Causes of Melanoma

When a melanocyte becomes malignant, it begins to proliferate uncontrollably. Melanoma can arise in an existing mole, or it can arise de novo, that is, by itself in otherwise unblemished skin.

Like other forms of skin cancer, excessive sun exposure is a primary cause of melanoma. Fair-skinned people (e.g., those with red or blond hair, blue eyes, and freckles) sunburn easily, causing cell damage. Eventually, the cell's genetic material can change (mutate) and the melanocyte becomes a malignant cell.

There are several notable risk factors for melanoma, including the following:

Number and Thickness of Common Nevi (Moles)

Nevus (pl., nevi) is the medical term for a mole. Various studies have been conducted to determine if the number and thickness of nevi and their location on the body increases the risk for melanoma. Most people have 10 to 40 common moles on their body, which are not precancerous. People with more than 100 moles are at increased risk for developing melanoma simply because there is a greater chance that one of the moles will become cancerous.

Mole thickness is another important factor in determining risk. Studies have shown that a person with more than 120 nevi between 1 mm and 5 mm thick has nearly 20 times the risk of developing melanoma. A person with more than 5 nevi that are between 5 mm and 10 mm thick is at 10 times the risk. There may be a correlation between the number of raised nevi (a nevus that is greater than 2 mm high) on the arms and the development of melanoma. There may also be a correlation between the number of nevi on the buttocks and the development of melanoma.

Presence of Atypical (Dysplastic) Nevi

People with more than three to six atypical nevi have an increased risk of developing melanoma. People with more than six atypical nevi also tend to have an extensive history of UV radiation exposure. Thus, the correlation of atypical nevi quantity and melanoma might be directly related to exposure to ultraviolet radiation. Importantly, most atypical nevi never develop into melanoma. When melanoma does develop, it is not necessarily located in the mole itself; it can develop in normal skin.

Family History of Melanoma

Five to 10 percent of all melanoma patients have a first-degree relative who has or has had melanoma. If a person has at least one first-degree relative who has had melanoma, the risk may be 8 times greater than for people with no family history.

Presence of Dysplastic Nevus Combined with a Family History of Melanoma

A person with a dysplastic nevus who has two or more first-degree relatives with melanoma may be 100 times more likely to develop melanoma.

Light Skin and Freckling Tendency

In the United States, Caucasians are 20 times more likely to develop melanoma than African Americans. People with light skin who sunburn easily and tan poorly are more likely to develop melanoma than those who have more pigment. This is true whether or not a person has ever had a sunburn.

Intermittent Sun Exposure and Severe Sunburns

One study showed that people who have had three or more episodes of severe sunburn during their childhood are at an increased risk for developing melanoma. It is believed that the amount of time spent outdoors between the ages of 10 and 24 is most crucial to the incidence of melanoma. Brief, intermittent, intense periods of UV radiation exposure are considered more dangerous than continuous but less intense exposure, even if the total amount of exposure is the same.

Exposure to Ultraviolet Radiation Due to Atmospheric Ozone Depletion

People who live in areas of the world where the ozone layer has been depleted (e.g., the poles, large cities) may be at a greater risk for developing melanoma, but the association is not clear. The ozone layer prevents harmful ultraviolet rays from penetrating the atmosphere, and ozone depletion may result in greater radiation exposure.

Excessive Use of UV Tanning Beds

Studies have shown that the use of UV tanning beds and exposure to ultraviolet radiation from UV lamps increase the risk for developing melanoma. There are no safe tanning rays and any exposure, whether in a tanning booth or outdoors, is simply exposure to harmful ultraviolet radiation.

History of Melanoma

A person who has already been treated for melanoma has a 3 to 7 percent greater risk of developing it a second time.

Xeroderma Pigmentosum

Xeroderma pigmentosum is a rare, hereditary skin disease that can affect all races and starts in childhood. It results from a defect in an enzyme that normally repairs ultraviolet-damaged DNA. People with this disease are at 1000 times the risk for developing melanoma.

Suppression of the Immune System

People whose immune systems have been suppressed by medication (e.g., organ transplant patients) have a greater risk for developing melanoma.


The simple truth is that the longer skin is around, the more sun exposure it receives. Therefore, skin cancers, especially melanomas, are more prevalent in older people. For this reason, physicians recommend that people begin protecting their skin from sun exposure at an early age.

Carcinogenic Risk Factors

The American Cancer Society has identified the following environmental carcinogens as risk factors for melanoma: coal, tar, and pitch (which are used in road paving and in some dyes); creosote (which is a wood preservative derived from tar); arsenic compounds (which are used in pesticides); and radium.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 14 Aug 1999

Last Modified: 24 Aug 2015