I have actinic prurigo an form of hereditary polymorphous light eruption. I'm American Indian and we're more prone to it than other ethnic groups. My actinic prurigo started manifesting itself when I was about 12. I have a younger sister and other relatives who also have AP but to varying degrees.
Unlike like my sister, and most other people with AP, mine borders more on solar urticaria because it is pretty severe and isn't just my exposed skin that breaks out in red, itchy, burning hives. Every year I have to "harden off" my limbs by wearing different types of long-sleeved shirts and pants. I have to start off with dark, long-sleeved shirts and pants and then gradually move to lighter and thinner long-sleeved shirts and pants. Most years I'm lucky and can eventually wear t-shirts and shorts outside. If I don't do this ritual every year I end up with red bumps that make my skin hot to the touch and itch like crazy. Some years it is worse than others and there have been years when my skin wouldn't harden until August or September.
Here's some information PMLE, actinic prurigo and solar urticaria
Polymorphous light eruption (PMLE) PMLE, which usually appears as an itchy rash on sun-exposed skin, is the second most common sun-related skin problem seen by doctors, after common sunburn. It occurs in an estimated 10% to 15% of the U.S. population, affecting people of all races and ethnic backgrounds. Women are affected by PMLE more often than men, and symptoms typically begin during young adult life. In temperate climates, PMLE is usually rare in the winter, but common during the spring and summer months. In many cases, the PMLE rash returns every spring, immediately after the person begins spending more time outside. As spring turns into summer, repeated sun exposure may cause the person to become less sensitive to sunlight, and the PMLE rash either may disappear totally or gradually become less severe. Although the effects of this desensitization process, called "hardening," usually last through the end of the summer, the PMLE rash often returns at full intensity the following spring.
Actinic prurigo (hereditary PMLE) This inherited form of PMLE occurs in people of American Indian background, including the American Indian populations of North, South and Central America. Its symptoms are usually more intense than those of classic PMLE, and they often begin earlier, during childhood or adolescence. Several generations of the same family may have a history of the problem.
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Solar urticaria This form of sun allergy produces hives (large, itchy, red bumps) on sun-exposed skin. It is a rare condition that most often affects young women.
http://www.intelihealth.com/IH/ihtIH/WSIHW000/9339/10710.html And, fwiw, this article says it is the second most skin-related problem seen by doctors but I wasn't diagnosed until a few years ago. I did some research on the internet and then took it to my doctor. He read the materials and then he did some photo-sensitivity tests. As soon as he told me I was on the phone to my relatives to tell them what they had. We'd all been suffering for years and never knew why.