Part of the sun's energy that reaches earth is composed of rays of invisible ultraviolet (UV) light. When ultraviolet light rays (UVA and UVB) enter the skin, they inflict both visible and invisible damage to the skin cells. Sunburn is a visible type of damage, which appears just a few hours after sun exposure. In many people, this type of damage also causes tanning. Freckles, which occur in people with fair skin, are usually due to sun exposure. Freckles are nearly always a sign that sun damage has occurred, and therefore show the need for sun protection.

Of the invisible damage to skin cells, some is repaired but some adds up year after year. After 20 to 30 years or more, the built-up damage appears as wrinkles and age spots. Although window glass blocks UVB light, UVA rays are able to pass through it.

Sun exposure during youth is a risk factor for certain skin cancers, including melanoma. Chronic sun exposure is also associated with squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). Severe burns, especially those that blister, increase the risk. Susceptible persons (light skinned, easily burned) playing or working outside in strong sunlight should be protected by clothing or sunscreen.