Does solar ultraviolet irradiation affect cancer mortality rates in China?

Solar ultraviolet B (UVB) has been found to correlate with reduced risk for 14 types of cancer in three or more observational studies and another 14 in one-to-two observational studies. The beneficial role of UVB is thought to be mediated through vitamin D production. Few such studies have been conducted in Southeast Asia. Data on cancer mortality rates for 65 counties in China in 1978 and approximately 300 geographic, dietary, serum, occupation, and lifestyle factors from 1983-4 are available in Diet, Life-style and Mortality in China (Chen et al., Oxford University Press, 1990). The data for 39 counties away from the east coast of China were used in multiple linear regression analyses. The indices of solar UV radiation (UVR), latitude and heat index, were correlated with reduced mortality rates for cervical, colorectal (females), esophageal, gastric, and lung (males) cancer. Latitude was inversely correlated with liver cancer (males) and nasopharyngeal carcinoma (NPC). Lung cancer, the index used for smoking, was correlated with all less lung (males), cervical, liver (males), and NPC. Several other factors were also correlated with some of the cancers. However, no other factors could explain the latitudinal variation for these seven cancers. Thus, it is concluded that solar UVB, through production of vitamin D, reduces the risk of some types of cancer in China. Liver cancer and NPC are linked to viruses, and UVR may increase the risk through immunosuppression. Further studies are warranted.