The sun is the most important cause of prematurely aging skin (photoaging) and skin cancers.
Long-term, repeated exposure to sunlight appears to be responsible for most undesirable consequences of aging skin, including basal cell and squamous cell cancers.
Melanoma is more likely to be caused by intense exposure to sunlight in early life.
UVA and UVB Radiation. When sunlight penetrates the top layers of the skin, ultraviolet (UVA or UVB) radiation strikes the DNA inside the skin cells and damages it.
- UVB is the main type of radiation responsible for sunburns. It primarily affects the outer skin layers. This type of ultraviolet light is most intense at midday when sunlight is brightest.
- UVA penetrates more deeply and efficiently. Unlike with UVB, window glass does not filter out UVA rays.
Damaging Effects of UV Radiation. Both UVA and UVB rays cause damage, including genetic injury, wrinkles, lower immunity against infection, aging skin disorders, and cancer, although the mechanisms are not yet fully clear. The following are some ways in which cancer may develop, and some actions the skin uses to defend itself against DNA damage.
- Oxidation and Antioxidants. The effects of UV radiation are implicated in the production of oxidants, also called free radicals. Free radicals are unstable molecules produced by normal chemical processes in the body that, in excess, can damage the body's cells and even alter the DNA. This contributes to the aging process and sometimes to cancer.
- Defective DNA Repair and Protective Enzymes. Some skin cancers are caused by a breakdown in the body's mechanisms that help repair DNA damage. For example, xeroderma pigmentosum (XP) is a rare genetic disease in which the body cannot repair damage caused by ultraviolet light. Normally, a number of enzymes in the skin help protect against this damage.
- Breakdown of Immune Protection. Specific immune factors protect the skin, including white blood cells called T lymphocytes and specialized skin cells called Langerhans cells. These immune system cells attack developing cancer cells at the very earliest stages. However, certain substances in the skin, particularly a chemical called urocanic acid, can suppress such immune factors when exposed to sunlight.
Defective Cell Death (Apoptosis). Apoptosis is the last defense of the immune system. It is a natural process of cell-suicide, which occurs when cells are very severely damaged. Apoptosis in the skin kills off cells harmed by UVA so that they do not turn cancerous. The peeling after sunburn is the result of these dead skin cells. However, some gene defects or other factors can interfere with apoptosis. If this occurs, damaged cells can continue to spread, resulting in skin cancer.
Review Date: 07/04/2010
Reviewed By: Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.