See this comprehensive link http://www.nlm.nih.gov/medlineplus/porphyria.html
"... Porphyrias are a group of genetic disorders caused by problems with how your body makes a substance called heme. Heme is found throughout the body, especially in your blood and bone marrow, where it carries oxygen.
Porphyrias affect the skin or the nervous system. People with the skin type develop blisters, itching, and swelling of their skin when it is exposed to sunlight. The nervous system type is called acute porphyria. Symptoms include pain in the chest or abdomen, vomiting, and diarrhea or constipation. During an attack, symptoms can include muscle numbness, tingling, paralysis, cramping, and personality or mental changes.
Certain triggers can cause an attack, including some medicines, smoking, drinking alcohol, infections, stress and sun exposure. Attacks develop over hours or days. They can last for days or weeks. "
Sun exposure may be a particular trigger for you and may be the reason you experience an outbreak towards the end of summer.
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