A risk factor is anything that affects your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, smoking is a risk factor for cancers of the lung, mouth, larynx, bladder, kidney, 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 melanoma has a risk factor, it is often very hard to know how much that risk factor may have contributed to the cancer.

Scientists have found several risk factors that may make you more likely to develop melanoma.

Ultraviolet (UV) light exposure

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

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.

The nature of the UV exposure may play a role in melanoma development. Many studies have linked the development of melanoma in the trunk, legs, and arms to frequent sunburns (especially in childhood). The fact that these areas are not constantly exposed to UV light may also be important. Some experts think that melanomas in these areas are different from those on the face and neck, where the sun exposure is more constant. And different from either of these are melanomas that develop on the palms, soles, nails or internal surfaces such as the mouth and vagina, where there has been little or no sun exposure.

Moles

A nevus (the medical name for a mole) is a benign (non-cancerous) melanocytic tumor. Moles are not usually present at birth but begin to appear in children and teenagers. Most moles will never cause any problems, but a person who has many moles is more likely to develop melanoma.

A dysplastic nevus, or atypical mole, is a type of mole that particularly increases a person's risk of melanoma. Dysplastic nevi (nevi is the plural of nevus) often look a little like normal moles but also look a little like melanoma. (Refer to the section, "Can melanoma skin cancer be found early?" for descriptions of the appearance of moles and melanomas.) They can appear in areas that are exposed to the sun as well as those areas that are usually covered, such as the buttocks and scalp. They are often larger than other moles.

A small number of dysplastic nevi may develop into melanomas. But most dysplastic nevi never become cancerous, and many melanomas seem to arise without a pre-existing dysplastic nevus.

Dysplastic nevi often run in families. If you have family members with many dysplastic nevi you have about a 50% chance of developing these nevi. Someone with 1 or more dysplastic nevi and with at least 2 close relatives with melanoma has a 50% or greater risk of developing melanoma.

Lifetime melanoma risk is estimated to be between 6% and 10% for those with many dysplastic nevi (sometimes referred to as dysplastic nevus syndrome). People with this condition should have very thorough periodic skin exams by a dermatologist (a doctor who specializes in skin problems). In some cases, full body photographs are taken at regular intervals to help the doctor recognize which moles are changing and growing. Many doctors recommend that patients be taught to do monthly skin self-exams and be counseled about sun protection.

Moles present at birth are called congenital melanocytic nevi. The lifetime risk of getting melanoma for people with congenital melanocytic nevi has been estimated to be between 0 and 10%, depending on the size of the nevus. People with very large congenital nevi have a greater risk, while the risk is smaller for those with small nevi. Congenital nevi are sometimes removed by surgery so that they do not have a chance to become cancerous. Whether or not doctors advise removing a congenital nevus is influenced by several factors including its size, location, color, and texture. Many doctors recommend that congenital nevi that are not removed should be examined at regular intervals by a dermatologist and that the patient should be taught how to do monthly skin self-exams.

The chance of any single mole turning into cancer is very low. However, anyone with lots of irregular or large moles has an increased risk for melanoma.

Fair skin, freckling, and light hair

The risk of melanoma is more than 10 times higher for whites than for African Americans. This is because skin pigment has a protective effect. Whites with red or blond hair or fair skin that freckles or burns easily are at increased risk. Red-haired people have the highest risk.

Family history of melanoma

Your risk of melanoma is greater if 1 or more of your first-degree relatives (mother, father, brother, sister, child) has been diagnosed with melanoma. Around 10% of all people with melanoma have a family history of melanoma.

The increased risk may be due to a shared family lifestyle of frequent sun exposure, a family tendency to have fair skin, or a combination of both factors. It may also be due to inherited gene changes (mutations) in a family. Gene mutations have been found in anywhere from about 10% to 40% of families with a high rate of melanoma. Most experts do not recommend genetic testing in these families at this time. Rather, they advise that people with a strong family history of melanoma do the following:

• have regular skin exams by a dermatologist
• do thorough skin self-exams once a month
• be particularly careful about sun protection

Personal history of melanoma

A person who has already had melanoma has an increased risk of getting melanoma again. About 5% to 10% of people with melanoma will develop a second one at some point.

Immune suppression

People who have been treated with medicines that suppress the immune system, such as organ transplant patients, have an increased risk of developing melanoma.

Age

Although melanoma is less related to aging than most other cancers, it is still more likely to occur in older people. But this is one of the few cancers that is also found in younger people. In fact, melanoma is one of the most common cancers in people younger than 30. Melanoma that runs in families may occur at a younger age.

Gender

Men have a higher rate of melanoma than women.

Xeroderma pigmentosum

Xeroderma pigmentosum (XP) is a rare, inherited condition resulting from a defect in an enzyme that normally repairs damage to DNA. People with XP have a high risk for developing both melanoma and basal cell and squamous cell skin cancers at a young age. Because people with XP are less able to repair DNA damage caused by sunlight, they can develop many cancers on sun-exposed areas of their skin.