A risk factor is anything that affects your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, unprotected exposure to strong sunlight is a risk factor for skin cancer, and smoking is a risk factor for cancers of the lung, mouth, throat, kidneys, bladder, and several other organs.

But risk factors don't tell us everything. Having a risk factor, or even several risk factors, does not mean that you will get the disease. And many people who get the disease may not have had any known risk factors. Even if a person with basal or squamous cell skin cancer has a risk factor, it is often very hard to know how much that risk factor may have contributed to the cancer.

The following are known risk factors for basal cell and squamous cell carcinomas (but do not apply to other forms of non-melanoma skin cancer such as Kaposi sarcoma and cutaneous lymphoma).

Ultraviolet (UV) light exposure

Ultraviolet (UV) radiation is thought to be the major risk factor for most skin cancers. Sunlight is the main source of UV radiation, which can damage the genes in your skin cells. Tanning lamps and booths are another source of UV radiation. People with excessive exposure to light from these sources are at greater risk for skin cancer.

The amount of UV exposure depends on the intensity of the radiation, length of time the skin was exposed, and whether the skin was protected with clothing and sunscreen. Many studies also point to exposure at a young age as an added risk factor.

Ultraviolet radiation is divided into 3 wavelength ranges:

• UVA rays are involved in the aging of cells and cause some damage to cells' DNA. They are mainly linked to long-term skin damage such as wrinkles, but are also thought to play a role in some skin cancers.

• UVB rays are mainly responsible for direct damage to the DNA, and are the rays that primarily cause sunburns. They are also thought to cause most skin cancers.

• UVC rays don't penetrate our atmosphere and therefore are not present in sunlight. They are not normally a risk factor for skin cancer.

While UVA and UVB rays make up only a very small portion of the sun's wavelengths, they are mainly responsible for the damaging effects of the sun on the skin. UVB radiation damages the DNA of skin cells. Skin cancers begin when this damage affects the DNA of genes that control skin cell growth. Recent research has found that UVA also contributes to skin cancer formation. Scientists now believe that both UVA and UVB rays damage skin and cause skin cancer. UVB rays are a more potent cause of at least some skin cancers, but there are no safe UV rays.

People who live in areas with year-round, bright sunlight have a higher risk. For example, the risk of skin cancer is twice as high in Arizona compared to Minnesota. The highest rate of skin cancer in the world is in Australia. Spending a lot of time outdoors for work or recreation without protective clothing and sunscreen increases your risk.

Having fair skin

The risk of skin cancer is much higher for whites than for African Americans or Hispanics. This is due to the protective effect of melanin (skin pigment). Whites with fair (light-colored) skin that freckles or burns easily are at especially high risk. This is another reason for the high skin cancer rate in Australia, as the major settlers were fair-skinned immigrants from the British Isles.

Albinism is a congenital (present at birth) absence of skin pigment. People with this condition may have pink-white skin and white hair. They have a high risk of getting skin cancer unless they are careful to protect their skin.

Older age

The risk of basal and squamous cell skin cancers grows as people get older. This is likely due to the accumulation of sun exposure over time. Still, these cancers are now being seen in younger people as well, probably because they are spending more time in the sun with their skin exposed.

Male gender

Men are about 2 times as likely as women to have basal cell cancers and about 3 times as likely to have squamous cell cancers of the skin. This is thought to be due mainly to higher levels of sun exposure.

Exposure to certain chemicals

Exposure to large amounts of arsenic increases the risk of developing skin cancer. Arsenic is a heavy metal found naturally in well water in some areas. It is also used in making some pesticides and certain medicines. Workers exposed to industrial tar, coal, paraffin, and certain types of oil may also have an increased risk for non-melanoma skin cancer.

Radiation exposure

People who have had radiation treatment have a higher risk of developing skin cancer in the area that received the treatment. This is particularly a problem in children who have had cancer treatment. Almost all of these cancers are basal cell carcinomas.

Previous skin cancer

Anyone who has had a skin cancer has a much higher chance of developing another one.

Long-term or severe skin inflammation or injury

Scars from severe burns, areas of skin over severe bone infections, and skin damaged by some severe inflammatory skin diseases are more likely to develop keratinocyte skin cancers, although this risk is generally small.

Psoriasis treatment

Psoralen and ultraviolet light treatments (PUVA) given to some patients with psoriasis (a long-lasting inflammatory skin disease) can increase the risk of developing squamous cell skin cancer and probably other skin cancers also.

Xeroderma pigmentosum (XP)

This very rare inherited condition reduces the skin's ability to repair damage to DNA caused by sun exposure. People with this disorder often develop many skin cancers, sometimes starting in childhood.

Basal cell nevus syndrome

This rare congenital (present at birth) condition causes multiple basal cell cancers. Most, but not all, cases are inherited. Affected people may also have abnormalities of the jaw and other bones, eyes, and nervous tissue. One clue to having this syndrome in your family is if the affected person began developing basal cell cancers when he or she was young (for example, under age 20).

Reduced immunity

The immune system helps the body fight cancers of the skin and other organs. People with weakened immune systems (due to certain diseases or medical treatments) are more likely to develop non-melanoma skin cancer, particularly squamous cell cancer.

For example, organ transplant patients are usually given medicines that weaken their immune system to prevent their body from rejecting the new organ. This increases their risk of developing skin cancer. The rate of skin cancer in people who have had transplants can be as high as 80% within 20 years after the transplant. Skin cancers in people with weakened immune systems grow faster and are more likely to be fatal.

Treatment with corticosteroid drugs can also depress the immune system. This may also increase a person's risk of skin cancer.

Human papilloma virus (HPV) infection

Human papilloma viruses (HPVs) are a group of more than 100 viruses that can cause papillomas, or warts. The types of warts that people commonly get on their hands and feet appear to be unrelated to any form of cancer. But some of the HPV types that people get in their genital and anal area appear to be related to skin cancers in these areas.

Smoking

People who smoke are more likely to develop squamous cell skin cancer, especially on the lips. Smoking is not a risk factor for basal cell cancer.

Genetic susceptibility

Scientists have found that certain people are more likely to develop (susceptibility) skin cancer than others after sun exposure. In these people, the chromosomes are more sensitive to being damaged by sunlight.