Causes

Causes

You cannot overestimate the role of the sun as the most important cause of prematurely aging skin (called photoaging) and skin cancers. Overall, exposure to ultraviolet (referred to as UVA or UVB) radiation emanating from sunlight accounts for about 90% of the symptoms of premature skin aging, and most of these effects occur by age 20:

• Even small amounts of UV radiation can lead to skin wrinkles.
• Long-term repetitive and cumulative exposure to sunlight appears to be responsible for the vast majority of undesirable consequences of aging skin, including basal cell and squamous cell carcinomas.
• 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 (referred to as UVA or UVB) radiation bombards the genetic material, the DNA, inside the skin cells and damages it.

• UVB is the primary agent in sunburning and primarily affects the outer skin layers. UVB is most intense at midday when sunlight is brightest. Slightly over 70% of the yearly UVB dose is received during the summer and only 28% is received during the remainder of the year.
• UVA penetrates more deeply and efficiently, however. UVA's intensity also tends to be less variable both during the day and throughout the year than UVB's. For example, only about half of the yearly UVA dose is received during the summer months, and the balance is spread over the rest of the year. UVA is also not filtered through window glass (as UVB is).

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 defensive actions that 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. These are unstable molecules produced by normal chemical processes in the body that, in excess, can damage the body's cells and even alter their genetic material, contributing to the aging process and sometimes to cancer. The large surface area of the skin makes this organ a prime target for oxidants.
• Defective DNA Repair and Protective Enzymes. Some melanomas and other skin cancers are caused by a breakdown in the mechanisms that help repair DNA damage. This can occur from various causes including an inherited condition called xeroderma pigmentosum (XP). A number of enzymes in the skin help protect against this damage. One repair enzyme called T4 endonuclease 5 (T4N5) is, in fact, being investigated in lotions to protect against skin cancers.
• Breakdown of Immune Protection. Specific immune factors protect the skin, including white blood cells called T lymphocytes and specialized skin cells called Langerhans cells. Such immune factors attack developing cancer cells at the very earliest stages. Unfortunately, certain substances in the skin -- of note a chemical called urocanic acid -- suppress such immune factors when exposed to sunlight, setting the stage for skin cancers.