Actinic keratoses (AKs) are known as the early beginnings of skin cancer. This most common lesion of the outermost layer of the skin (epidermis) is caused by long-term exposure to sunlight (specifically to ultraviolet wavelengths). AKs are most likely to appear after age 40-50, and years of chronic exposure to the sun. However, in geographic areas with year-round high-intensity sunlight (e.g., Florida, southern California) AKs are now found in persons as young as the teens and twenties. The incidence of AKs is over 50 percent in older, fair-skinned persons in hot, sunny geographic areas.

AKs are defined as a cutaneous dysplasia of the epidermis (the outermost layer of skin). In everyday terms, AKs are an alteration in size, shape and organization of skin cells. The cells most affected in AKs are the keratinocytes, the tough-walled cells that make up more than 90 percent of the epidermis and give the skin its texture. Cellular alterations in AKs may extend into the dermis, the layer of skin under the epidermis. The most significant cause of actinic keratoses is long-term exposure to sunlight, and specifically to the ultraviolet wavelengths of solar radiation. The most significant predisposing factor to AKs is fair skin.