks if you stay out of the sun. Phototoxic reactions heal like a sunburn in about a week but can leave dark marks for months. Photoallergic eczema can persist longer if the trigger is still being used.

Can you suddenly become allergic to the sun?

Yes. Polymorphous light eruption often starts in adulthood, especially in the teens to thirties. A new medicine, a new sunscreen or skin product, or a developing condition such as lupus can also cause sudden new sun sensitivity, so mention any recent changes to your doctor.

What is the difference between sunburn and sun allergy?

Sunburn is direct UV damage and depends on how much sun you got. It appears within hours as red, hot, sore skin. Sun allergy is an abnormal reaction to modest amounts of sun and is mainly itchy, with bumps, blisters or hives that may appear a day or two later.

Can sunscreen cause a sun rash?

Sometimes. Some chemical UV filters and fragrances in sunscreen can cause allergic or photoallergic dermatitis. If your rash is worse where sunscreen was applied, switch to a fragrance-free mineral sunscreen with zinc oxide or titanium dioxide and ask a dermatologist about photopatch testing.

Which medicines cause sun sensitivity?

Common ones include doxycycline and other tetracyclines, ciprofloxacin and related antibiotics, thiazide water pills, amiodarone, some anti-inflammatory painkillers, isotretinoin and St John's wort. If you take one, use strict sun protection and ask your pharmacist about the risk.