Sunburns are caused by exposure to too much ultraviolet (UV) light. UV radiation is a wavelength of sunlight in a range too short for the human eye to see. UV light is divided into three wavelength bands â€” ultraviolet A (UVA), ultraviolet B (UVB) and ultraviolet C (UVC). Only UVA and UVB rays reach the earth. Commercial tanning lamps and tanning beds also produce UV light and can cause sunburn.
When you're exposed to UV light, your skin accelerates its production of melanin. Melanin is the dark pigment in the epidermis that gives your skin its normal color. The extra melanin â€” produced to protect the skin's deeper layers â€” creates the darker color of a "tan." A suntan is actually your body's way of blocking the UV rays to prevent sunburn and other skin damage. But the protection only goes so far. The amount of melanin a person produces is determined genetically, and many people simply can't produce enough melanin to protect the skin well. Eventually, UV light causes the skin to burn, bringing pain, redness and swelling.
You can get sunburn on hazy or cloudy days. As much as 90 percent of UV rays pass through clouds. UV rays can also reflect off snow, ice, sand, water and other reflective surfaces, burning your skin as severely as direct sunlight.