|
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
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 metalloproteinase's are produced in large quantities. Normally,
metalloproteinase remodel sun-injured skin by manufacturing and reforming
collagen. However, this process does not always work well and some of the
metalloproteinase's 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 metalloproteinase's 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 end nuclease 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.
Visible Skin Changes Caused by UV Radiation
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
melanocyte 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 seborrhea keratoses, which are
warty looking lesions that appear to be “stuck on” the skin. In contrast to
actinic keratoses, seborrhea 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.
|
