UV rays and cancer

Most people have heard the message: It's important to apply sunscreen to protect the skin from the sun’s damaging ultraviolet (UV) rays. Using sunscreen is the best way to reduce the risk of developing skin cancer, including melanoma.

Natural sunlight is a major source of damaging UV rays, but there are other ways people may be exposed, too.

What are UV rays?

UV rays are a form of ionizing radiation, which means they have an elevated level of energy—enough energy to remove an electron from an atom or molecule. They damage the genetic material in cells, which may lead to skin cancer. The risk only runs skin-deep, though: They don’t tend to penetrate deeper than that, so the main risk of UV radiation is skin damage, including skin cancer.

Types of UV rays

The three types of ultraviolet rays are identified as UVA, UVB and UVC:

Ultraviolet A (UVA) rays are known to cause premature aging and wrinkles and may play a role in some skin cancers. These make up most of the sun’s rays that reach the skin.

Ultraviolet B (UVB) rays may damage the DNA or genetic material within skin cells. Registering a bit more energy than UVA rays, they’re thought to be responsible for most skin cancers. These make up about 5 percent of the sun’s rays that reach the skin.

Ultraviolet C (UVC) rays, the most dangerous, are absorbed by the ozone layer and don’t reach the Earth’s surface.

Do UV rays from the sun cause cancer?

UVB rays stimulate skin cells known as melanocytes to produce melanin, causing a sunburn or tan, both of which are signs of damage. As many as 90 percent of melanomas are thought to be caused by UV exposure, and both basal cell and squamous cell cancers—the two most common skin cancers—are most often found on sun-exposed parts of the body, suggesting a strong causative role for the sun’s rays.

What affects UV rays?

Clouds: Although clouds do block visible light, they’re not as effective as blocking UV rays. In fact, some research suggests that UV rays are stronger on cloudy days, so it’s still important to wear sunscreen when it’s cloudy outside.

Windows: Most types of glass, including windows, are effective at blocking UVB rays. However, they aren’t as useful in blocking UVA rays, so it’s a good idea to sit a few feet away from windows, or wear sunscreen when sitting near windows.

Clothes: UV rays can often penetrate clothing, so it’s important to wear clothes with high UPF (Ultraviolet Protection Factor) ratings.

Man-made UV rays

The sun is the main source of cancer-causing UV radiation, but it’s not the only one. Tanning beds and sun lamps also emit UV rays. The World Health Organization has declared that use of UV-emitting tanning devices are "carcinogenic to humans."

Beyond tanning beds and booths, other man-made UV rays may come from different lighting sources.

• Phototherapy, also called UV light therapy, uses UVA or UVB light to help treat certain skin diseases, such as psoriasis, in a controlled setting.
• Mercury vapor lighting, often used in stadiums and school gyms, emits UV light, too. They have an outer bulb that filters UV light, but if it breaks, intense UV radiation may cause burns. (Some are designed to turn off in the event of a rupture.)
• Halogen, fluorescent and incandescent lightbulbs, such as black lights, emit a small amount of UV light, though questions about cumulative damage remain.
• Some specialty lights, such as high-pressure xenon and xenon-mercury arc lamps, plasma torches and welding arcs, are used for particular tasks—including disinfecting or curing a surface or stimulating sunlight to test solar panels. Exposure to the UV rays from these types of light tends to be a concern if they’re in the workplace.
• Manicure salons use UV drying devices that release a low level of mainly UVA rays. These devices emit much less UV light than tanning beds, but it may be a good idea to limit the amount of time a person's hand is under this type of light.

Skin cancer risks

A person's skin type, when he or she was exposed to the sun, and whether he or she wore sunscreen all affect the chance of developing skin cancer. People with fair skin tend to be more susceptible to sunburns and the damage that leads to skin cancer, but those with darker skin may develop it, too.

Risk factors include:

• Light skin that burns or gets freckles easily
• Light eyes, such as blue or green eyes
• Light hair, such as blond or red hair
• Certain types of types of moles, or many moles
• Previous skin cancer diagnosis
• Family history
• Older age

Being exposed to UV rays at a young age may be particularly damaging. A history of sunburns in childhood increases skin cancer risk. Using tanning beds before age 30 raises the risk for developing melanoma by 75 percent, according to the Skin Cancer Foundation.

Skin cancer isn’t the only cancer caused by UV rays. They may also cause cancers of the eye and other eye issues, including cataracts.

Skin cancer prevention

When it comes to skin cancer, prevention is key. This starts with protecting against both UVA and UVB rays by using a broad-spectrum sunscreen with an SPF (Sun Protection Factor) of 30 or higher.

• Apply roughly a shot glass-sized amount to exposed skin 30 minutes before going outside.
• Reapply every two hours or after swimming or sweating. This should be done every day, regardless of the season or plans. UV radiation is reflected by snow and ice and may go through windshields and windows.

It's a good idea to also:

• Stay in the shade when the sun’s rays are strongest, typically between 10 a.m. and 4 p.m.
• Wear a wide-brimmed hat in the sun to protect facial skin.
• Wear sunglasses that block UV radiation to protect the eyes and the thin skin around them.
• Wear long sleeves and long pants while in the sun.
• Choose fabrics with high UPF ratings.
• Visit a dermatologist yearly for skin checks, and perform a skin examination each month.

Pay attention to warning signs, such as a new spot on the skin or one that’s changing size, color or shape. Use the ABCDE rule, detailed below.

Asymmetry (A): One side is bigger than the other.

Border (B): The border of the mole is irregular.

Color (C): Melanomas tend to be multicolored.

Diameter (D): Melanomas tend to be about the size of a pencil eraser.

Evolving (E): The mole is changing in size, shape or color.

The ABCDE rules don’t cover all the potential warning signs of skin cancer, so inform the doctor about any skin changes that are different or concerning.

Learn more about preventing skin cancer