Introduction

Initial Damaging Effects of Sunlight. Ultraviolet radiation penetrates the layers of the skin. Both UVA and UVB rays cause damage leading to wrinkles, lower immunity against infection, aging skin disorders, and cancer. They appear to damage cells in different ways, however.

• UVB is the main cause of sunburns, 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. Only 28% is received during the remainder of the year. Window glass filters out UVB .
• UVA penetrates more deeply and efficiently. The intensity of UVA rays is less dependent on the time of day and season of the year than UVB rays. For example, you receive only about half of your yearly UVA dose during the summer months, with the balance spread over the rest of the year. Window glass does NOT filter out UVA.

Both UVA and UVB rays cause damage to the body, including genetic injury, wrinkles, aging skin disorders, and skin cancers. Exactly how they cause this damage is not yet fully understood.

Processes Leading to Wrinkles. Even small amounts of UV radiation trigger the processes that can cause wrinkles:

• Sunlight damages collagen fibers (the major protein that gives structure to the skin). Sunlight also causes accumulation of abnormal elastin (the protein that causes tissue to stretch) in the skin.
• In response to this sun-induced elastin accumulation, the body produces large amounts of enzymes called metalloproteinases. One study indicated that when people with light to moderate skin color are exposed to sunlight for just 5 - 15 minutes, the metalloproteinase levels in their body remain high for about a week.
• The normal function of these metalloproteinases is generally positive -- to remodel the sun-injured tissue by producing and repairing collagen. This is an imperfect process, however, and some of metalloproteinases produced by sunlight actually degrade (break down) collagen. The result is an uneven formation (matrix) of disorganized collagen fibers called solar scars. Repetition of this imperfect skin rebuilding causes wrinkles.
• An important event in this process is the over-production of oxidants, also called free radicals. These are unstable molecules that are normally produced by chemical processes in the body, a process called oxidation. Environmental damage, however, causes an overproduction of oxidants. Excessive amounts of oxidants damage the body's cells and even alter their genetic material. Oxidation may specifically contribute to wrinkling by activating the specific metalloproteinases that degrade connective tissue.