Quick Answer
Sun allergy (photosensitivity) is an immune-mediated skin reaction to ultraviolet radiation, occurring in approximately 10–15% of the population. The most common form is polymorphous light eruption (PMLE) — an itchy, bumpy rash on sun-exposed skin appearing within hours of sun exposure. Solar urticaria causes hives within minutes of UV exposure. Both are manageable with sun protection and medical treatment.
Polymorphous Light Eruption (PMLE): The Most Common Sun Allergy
PMLE is an immune-mediated photosensitivity reaction affecting approximately 10–15% of the population, more commonly women and individuals with fair skin. The rash appears on sun-exposed areas (chest, neck, arms, legs) within 30 minutes to 24 hours of UV exposure — typically UVA radiation. It presents as itchy papules, vesicles, or plaques and resolves over 7–10 days without further sun exposure.
PMLE is most common in spring and early summer, when the skin is not yet adapted to UV after winter. Many patients develop increasing tolerance with repeated UV exposure throughout summer — a phenomenon called 'hardening.' PMLE tends to be a recurrent but manageable condition. Broad-spectrum sunscreen and sun-protective clothing are the foundations of PMLE prevention.
Solar Urticaria: Hives from Sun
Solar urticaria is a rare but well-defined condition in which hives (urticaria) develop within minutes of skin exposure to sunlight — including visible light in some patients, not just UV. Individual wheals appear at sun-exposed sites, cause intense itching, and typically resolve within 1–2 hours of shade. Extensive solar urticaria can cause systemic symptoms including headache, nausea, and rarely anaphylaxis.
Solar urticaria involves IgE-like mechanisms with mast cell activation triggered by a photoallergen produced when UV irradiates the skin. Antihistamines provide partial relief; omalizumab has shown efficacy in refractory cases. Phototolerance induction with controlled UV exposure (phototherapy) and strict photoprotection are the mainstays of management.
Photoallergic and Phototoxic Contact Dermatitis
Photoallergic contact dermatitis is a Type IV delayed hypersensitivity to a chemical that becomes an allergen only after UV activation. Common photoallergens include sunscreen ingredients (oxybenzone, PABA), topical NSAIDs, fragrances (musk ambrette), and some topical antimicrobials. The rash develops 24–72 hours after sun exposure and is confined to UV-exposed areas that contacted the chemical.
Phototoxic reactions are non-immune, direct UV-mediated chemical reactions that can affect anyone — no prior sensitization is required. Psoralens from plant juices (citrus, parsley, celery, hogweed) absorb UVA and release reactive oxygen species that cause burns and hyperpigmentation confined to areas of plant contact. Some antibiotics (tetracyclines, quinolones), diuretics (furosemide), and St. John's Wort sensitize skin to UV phototoxic reactions.
Key Takeaways
- PMLE affects 10–15% of the population — itchy papular rash on sun-exposed skin, most common in spring.
- Solar urticaria: hives within minutes of sun exposure — can cause systemic symptoms in severe cases.
- Photoallergic dermatitis requires UV activation of a chemical to become an allergen — oxybenzone is a common photoallergen.
- Phototoxic reactions (from citrus/plant psoralen + UVA) are not immune-mediated and can affect anyone.
- Broad-spectrum SPF 50 sunscreen and UV-protective clothing are essential for all sun allergy types.
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