Solar ultraviolet irradiance and cancer incidence and mortality

The solar ultraviolet-B (UVB)/vitamin D/cancer hypothesis was proposed by the brothers Cedric and Frank Garland in 1980. In 2002, the list was increased to 15 types of cancer using data in the 1999 version of the atlas of cancer mortality rates in the United States. Ecological studies of cancer incidence and/or mortality rates with respect to indices of solar UVB doses have also been reported for Australia, China, France, Japan, and Spain with largely similar findings. In addition, several studies using nonmelanoma skin cancer as the index of solar UVB dose have found reduced internal cancer incidence and/or mortality rates, especially in sunny countries. A study of cancer incidence with respect to 54 categories of occupation in five Nordic countries, using lip cancer less lung cancer as the UVB index, found this index inversely correlated with 14 types of internal cancers for males and four for females. Observational studies with respect to UVB doses and serum 25-hydroxyvitamin D [25(OH)D] concentrations also support the hypothesis. Hill's criteria for causality in a biological system to assess whether solar UVB and vitamin D can be considered causal in reducing risk of cancer. The primary criteria for this analysis include strength of association, consistent findings in different populations, biological gradient, plausibility (e.g., mechanisms), and experimental verification (e.g., randomized controlled trials). The totality of evidence is judged to satisfy the criteria very well for breast and colorectal cancer, and moderately well for several other types of cancer.