Any substance or condition that increases cancer risk is referred to as a risk factor. The most important risk factor for developing melanoma is exposure to sun and ultraviolet radiation. However, most cancers are the result of many risk factors.
*Risk factors are generally listed in order from most significant to least significant. In most cases, it is impossible to rank the relative significance of individual risk factors with absolute certainty.
Melanoma rates are increasing in both men and women. Older people have a greater risk of developing melanoma. However, melanoma is also found in younger people. In Canada, melanoma is slightly more common in men than women.
The following factors are known to increase the risk of developing melanoma.
Sun and ultraviolet radiation
Ultraviolet radiation (UVR) is the most important risk factor for developing skin cancer. The 3 types of UVR are UVA, UVB and UVC. The main source of UVR in the environment is the sun. Most cases of melanoma skin cancer are caused by intermittent exposure and exposure early in life to the sun's UVR. UVB is the UVR most strongly associated with developing melanoma.
Indoor tanning equipment, such as tanning beds and sun lamps, is also a source of UVR. A tan is evidence of skin damage from exposure to UVR. Just like the sun, indoor tanning equipment gives off UVR that can cause melanoma, squamous cell carcinoma, sunburns, premature aging and cataracts.
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Number of moles
A mole (melanocytic nevus) is a non-cancerous lesion (area of abnormal tissue) on the skin. Moles occur when melanocytes (cells that produce melanin, the substance that gives colour to our skin, hair and eyes) grow together in a group. Moles are not usually present at birth but often begin to appear in children.
Most moles are harmless, but a person who has many moles is more likely to develop melanoma.
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Atypical moles
An atypical mole, also known as a dysplastic nevus, has some of the same characteristics of a melanoma but is not cancerous. Atypical moles look different from normal moles. They tend to be larger (greater than 6 mm) and have an irregular shape with undefined borders. A normal mole is usually round and smaller than 6 mm.
Atypical moles are often multicoloured, ranging from pink to dark brown. Having these unusual moles may increase a person’s risk of developing melanoma. The risk of developing melanoma also increases with the number of atypical moles.
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Congenital melanocytic nevi
Congenital melanocytic nevi are those moles present at birth. They may be classified as small (less than 1.5 cm), medium (1.5–19.9 cm) or large (greater than 5% body surface area in preteens or greater than 20 cm in teens or adults). The risk of developing melanoma increases with the size of the mole. People with very large congenital melanocytic nevi have a greater risk, while the risk is lower for those with small nevi.
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Familial atypical multiple mole melanoma (FAMMM) syndrome
FAMMM syndrome is an inherited condition characterized by family members diagnosed with melanoma and a family history of large numbers of atypical moles of different sizes (asymmetrical, raised) and shades (tan, brown, black or red). These people have a very high risk of developing melanoma.
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Having a fair complexion
Having a fair complexion increases a person's risk of melanoma. People with fair skin or a light complexion are at a higher risk of developing melanoma than people with other skin types. This includes:
- skin that burns easily, freckles and doesn’t tan
- blue, green or other light-coloured eyes
- red or blond hair
The risk of skin cancer is greater for this group of people because the skin has less pigment (melanin) to act as a protective factor against UVR. People most at risk are those who had intense exposures to the sun at an early age that resulted in sunburns.
Melanoma in dark-skinned people is very rare. It is thought that the melanin (the substance that gives colour to our skin, hair and eyes) in the epidermis of the skin is a protective factor. Melanomas that do occur are most often on areas not exposed to the sun, such as the soles of the feet, palms of the hands and on mucous membranes.
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Personal history of skin cancer
People who have a history of melanoma have a higher risk of developing another primary melanoma. A previous history of basal cell carcinoma or squamous cell carcinoma is associated with a higher risk of melanoma.
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Family history of skin cancer
A person's risk of developing melanoma increases if one or more of their first-degree relatives (father, mother, brother, sister or child) has been diagnosed with melanoma. Skin colour, sun exposure and, to a lesser extent, genetics may all be factors related to this risk.
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CDKN2A gene mutation
If several family members have the same type of cancer, or if the family members have related types of cancer, they might have an inherited gene mutation. Approximately 5%–25% of families with a higher risk for melanoma have germ-line mutations in the CDKN2A tumour suppressor gene.
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Weakened immune system
People who have a weakened immune system or who must take drugs to suppress their immune system following an organ transplant have an increased risk of developing melanoma.
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History of severe blistering sunburn
People who have had at least one blistering sunburn as a child or teenager have an increased risk of developing melanoma later in life. The more sunburns a person has had, the greater the risk of melanoma.
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Other health conditions
The following conditions can increase the risk of developing melanoma:
- Xeroderma pigmentosum (XP) is a rare genetic disease that affects the skin's ability to repair damage caused by the sun. It is characterized by greater sensitivity to UVR, skin colour changes, premature skin aging and skin cancer. People with XP have a high risk of developing both melanoma and non-melanoma skin cancer.
- Werner syndrome is a rare inherited disorder that causes premature aging and is most common in people from Japan. People with this condition have an increased risk of developing melanoma.
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Possible risk factors
The following factors have some association with melanoma, but there is not enough evidence to say they are known risk factors. Further study is needed to clarify the role of these factors for melanoma.
- melanocortin 1 receptor (MC1R) gene mutation – MC1R is associated with pigmentation and certain variations of the MC1R gene determine red hair and fair skin. People with an MC1R variant may have a higher risk of developing melanoma.
- exposure to ionizing radiation – People who have had radiation therapy in the past or are victims of nuclear fallout are likely to have a higher risk of developing melanoma. Research has shown that some occupations that work with ionizing radiation, such as radiation technologists, have higher rates of melanoma.
- exposure to chemicals – Occupational exposure to polychlorinated biphenyl (PCB) is likely associated with a higher risk of melanoma.
- PUVA – PUVA (psoralen + UVA) therapy is a common treatment for skin conditions like psoriasis. The drug psoralen is given to make the skin more sensitive to UVA light. Long-term PUVA therapy may increase a person’s risk of developing melanoma.
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Unknown risk factors
The following are factors for which there is not enough evidence or the evidence is inconclusive. In other words, it can't be determined for sure whether these risk factors are or are not associated with melanoma.
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See a list of questions to ask your doctor about risks.