Ultraviolet (UV) Radiation
What is UV radiation?
Radiation is the emission (sending out) of energy from any source.
Ultraviolet (UV) radiation is a form of electromagnetic radiation. The main source of UV
radiation (rays) is the sun, although it can also come from man-made sources such as
tanning beds and welding torches.
Radiation exists across a spectrum from very high-energy (high-frequency) radiation –
like x-rays and gamma rays – to very low-energy (low-frequency) radiation – like radio
waves. In terms of energy, UV rays have more energy than visible light, but not as much
as x-rays.
Higher energy UV rays often have enough energy to remove an electron from (ionize) an
atom or molecule, making them a form of ionizing radiation. Ionizing radiation can
damage the DNA in cells, which in turn may lead to cancer. But because UV rays don’t
have enough energy to penetrate deeply into the body, their main effect is on the skin.
Scientists often divide UV radiation into 3 wavelength ranges:
• UVA rays are the weakest of the UV rays. They can cause skin cells to age and can
cause some indirect damage to cells’ DNA. UVA rays are mainly linked to long-term
skin damage such as wrinkles, but are also thought to play a role in some skin
• UVB rays are slightly stronger. They are mainly responsible for direct damage to the
DNA, and are the rays that cause sunburns. They are also thought to cause most skin
• UVC rays are the strongest UV rays. Fortunately, because of this, they react with
ozone high in our atmosphere and do not reach the ground. Therefore UVC rays are
not present in sunlight and are not normally a risk factor for skin cancer. But they can
be found in some man-made sources, such as arc welding torches and mercury lamps.
In the past, sunbeds were also a source of UVC rays. How are people exposed to UV radiation?
Sunlight is the main source of UV radiation, even though UV rays make up only a small
portion of the sun’s rays. About 95% of the UV radiation from the sun that reaches the
earth is UVA, with the remaining 5% being UVB. The amount of UV radiation you may
be exposed to at any point depends on a number of factors, such as:
• Time of day: Almost a third of the day’s UV rays from the sun comes down between
11AM and 1PM, with three-quarters between 9AM and 5PM.
• Season of the year: UV rays are strongest during summer months. This is less of a
factor near the equator.
• Distance from the equator (latitude): The amount of UV exposure per year goes
down as you get further from the equator.
• Altitude: People burn more easily at higher elevation because more UV rays get
• Clouds: The effect of clouds can vary – sometimes cloud cover blocks some UV
from the sun and lowers UV exposure, while some types of clouds can reflect UV and
so can increase UV exposure. What is important to know is that UV can get through,
even on a cloudy day.
• Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, or
grass, leading to an increase in UV exposure.
• Contents of the air: Ozone in the upper atmosphere, for example, filters out some
UV radiation.
Man-made sources of UV rays
Man-made sources of UV rays can also be important. These include:
• Sunlamps and sunbeds (tanning beds and booths): The amount and type of UV
radiation someone is exposed to from a tanning bed (or booth) depends on the
specific lamps used in the bed, how long a person stays in the bed, and how many
times the person uses it. Most modern UV tanning beds emit mostly (more than 95%)
UVA rays, with the rest being UVB.
• Phototherapy (UV therapy): Some skin problems (psoriasis, for example) can be
treated with UV light. For a treatment known as PUVA, a drug called a psoralen is
given first. The drug collects in the skin and makes it more sensitive to UV. Then the
patient is treated with UVA radiation. Another treatment option is the use of UVB
alone (without a drug). • Black-light lamps: These lamps use bulbs that emit UV. The bulb may also make
some visible light, but it is made with a filter that blocks most of that out, letting
through UV rays (mostly UVA). They have a purple glow and are used to view
fluorescent material. Bug zapping insect traps also use “black light,” but the bulbs use
a different filter that causes them to glow blue.
• Mercury-vapor lamps: Mercury vapor lamps can be used to light large public areas
such as streets or gyms. They do not expose people to UV rays if they are working
properly. They are actually made up of 2 bulbs – an inner bulb that emits light and UV
rays, and an outer bulb that filters out the UV. UV exposure can only occur if the
outer bulb is broken. Some mercury vapor lamps are designed to turn themselves off
when the outer bulb breaks. The ones that don’t have this feature are only supposed to
be installed behind a protective layer or in areas where people wouldn’t be exposed if
part of the bulb breaks.
• High-pressure xenon and xenon-mercury arc lamps, plasma torches, and
welding arcs: Xenon and xenon-mercury arc lamps are used as sources of light and
UV rays for many things, such as UV “curing” (of inks, coatings, etc), video
projection, fiber optics, disinfection, to simulate sunlight (to test solar panels, for
example), and even in some car headlights. Most of these, along with plasma torches
and welding arcs, are mainly of concern in terms of workplace UV exposure.
Does UV radiation cause cancer?
Yes. In fact, most skin cancers are a direct result of exposure to the UV rays in sunlight.
Both basal cell and squamous cell cancers (the most common types of skin cancer) tend
to be found on sun-exposed parts of the body, and their occurrence is related to lifetime
sun exposure. The risk of melanoma, a more serious but less common type of skin cancer,
is also related to sun exposure, although perhaps not as strongly. Skin cancer has also
been linked to exposure to some artificial sources of UV.
Studies in people
Sun exposure
Basal and squamous cell skin cancer
Many observational studies have looked at the link between basal and squamous cell skin
cancers and sun exposure. These studies have found that basal and squamous cell skin
cancers are linked to certain behaviors that put people in the sun, as well as a number of
markers of sun exposure, such as:
• Spending time in the sun for recreation (including going to the beach)
• Spending a lot of time in the sun in a swim suit
• Living in an area with a high amount of sun • Serious sunburns in the past (with more sunburns linked to a higher risk)
• Signs of sun damage to the skin, such as liver spots, actinic keratoses (rough skin
patches that can be precancerous), and solar elastosis (thickened, dry, wrinkled skin
caused by sun exposure) on the neck
Observational studies have also found links between certain behaviors and markers of sun
exposure and melanoma of the skin, including:
• Activities that lead to “intermittent sun exposure,” like sunbathing, water sports, and
taking vacations in sunny places
• Previous sunburn
• Signs of sun damage to the skin, such as liver spots, actinic keratoses, and solar
Melanoma of the eye has also been linked to sun exposure in some studies.
Other cancers
It is not clear if there is a link between cancer of the lip and sun exposure. One study
found a link, but another did not.
A type of cancer affecting the eye, squamous cell of the conjunctiva, has been linked to
sun exposure in some studies, but again the link is not clear.
Artificial sources of UV rays
Indoor tanning
Studies have found that people who use tanning beds (or booths) have a higher risk of
skin cancer, including melanoma, and squamous and basal cell skin cancers. The risk of
melanoma is higher if the person started indoor tanning before age 30 or 35 and the risk
of basal and squamous cell skin cancer is higher if indoor tanning started before age 20 or
Some studies have also found a higher risk of melanoma of the eye in people who have
used UV tanning beds.
Welding and metal work
Studies have found that welders and sheet metal workers have a higher risk of melanoma
of the eye.
People exposed to UVA as a treatment for skin conditions such as psoriasis (as a part of
PUVA therapy) have an increased risk of squamous cell skin cancers. Treatment of skin conditions with UVB alone (not combined with PUVA) has not been
linked to an increased risk of cancer.
Studies in the lab
Studies of cells
Studies of cells in lab dishes and test tubes have shown that sunlight and simulated
sunlight (for example, from xenon or xenon-mercury arc lamps) can cause DNA damage
One study compared a commercial tanning bed to natural sunlight, and found that the
tanning bed was more likely to cause mutations than sunlight.
Studies in animals
Exposure of mice, rats, and some other lab animals to sunlight and artificial sources of
UV rays has been shown to lead to skin cancers. Some exposed animals have also
developed cancers of the eye (affecting the cornea and conjunctiva). No type of UV
radiation has been shown to be safe – cancers have developed after exposure to UVA
(alone), UVB (alone), and UVC (alone). Most of the cancers produced in these studies
were squamous cell carcinomas.
What expert agencies say
Several national and international agencies study different substances in the environment
to determine if they can cause cancer. (A substance that causes cancer or helps cancer
grow is called a carcinogen.) The American Cancer Society looks to these organizations
to evaluate the risks based on evidence from laboratory, animal, and human research
Based on animal and human evidence like the examples above, several expert agencies
have evaluated the cancer-causing nature of UV radiation.
The International Agency for Research on Cancer (IARC) is part of the World Health
Organization (WHO). Its major goal is to identify causes of cancer. Based on the data
available, IARC classifies UV radiation as “carcinogenic to humans.”
The National Toxicology Program (NTP) is formed from parts of several different US
government agencies, including the National Institutes of Health (NIH), the Centers for
Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA).
The NTP has classified UV radiation as “known to be a human carcinogen.”
(For more information on the classification systems used by these agencies, see our
document Known and Probable Human Carcinogens.) What about tanning beds?
Some people think that getting UV rays from tanning beds is a safe way to get a tan, but
this isn’t true.
IARC classifies the use of UV-emitting tanning devices as “carcinogenic to humans.”
This includes sunlamps and sunbeds (tanning beds).
The NTP has stated that exposure to sunlamps or sunbeds is “known to be a human
The US Food and Drug Administration (FDA) which now refers to all UV lamps used for
tanning as “sunlamps,” requires them to carry a label that states, “This sunlamp product
should not be used on persons under the age of 18 years.” It also requires that user
instructions and sales materials directed at consumers (including catalogs, specification
sheets, descriptive brochures, and webpages) carry the following statements:
• Contraindication: This product is contraindicated for use on persons under the age of
18 years.
• Contraindication: This product must not be used if skin lesions or open wounds are
• Warning: This product should not be used on individuals who have had skin cancer or
have a family history of skin cancer.
• Warning: Persons repeatedly exposed to UV radiation should be regularly evaluated
for skin cancer.
Does UV radiation cause any other health
In addition to cancer, exposure to UV rays can cause other problems. UV rays, either
from the sun or from artificial sources like tanning beds, can cause sunburn. In some
people, exposure to UV rays can cause a rash or a type of allergic reaction. Exposure to
UV rays can also cause premature aging of the skin and signs of sun damage such as liver
spots, actinic keratosis, and solar elastosis.
UV rays can also cause eye problems. They can cause the cornea (on the front of the eye)
to become inflamed or burned. They can also lead to the formation of cataracts (clouding
of the lens of the eye) and pterygium (tissue growth on the surface of the eye), both of
which can impair vision. UV rays can also affect an area of the retina (the part of the eye
that senses light) called the macula, which can also affect vision.
Exposure to UV rays can also weaken the immune system, so that the body has a harder
time fending off infections. This can lead to problems such as reactivation of herpes
triggered by exposure to the sun or other sources of UV rays. It can also cause vaccines to
be less effective. Some medications can make you more sensitive to UV radiation, making you more likely
to get sunburned. Certain medical conditions can be made worse by UV radiation.
How can I avoid exposure to UV radiation?
UV rays in sunlight
Simply staying in the shade is one of the best ways to limit your UV exposure from
sunlight. If you are going to be in the sun, protect your skin with clothing and wear a hat.
Wear sunglasses that block UV to protect your eyes and the skin around them. Use
sunscreen to help protect skin that isn’t covered with clothing.
For more information, see our document Skin Cancer Prevention and Early Detection.
Artificial sources of UV rays
Many people believe the UV rays of tanning beds are harmless. This is not true. The best
thing to do is not use tanning beds (or booths).
People who may be exposed to artificial sources of UV at their job should follow
appropriate safety precautions, including protective clothing and the use of shields and
Additional resources
The following information may also be helpful to you. These materials may be ordered
through our toll-free number, 1-800-227-2345.
Skin Cancer Prevention and Early Detection
Melanoma Skin Cancer
Basal and Squamous Cell Skin cancer
National organizations and websites*
In addition to the American Cancer Society, other sources of information and support
American Academy of Dermatology
Toll-free number: 1-888-462-3376 (1-888-462-DERM)
Website: www.aad.org
Environmental Protection Agency
Website: www.epa.gov/sunwise Melanoma Research Foundation
Toll free number: 1-800-673-1290
Website: www.melanoma.org
National Cancer Institute
Toll-free number: 1-800-422-6237 (1-800-4-CANCER); TYY: 1-800-332-8615
Website: www.cancer.gov
Skin Cancer Foundation
Toll-free number: 1-800-754-6490 (1-800-SKIN-490)
Website: www.skincancer.org
*Inclusion on this list does not imply endorsement by the American Cancer Society.
No matter who you are, we can help. Contact us anytime, day or night, for information
and support. Call us at 1-800-227-2345 or visit www.cancer.org.
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Last Medical Review: 7/2/2013
Last Revised: 5/30/2014
2013 Copyright American Cancer Society