[Role of sun exposure on benign melanocytic nevi. A first study in populations controlled for age, sex and phenotype].

BACKGROUND: These studies were designed to assess the influence of sun exposure on nevi in white people. In order to eliminate the confounding effect of age, sex and phenotype, two parallel studies were conducted in people of the same age (17 to 24 years), sex (males) and phenotype: one in people with "red" phenotype and one in people with "dark" phenotype. RESULTS: In both groups, comparison of nevus counts on the inner and outer side of the upper extremities and comparison of mean density of nevi (number per square meter) in always-exposed and never-exposed skin show that number of nevi is higher in sun exposed areas. In both groups the density of large and atypical nevi was maximal on intermittently sun-exposed skin while the density of small nevi was maximal on always-exposed skin. The number of large nevi on intermittently-exposed skin correlated with cumulative intensive exposure during beach recreation in the red phenotype group. The number of large nevi was significantly higher in red phenotypes who repeatedly experienced severe sunburns in their first 20 years of life. CONCLUSIONS: The number of nevi at the end of the second decade is influenced by cumulative sun exposure from birth. "Traumatizing" sun exposure, which is more frequent in "red" phenotype than in "dark" one, has an influence on the number of large nevi and is therefore likely to make small nevi grow.