Sunburn is caused by overexposure to UVR, mostly UVB. In fact, sunburn is almost exclusively a UVB phenomenon.This is important because it means that the SPF system measures UVB protection and not UVA (see SPF below). During a sunburn the skin turns red, swells and, in severe cases, blisters.
On a cellular level there is an increase in inflammatory cells as well as many chemicals associated with inflammation. A sunburn continues to develop for 12 to 24 hours after the exposure. One blistering sunburn will double a person’s chance of developing skin cancer. Remember however, that avoiding sunburn is not the same as avoiding sun damage. Sunburn is just the immediate sign of UVR overexposure. People who have never been sunburned can still develop the delayed problems such as skin cancer and the sun related skin changes such as deep lines, sagging skin . . .