People who get sun rash are photosensitive, which means they're sensitive to sunlight. Why certain people are more sensitive to sunlight is unknown, but what triggers the rash is ultraviolet light [source: Mayo Clinic].
The sun's rays are composed of three different types of ultraviolet, or UV, radiation: UVA rays, UVB rays and UVC rays. UVC rays aren't a factor in sun rash because they never reach the Earth. UVA rays most commonly cause PMLE; however, some people react to UVB rays or to a combination of UVA and UVB rays. People with PMLE can also have allergic reactions to tanning beds because they emit UV radiation.
If you have PMLE and live in a northern climate, your first exposure to sunlight in spring or early summer is more likely to cause sun rash because your skin hasn't seen the sun for months. Taking a sunny vacation during the winter can also trigger PMLE for the same reason. Fortunately, each time the skin is exposed to the sun, sensitivity to UV light usually decreases. By the end of the summer, people with PMLE often are able to enjoy the sun's rays without getting a rash.
Anyone can be affected by PMLE; however, some common risk factors include being female, being under 30, being fair-skinned, living in a northern climate and having a family history of PMLE [source: Mayo Clinic].
Now that you know what causes sun rash, read on to learn how to treat and prevent it.