After reading some articles on the subject I understand that 10-15% of the population have sun allergy.
This is more common in the northern parts of the earth.
A few of the most common types of sun allergy are: 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.
About 10 to 15 per cent of people in Britain have this condition, which is not so much caused by warmth but by ultraviolet radiation. Like you, more women than men are affected often from adolescence or early adulthood although sometimes also later in life too.
http://www.intelihealth.com/IH/ihtIH/WSIHW000/9339/25934.html
http://dermnetnz.org/reactions/photosensitivity.html
http://www.ecureme.com/emyhealth/data/Sun_Allergy.asp
http://www.netdoctor.co.uk/ate/asthmaandallergy/203474.html