The stratospheric
ozone layer absorbs some but not all of these types of UV
radiation:
UVA
Not absorbed by the ozone layer
UVB
Partially absorbed by the ozone layer
UVC
Completely absorbed by oxygen and ozone in the atmosphere
UVA
and especially UVB penetrate into the skin and eyes, and can
cause the adverse health effects listed above.
UV Levels Depend on a Number of Factors
* Stratospheric
Ozone
The
ozone layer absorbs most of the sun's harmful UV rays, but
its thickness varies depending on the time of year and changing
weather patterns. The ozone layer has thinned in certain areas
due to the emission of ozone-depleting chemicals.
* Time
of Day
The
sun is at its highest in the sky around noon. At that time,
the sun's rays have the least distance to travel through the
atmosphere, and UVB levels are at their highest. In the early
morning and late afternoon, the sun's rays pass obliquely
through the atmosphere, and the intensity of UVB is greatly
reduced. UVA levels are not sensitive to ozone and vary throughout
the day much like visible sunlight does.
* Time
of Year
The
sun's angle varies with the seasons, causing the intensity
of UV rays to vary. UV intensity tends to be highest during
the summer months.
* Latitude
The
sun's rays are strongest at the equator where the sun is most
directly overhead and where UV rays must travel the least
distance through the atmosphere. Ozone also is naturally thinner
in the tropics as compared to the mid- and high-latitudes,
so there is less ozone to absorb the UV radiation as it passes
through the atmosphere. At higher latitudes the sun is lower
in the sky, so UV rays must travel a greater distance through
ozone-rich portions of the atmosphere and in turn expose those
latitudes to less UV radiation.
* Altitude
UV
intensity increases with altitude because there is less atmosphere
to absorb the damaging rays.
* Weather
Conditions
Cloud
cover reduces UV levels, but not completely. Depending on
the thickness of the cloud cover, it is possible to burn on
a cloudy day even if it does not feel very warm.
*
Ozone Depletion
The ozone layer forms a thin shield in the stratosphere, protecting
life on earth from the sun's harmful UV rays. In the 1980s,
scientists began accumulating evidence that the ozone layer
was being depleted. Depletion of the ozone layer can result
in increased UV radiation reaching the earth's surface, which
can lead to a greater chance of overexposure to UV radiation
and consequent health effects including skin cancer, cataracts,
and immune suppression.
How Stratospheric Ozone Protects Us
Ozone
is a naturally occurring gas that is found in two layers in
the atmosphere. In the layer surrounding the earth's surface-the
Stroposphere-ground-level or "bad" ozone is an air
pollutant that damages human health and vegetation and is
a key ingredient of urban smog. The troposphere extends up
to the stratosphere, which is where the "good" ozone
protects life on earth by absorbing some of the sun's ultraviolet
rays. Stratospheric ozone is most concentrated between 6 to
30 miles above the earth's surface.
Ozone
is formed when oxygen molecules absorb UV radiation and split
apart into two oxygen atoms (0), which combine with other
oxygen molecules (02) to form ozone molecules (03). Ozone
also is broken apart as it absorbs UV radiation. In this way,
UV radiation helps sustain the natural balance of ozone in
the stratosphere, while ozone in turn absorbs UV radiation,
protecting life on earth from harmful radiation.
How
Ozone Is Depleted
Until
recently, chlorofluorocarbons (CFCs) were used widely in industry
and elsewhere as refrigerants, insulating foams, and solvents.
They migrate into the upper atmosphere after use, carried
by air currents into the stratosphere. This process can take
as long as 5 to 10 years. These chemicals absorb UV radiation,
break apart, and react with ozone, taking one oxygen atom
away and forming highly reactive chlorine monoxide. Chlorine
monoxide in turn breaks down 03 again
by pulling away a single oxygen atom, creating two 02 molecules,
and allowing the chlorine to move freely to another ozone
molecule. In this way, each chlorine atom acts as a catalyst,
repeatedly combining with and breaking apart as many as 100,000
ozone molecules during its stratospheric life.
Other
compounds also damage the ozone layer in much the same way
as do CFCs. These ozone-depleting substances include pesticides
such as methyl bromide, halons used in fire extinguishers,
and methyl chloroform used in industrial processes.
Effect
of Ozone Depletion on UV Radiation Levels
Current
studies predict that CFC levels in the atmosphere should peak
by around 2000 and should fall to pre-1980 levels by about
2050. As international control measures reduce the release
of CFCs and other ozone-depleting substances, natural atmospheric
processes will repair the ozone layer. Until that time, we
can expect increased levels of UV radiation at the earth's
surface. These increased UV radiation levels can lead to a
greater chance of overexposure to UV radiation and the consequent
health effects.
Health Effects From Overexposure to the Sun
Americans
love the sun and spend a great deal of time outside-working,
playing, exercising-often in clothing that exposes a lot of
skin to the sun. Most people are now aware that too much sun
has been linked to skin cancer. However, few know the degree
of risk posed by overexposure, and fewer are aware that the
risks go beyond skin cancer. Recent medical research has shown
that overexposure to the
sun's UV radiation can contribute to serious health problems.
Each year, for example, more than 1 million cases of skin
cancer are diagnosed in the United States, and one person
dies every hour from melanoma or nonmelanoma skin cancer.
This section provides a quick overview of the major problems
linked to excess UV exposure: skin
cancer (i.e., melanoma, basal cell carcinoma, and squamous
cell carcinoma); other skin problems; cataracts and other
eye damage; and immune system suppression. Understanding these
risks and taking a few sensible precautions described in this
article will help you to enjoy the sun while lowering your
chances of sun-related health problems later in life.
A
Word About Risk
Overexposure
to UV radiation poses the risk of serious health effects for
everyone, but not everyone is equally at risk. For example,
you may be at greater risk of contracting skin cancer if your
skin always burns, or burns easily, and if you have blond
or red hair, or blue, green, or gray eyes. Other factors indicating
an increased risk of skin cancer include: a history of blistering
sunburns in early childhood, usually from acute sun overexposure;
the presence of many moles; or a family history of skin cancer.
Also, people who work or otherwise spend a large amount of
time outdoors (i.e., chronic exposure to the sun) may be at
higher risk of health effects. It's a good idea to remember
that anyone can contract skin cancer, and that all people,
no matter what skin type, are equally at risk of eye
damage.
Melanoma
Melanoma,
the most serious form of skin cancer, also is one of the fastest
growing types of cancer in the United States. Many scientists
believe there might be a link between childhood sunburns and
malignant melanoma later in life. Melanoma cases in this country
have more than doubled in the past 2 decades; according to
the American
Cancer Society, about 44,200 new cases of melanoma and 7,300
deaths are currently reported each year.
Nonmelanoma
Skin Cancers
Unlike
melanoma, nonmelanoma skin cancers are rarely fatal. Nevertheless,
they should not be taken lightly. Untreated, they can spread
and cause more serious health problems. An estimated 1 million
Americans will develop nonmelanoma skin cancers this year,
while 1,900 will die from the disease.
Actinic
Keratoses
These
sun-induced skin growths occur on body areas exposed to the
sun. The face, hands, forearms, and the "V" of the
neck are especially susceptible to this type of blemish. They
are premalignant, but if left untreated, actinic keratoses
can become malignant. Look for raised, reddish, rough-textured
growths. See a dermatologist promptly if you notice these
growths.
Premature Aging of the Skin
Chronic
exposure to the sun causes changes in the skin called actinic
(or solar) degeneration. Over time, the skin becomes thick,
wrinkled, and leathery. Since it occurs gradually, often manifesting
itself many years after the majority of a person's exposure
to the sun, this condition is often regarded as unavoidable,
a normal part of growing older. With proper protection from
W radiation, however, premature aging of the skin can be substantially
avoided.
Cataracts and Other Eye Damage
Cataracts
are a form of eye damage, a loss of transparency in the lens
that clouds vision. Left untreated, cataracts can rob people
of vision. Research has shown that W radiation increases the
likelihood of certain cataracts. Although curable with modern
eye surgery, cataracts diminish the eyesight of millions of
Americans and necessitate billions of dollars of eye surgery
each year. Other kinds of eye damage include: pterygium (tissue
growth on the white of the eye that can block vision), skin
cancer around the eyes, and degeneration of the macula (the
part of the retina near the center, where visual perception
is most acute). All of these problems could be lessened with
proper eye protection from W radiation.
Immune Suppression
Scientists
have found that sunburn can alter the distribution and function
of disease-fighting white blood cells in humans for up to
24 hours after exposure to the sun. Repeated exposure to W
radiation might cause more long-lasting damage to the body's
immune system. Mild sunburns can suppress immune functions
in people of all skin types.
Action Steps for Sun Protection
*Be SunWise
Protecting
yourself from overexposure to UV radiation is simple if you
take the precautions listed below.
