Sun blisters are small fluid-filled bumps that appear on seriously burned skin. They are caused by a sunburn, which is a type of radiation burn due to overexposure to ultraviolet (UV) radiation from the sun.
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There are three degrees of sunburn that are based on the size and depth of burned skin. The higher the degree, the more severe the burn is.
Sun blisters are second-degree burns, which affect the epidermis (outer layer of the skin) and dermis (layer below the epidermis that includes blood capillaries and nerve endings). As such, they can be highly painful.
First-, Second-, and Third-Degree Burns
First-degree burns, which are typically mild, only affect the epidermis, or outer layer of the skin. Second-degree burns affect the epidermis and dermis, which is the layer of skin below the epidermis. Third-degree burns, which are the most severe, destroy the epidermis and dermis. In some cases, they may also damage bones, muscles, and tendons.