Arm sun allergy rash: recognition, relief and red flags

Polymorphic light eruption, sometimes called sun allergy, often causes itchy bumps, patches or small blisters after ultraviolet exposure. It commonly affects skin newly exposed after a less sunny period and may recur with a similar seasonal pattern.

A light-related eruption tends to appear on exposed outer arms while skin covered by sleeves is spared. The face and hands may react less in people whose skin there receives regular daylight. Timing can vary, so a rash may emerge after leaving the sun rather than during exposure.

Similar-looking arm rashes can come from sunscreen or fragrance reactions, heat, eczema, medicines that increase light sensitivity, infection or autoimmune disease. A clinician uses the distribution, timing, medicine history and examination to distinguish them. Sun allergy describes an abnormal skin response to light; it does not mean sunlight contains an allergen.

Move out of direct sun, cool the skin gently and avoid scratching. Use a plain fragrance-free moisturiser. A pharmacist or doctor can advise whether an antihistamine for itch or a topical corticosteroid is appropriate, especially when other health conditions, pregnancy or medicines affect the choice.

For prevention, combine shade, tightly woven long sleeves and broad-spectrum sunscreen on exposed skin. Reapply sunscreen after sweating or washing and check whether any medicine carries a photosensitivity warning. People with troublesome recurring eruptions may be offered medically supervised light treatment before a sunny season; this is different from commercial tanning.

Arrange a clinical review if this is the first severe episode, the rash repeatedly returns, it involves covered skin, or it persists despite avoiding light. Assessment is also appropriate when there is joint pain, marked fatigue, mouth sores, facial rash, a newly started medicine or signs of infection such as warmth, pus and fever.

Seek emergency help for breathing difficulty, faintness, swelling of the lips or tongue, widespread blistering, skin peeling, severe eye involvement or a rapidly worsening reaction. These features are not typical of a simple sun rash and should not be managed with home care alone.

Photograph the rash in good light and note which areas were exposed, when it began, products applied and all medicines or supplements taken. Ask whether the distribution fits polymorphic light eruption, whether tests are needed to exclude another photosensitive disorder, and exactly how to use any prescribed cream.