Re: “Precipitation and Climate Zone Explains the Geographical Disparity in the Invasive Cancer Incidence Rates in the United States” by Shah,Rieger,and Pan ( Environ. Eng. Sci. 2019;36(12):1452–1458;DOI: 10.1089/ees.2019.0241)

Dear Editor:

The article by Shah, Rieger, and Pan proposed that precipitation and climate zone explain the geographical disparity in the invasive cancer incidence rates in the United States (Shah et al., 2019). Although there may be some merit to their hypothesis, they overlooked the most important risk-modifying factor, solar ultraviolet B (UVB) dose, an important source of vitamin D. A quick glance at their figure 1 indicates that the cancer incidence rates have a much higher correlation with solar UVB doses in summer (Leffell and Brash, 1996) than with climate regions in the United States shown in figure 2. Vitamin D has been found to reduce the risk of cancers through observational studies (Grant and Boucher, 2017; McCullough et al., 2019) and the secondary analyses of a recent large-scale clinical trial (Manson et al., 2019). Articles cited in Mandal et al. (2009), which is cited in Shah et al. (2019), relate to UVB exposure, vitamin D, and risk of cancer. An article cited by Mandal et al. reported that the geographical variation in incidence and mortality rates for many types of cancer had similar relationships with respect to solar UVB doses (Boscoe and Schymura, 2006).

The seminal study reporting that annual solar radiation was inversely correlated with colon cancer mortality rates and that vitamin D production was the likely reason was by the brothers Cedric and Frank Garland (Garland and Garland, 1980). My first article on the topic reported that 13 types of cancer had mortality rates significantly inversely correlated with solar UVB doses (Grant, 2002). The second study included several additional cancer risk-modifying factors [alcohol consumption, Hispanic heritage, poverty level, urban/rural residence, and lung cancer (an index of the health effects of smoking)] and confirmed the findings regarding UVB from the first study (Grant and Garland, 2006). Another study found similar results for black Americans, although for fewer types of cancer (Grant, 2006). There is also one related to acid precipitation, thought to be an index of particulate air pollution acting such as tobacco smoke (Grant, 2009). A number of other single-country ecological studies are discussed in a later review (Moukayed and Grant, 2013).

Returning to the study by Shah et al., precipitation may be directly correlated with cancer incidence, but probably through reduction in exposure to UVB rather than any other effects. Both climate and solar UVB doses are related, so most of the effects of climate are probably related to lower UVB exposure.