The incidence of malignant melanoma has risen significantly over the last 30 to 40 years, and continues to increase in the United States, Canada, Australia and Europe, becoming a major public health concern nevi.pdf. A number of studies have identified three categories of major risk factors:
- Environmental - Solar ultraviolet (UV) exposure is the major environmental risk.
- Constitutional - Light skin and hair colour and propensity to burn rather than tan in the sun are genetically based constitutional risk factors.
- Combination - Acquired melanocytic nevi and freckles combine the risks of constitutional susceptibility and environmental UV exposure.
The single greatest predictor of risk for developing melanoma is the total number of nevi. Studies over the last 15 years have revealed a great deal about the way nevi develop and the relationship between nevi and melanoma. The studies can be divided into adult and child investigations to determine the prevalence of nevi by sex and age, and to identify factors associated with a high number of nevi.
Studies of Nevi in Adults
The findings on adult nevi are somewhat contradictory, because many of the studies were conducted over broad age ranges and may have differences in UV exposure and the number of nevi. Highlights include:
- Sunburn frequency up to ten years old showed the strongest relationship with the number of nevi in one study, suggesting childhood sun exposure is connected to nevus prevalence in young adults.
- Sun exposed surfaces had higher counts than less exposed areas.
Different results may arise because people who are most likely to burn avoided the sun.
Studies of Nevi in Children
Recent research focuses on childhood and adolescence since most nevi develop by age 20. Results have shown:
- Boys developed more nevi than girls, and the number of nevi increased with age. Girls showed substantial gains between 13 and 18 in one study.
- Nevus density increased to about age 14.
- Nevi were more common in sun sensitive children, based on predisposition to burn or tan, light eye and hair colour, freckling, and a history of childhood sunburns.
- Higher counts were seen in children with a family history of skin cancer.
- The number of nevi increased among children who lived closer to the equator.
Results among children are clearer than for adults. Nevus density increases up to ages 9-14, and peaks in areas with higher sun exposure, which suggests exposure is involved in nevus development. It's also been suggested sunlight may be involved in the disappearance of nevi in older people. Nevus counts appear to be higher in children with light skin and a propensity to burn. Children with a history of sunburns, particularly with blisters, have higher counts than those who have not burned.
Pre-existing nevi are present in about 50% of adult melanomas, suggesting some nevi may be precursors of melanoma. As a result, reducing nevi in children may substantially lower melanoma rates as they move into adulthood. Regularly covering the skin with clothing, using a high SPF sunscreen containing Parsol 1789, and minimizing sun exposure between 11 am and 3 pm can prevent the development of some nevi.
Refer to the complete journal article for additional details on individual studies nevi.pdf.