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Individuals with photosensitivity often develop a skin rash triggered by exposure to ultraviolet radiation (UVR). This may occur as a result of the following conditions:

• Immune system reaction. For unknown reasons, certain individuals’ immune systems recognize sun-altered skin as a threat to the body and respond by triggering a skin reaction. This is also called a sun allergy. Sun allergies include:
  
  
• • Polymorphous light eruption (PMLE). Multiple red papules or patches on sun-exposed skin (e.g., neck, chest, arms, lower legs) that appear within the first two hours of sun exposure. Other symptoms include itchiness (pruritus), chills, headache, nausea and malaise (general ill feeling). PMLE is more common in women than men.
    
    
  • Solar urticaria. Itchy hives that appear on skin within minutes of sun exposure. The hives may vary in size from as small as a pencil eraser to as large as a dinner plate and may overlap to create large areas of swelling. People with large affected areas often experience headaches, weakness and nausea. Though hives will often disappear without treatment, they usually return with further sun exposure. Solar urticaria most often affects young women.
    
    
• A defect in skin cell repair (rare). Exposure to UVR damages the DNA (the genetic material within a cell) in skin cells and disrupts normal cell functioning. Generally, in healthy people damaged DNA is repaired by the DNA repair system. Patients with rare genetic diseases including xeroderma pigmentosum (a rare degenerative disease involving sensitivity to light that develops in the first few years of life) and Cockayne syndrome (genetic condition characterized by short stature, premature aging, sensitivity to light and possible deafness and mental retardation) often experience severe, life-threatening photosensitivity due to a defect in their body’s ability to repair DNA damaged by UVR.

The use of certain medications and substances may increase the skin’s sensitivity to sunlight or other sources of UVR in some people. In such cases, hyperpigmentation (a condition in which patches of skin become darker than surrounding skin) may accompany the photosensitive rash. Medications that may cause photosensitivity include:

• Antibiotics
  
  
• Birth control pills
  
  
• Nonsteroidal anti-inflammatory drugs ([NSAIDs], which include ibuprofen and naproxen)
  
  
• Diuretics
  
  
• Coal tar (topical medication used to treat skin disorders)
  
  
• Antiarrhythmics (medications used to treat abnormal heart rhythms)
  
  
• Antidepressants
  
  
• Oral antidiabetics (medications used to treat diabetes)

Substances applied directly to the skin (e.g., lotions, creams, perfumes) that may induce photosensitivity include:

• Sunscreens that contain para-aminobenzoic acid (PABA), cinnamates, benzophenones or salicylates
  
  
• Fragrances such as musk ambrette and 6-methylcoumarin

Photosensitivity may also occur following exposure to certain plants. Plants that may trigger photosensitivity after being touched include:

• Celery
• Parsley
• Parsnips (leaves)
• Carrots (leaves)
• Figs
• Citrus fruit
• Hogweed

In addition, individuals with certain skin conditions may be prone to developing photosensitivity. Skin conditions that may be aggravated by UVR exposure include:

• Acne. An inflammatory skin disorder characterized by outbreaks of whiteheads, blackheads and pimples.
  
  
• Atopic dermatitis. An inflammation of the skin caused by allergies.
  
  
• Seborrheic dermatitis. An inflammation of the upper layers of the skin that causes a scaly, itchy rash with or without reddened skin.
  
  
• Rosacea. A condition that leads to redness, inflammation and acne-like lesions on the nose, forehead, cheeks, chin and sometimes the eyes.
  
  
• Psoriasis. A common disorder that can affect any part of the body. It is characterized by raised, inflamed lesions that join together to form plaques (patches) with distinct borders that are covered with flaky scales.
  
  
• Bullous pemphigoid. Chronic blistering of the skin that ranges from mildly itchy welts to severe blisters and infection.
  
  
• Lupus. An autoimmune disease that affects various organs in the body including the skin.
  
  
• Lichen planus. A skin condition characterized by angular, purple papules that are inflammatory and pruritic (itchy).
  
  
• Herpes. A sexually-transmitted disease characterized by the appearance of blisters or sores on the mouth, lips, gums and genitals.
  
  
• Porphyria. A group of diseases characterized by increased amounts of porphyrins (a naturally occurring chemical) in the skin.