Effects of Sun on the Skin
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.
The sun gives off ultraviolet radiation that we divide into categories based on the wavelength.
UVC Radiation
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 Radiation
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 Radiation
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.
Collagen Breakdown
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.
Free Radicals
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.
DNA Repair
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.
Cell Death
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.
Texture Changes Caused by the Sun
UV exposure causes thickening and thinning of the skin. Thick skin is found in coarse wrinkles especially on the back of the neck that do not disappear when the skin is stretched. A condition called solar elastosis is seen as thickened, coarse wrinkling and yellow discoloration of the skin. A common effect of UV exposure is thinning of the skin causing fine wrinkles, easy bruising, and skin tearing.
Blood Vessel Changes Caused by the Sun
UV radiation causes the walls of blood vessels to become thinner leading to bruising with only minor trauma in sun-exposed areas. For example, most of the bruising that occurs on sun-damaged skin occurs on the backs of the hands and forearms not on the inside of the upper arm or even the inside of the forearm. The sun also causes the appearance of telangiectasias, tiny blood vessels, in the skin especially on the face.
Pigment Changes Caused by the Sun
The most noticeable sun-induced pigment change is a freckle or solar lentigo. Light-skinned people tend to freckle more noticeably. A freckle is caused when the melanin-producing cell, or melanocyte, is damaged causing it to get bigger. Large freckles, also known as age spots or liver spots, can be seen on the backs of the hands, chest, shoulders, arms, and upper back. These are not actually age related but sun-damage related. UV exposure can also cause white spots especially on the legs, but also on the backs of the hands and arms, as melanocytes are destroyed.
Skin Bumps Caused by the Sun
UV radiation causes an increased number of moles in sun-exposed areas. Sun exposure also causes precancerous lesions called actinic keratoses that develop especially on the face, ears, and backs of the hands. The are small crusty bumps that can often be felt better than they can be seen. Actinic keratoses are felt to be premalignant lesions because 1 in 100 cases per year will develop into squamous cell carcinoma. UV exposure also causes seborrheic keratoses, which are warty looking lesions that appear to be "stuck on" the skin. In contrast to actinic keratoses, seborrheic keratoses do not become cancerous.
Skin Cancer Caused by the Sun
The ability of the sun to cause skin cancer is a well-known fact. The 3 main skin cancers are melanoma, basal cell carcinoma, and squamous cell carcinoma. Melanoma is the most deadly skin cancer because it metastasizes more readily than the other skin cancers. It is believed that the amount of exposure of the skin to the sun before the age of 20 is actually the determining risk factor for melanoma. Basal cell carcinoma is the most common skin cancer and tends to spread locally, not metastasize. Squamous cell carcinoma is the second most common skin cancer, and it can metastasize although not as commonly as melanoma. The risk of getting basal cell carcinoma or squamous cell carcinoma is determined by a person's lifetime exposure to UV radiation and the person's pigment protection.
