Photosensitivity (Sun Sensitivity of the Skin)

What photosensitivity is

Sunlight contains UVB, which causes ordinary sunburn, UVA, which passes through window glass and penetrates deeper, and visible light. In photosensitivity, something makes the skin react to these wavelengths at doses that would not harm most people. The trigger may be a chemical in the skin that absorbs light, an immune reaction, or a disease process.

Photosensitivity of the skin is different from light sensitivity of the eyes (photophobia), which is discomfort in bright light caused by eye or brain conditions such as migraine.

Types and causes

Phototoxic reactions (most common)

A chemical in the skin absorbs UV light and directly damages skin cells. It looks like an exaggerated sunburn: redness, burning, swelling and sometimes blisters, limited to sun-exposed skin, within minutes to hours of exposure. It can leave dark pigmentation. Anyone taking enough of the drug can be affected.

Common triggers:

• antibiotics: doxycycline and other tetracyclines, fluoroquinolones, sulfonamides
• diuretics: hydrochlorothiazide, furosemide
• NSAIDs such as naproxen and piroxicam
• heart medicines: amiodarone
• acne and skin treatments: oral and topical retinoids, isotretinoin
• voriconazole, methotrexate, some chemotherapy
• St John's wort
• coal tar and psoralens

Phytophotodermatitis is a phototoxic reaction from plant chemicals called furocoumarins. Lime juice, celery, parsnips, figs and giant hogweed cause streaky burns and blisters where juice touched the skin, followed by brown marks lasting weeks to months.

See drug-induced photosensitivity for a fuller list.

Photoallergic reactions

The immune system reacts to a substance only after light has changed it. The result is an itchy, eczema-like rash that appears 1 to 3 days after exposure and can spread to covered skin. Only a small number of exposed people react. Triggers include sunscreen chemicals (such as oxybenzone and octocrylene), fragrances and topical NSAIDs such as ketoprofen. See photoallergic dermatitis.

Sun allergy rashes (idiopathic photodermatoses)

• Polymorphous light eruption (PMLE): the most common sun rash. Itchy red bumps or blisters appear hours to days after the first strong sun of spring, then fade over a week. It often improves as summer goes on ("hardening").
• Solar urticaria: hives within minutes of sun exposure that clear within hours of shade.
• Actinic prurigo: itchy bumps and crusts, often starting in childhood, more common in people of Indigenous American descent.
• Chronic actinic dermatitis: persistent eczema on sun-exposed skin, mainly in older men.

Diseases made worse by sunlight

• Lupus: a butterfly rash across the cheeks or scaly rings on sun-exposed skin. Sun can also trigger flares of systemic symptoms.
• Dermatomyositis: a purple rash on the eyelids, knuckles and chest, with muscle weakness.
• Rosacea, cold sores and melasma can all be triggered or worsened by sun.

Inherited and metabolic causes

• Porphyrias: erythropoietic protoporphyria causes burning pain within minutes of sun, often starting in early childhood. Other types cause fragile skin and blisters on the backs of the hands.
• Xeroderma pigmentosum: a rare genetic inability to repair UV damage, causing severe burns and skin cancers from early childhood.
• Pellagra: niacin deficiency causing a sunburn-like rash on exposed skin.

Diagnosis

A dermatologist will ask what medicines, supplements, sunscreens and cosmetics you use, when the rash appears after sun exposure, how long it lasts and whether it occurs through window glass. The pattern often gives the answer: affected face, neck, V of the chest and backs of hands, with sparing under the chin, behind the ears and under a watch strap.

Tests may include:

• Phototesting: small areas of skin are exposed to measured doses of UVA, UVB and visible light to see which wavelength causes a reaction.
• Photopatch testing: possible allergens are applied in duplicate; one set is exposed to UVA to detect photoallergy.
• Blood tests: antinuclear antibodies (ANA) and other lupus antibodies; porphyrin levels in blood, urine and stool.
• Skin biopsy in unclear cases.

Treatment

Remove the trigger

Stop or switch the responsible medicine if your doctor agrees. Do not stop a prescribed medicine on your own. Avoid plant juices and fragrances that caused a reaction.

Sun protection

• Use a broad-spectrum sunscreen of SPF 30 or higher with good UVA protection. Mineral sunscreens with zinc oxide or titanium dioxide are less likely to cause photoallergy. Tinted sunscreens with iron oxides also block visible light.
• Reapply every 2 hours and after swimming or sweating.
• Wear tightly woven clothing, UPF-rated garments, wide-brimmed hats and sunglasses.
• Seek shade between 10 am and 4 pm.
• Use UV-blocking film on car and home windows if UVA through glass triggers symptoms.

Treating reactions

• Cool compresses and moisturizers for phototoxic burns.
• Topical corticosteroids for itchy, inflamed rashes.
• Antihistamines for solar urticaria.
• Pain relief with acetaminophen (paracetamol) at the label dose. Avoid NSAIDs if they were the trigger.

Specific treatments

• Phototherapy "hardening": controlled low doses of light in early spring to build tolerance in PMLE and solar urticaria.
• Hydroxychloroquine for lupus and some sun allergies.
• Afamelanotide, an implant that increases skin pigment, for erythropoietic protoporphyria.
• Immune-suppressing medicines for severe chronic actinic dermatitis.

Living with photosensitivity

Most drug-induced reactions fade within days to weeks after stopping the medicine, though pigmentation can last longer. Sun allergy rashes tend to recur each year but can be controlled with protection and hardening. People with severe photosensitivity may need vitamin D supplements, because strict sun avoidance lowers vitamin D levels.

Frequently asked questions

What causes sudden photosensitivity?

The most common cause of new sun sensitivity is a medicine started in the past weeks, such as doxycycline, a thiazide diuretic, an NSAID or a retinoid. Lime juice or plant contact, a new sunscreen or fragrance, and the first strong sun of spring (polymorphous light eruption) are other frequent causes. New sun sensitivity with joint pain or fatigue can suggest lupus.

What does a photosensitivity rash look like?

Phototoxic reactions look like a bad sunburn, with redness, swelling and sometimes blisters, limited to exposed skin. Photoallergic and sun allergy rashes look like itchy red bumps, blisters or eczema. Typically, the face, neck, V of the chest and backs of the hands are affected, with covered skin spared.

How long does photosensitivity last?

Drug-related photosensitivity usually lasts as long as you take the medicine and for days to weeks after stopping, depending on how long it stays in the body. Brown marks can persist for months. Sun allergy conditions such as polymorphous light eruption tend to recur each spring and summer.

Is photosensitivity the same as sun allergy?

Not exactly. "Sun allergy" usually refers to immune-mediated sun rashes such as polymorphous light eruption and solar urticaria. Photosensitivity is broader and includes phototoxic drug reactions, light-worsened diseases such as lupus, and inherited conditions such as porphyria.

Can photosensitivity be cured?

It depends on the cause. Reactions to medicines or plants resolve once the trigger is removed. Sun allergy rashes often improve over years and can be controlled with hardening and protection. Photosensitivity from lupus or porphyria is long term but manageable with treatment and strict sun protection.