Sunlight has a profound effect on the skin causing premature skin aging, skin cancer, and a host of skin changes. Exposure to ultraviolet light, UVA or UVB, from sunlight accounts for 90% of the symptoms of premature skin aging. Many skin changes that were commonly believed to be due to aging, such as easy bruising, are actually a result of prolonged exposure to UV radiation.
What is UV Radiation?
The sun gives off ultraviolet radiation that we divide into categories based on the wavelength.
- UVC - 100 to 290 nm
- UVB - 290 to 320 nm
- UVA - 320 to 400 nm
UVC radiation is almost completely absorbed by the ozone layer and does not affect the skin. UVC radiation can be found in artificial sources such as mercury arc lamps and germicidal lamps.
UVB affects the outer layer of skin, the epidermis, and is the primary agent responsible for sunburns. It is the most intense between the hours of 10:00 am and 2:00 pm when the sunlight is brightest. It is also more intense in the summer months accounting for 70% of a person's yearly UVB dose. UVB does not penetrate glass.
UVA was once thought to have a minor effect on skin damage, but now studies are showing that UVA is a major contributor to skin damage. UVA penetrates deeper into the skin and works more efficiently. The intensity of UVA radiation is more constant than UVB without the variations during the day and throughout the year. UVA is also not filtered by glass.
Damaging Effects of UVA and UVB
Both UVA and UVB radiation can cause skin damage including wrinkles, lowered immunity against infection, aging skin disorders, and cancer. However, we still do not fully understand the process. Some of the possible mechanisms for UV skin damage are collagen breakdown, the formation of free radicals, interfering with DNA repair, and inhibiting the immune system.
In the dermis, UV radiation causes collagen to break down at a higher rate than with just chronologic aging. Sunlight damages collagen fibers and causes the accumulation of abnormal elastin. When this sun-induced elastin accumulates, enzymes called metalloproteinases are produced in large quantities. Normally, metalloproteinases remodel sun-injured skin by manufacturing and reforming collagen. However, this process does not always work well and some of the metalloproteinases actually break down collagen. This results in the formation of disorganized collagen fibers known as solar scars. When the skin repeats this imperfect rebuilding process over and over wrinkles develop.
UV radiation is one of the major creators of free radicals. Free radicals are unstable oxygen molecules that have only one electron instead of two. Because electrons are found in pairs, the molecule must scavenge other molecules for another electron. When the second molecule looses its electron to the first molecule, it must then find another electron repeating the process. This process can damage cell function and alter genetic material. Free radical damage causes wrinkles by activating the metalloproteinases that break down collagen. They cause cancer by changing the genetic material, RNA and DNA, of the cell.
UV radiation can affect enzymes that help repair damaged DNA. Studies are being conducted looking into the role a specific enzyme called T4 endonuclease 5 (T4N5) has in repairing DNA.
Immune System Effects
The body has a defense system to attack developing cancer cells. These immune system factors include white blood cells called T lymphocytes and specialized skin cells in the dermis called Langerhans cells. When the skin is exposed to sunlight, certain chemicals are released that suppress these immune factors.
The last line of defense of the immune system is a process called apoptosis. Apoptosis is a process of cell-suicide that kills severely damaged cells so they cannot become cancerous. This cell-suicide is seen when you peel after a sunburn. There are certain factors, including UV exposure, that prevent this cell death allowing cells to continue to divide and possibly become cancerous.