Ivanka Stajner
Traditional risk factors for breast cancer explain only a fraction of cases. Causes for trends in breast cancer incidence are not fully understood. Breast cancer incidence and mortality rates decrease with environmental conditions that promote Vitamin D synthesis in human skin including lower latitude and higher personal exposure to sunlight. Association of temporal variability in breast cancer incidence with changes in cloudiness, which decrease human Vitamin D synthesis is investigated. Association between temporal changes in breast cancer incidence and in the autumn cloudiness for preceding years is computed using data for the United States. There is a correlation of 0.96 (95%CI=0.92-1) between the time series of breast cancer incidence in the age group of 70-79 years and the average cloudiness in October during preceding 20 years. An empirical model for breast cancer incidence using autumn cloudiness in preceding years captures a rapid increase in breast cancer incidence in the 1980s and some of its yearto- year variability. Increased autumn cloudiness is associated with increased subsequent breast cancer incidence. Proposed mechanism includes blocking of solar ultraviolet radiation by thick clouds and decreasing the synthesis of Vitamin D in human skin. The findings suggest a new connection between climate variability and human health.
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