Spatiotemporal and Spatial Threshold Models for Relating UV Exposures and Skin Cancer in the Central United States

Testing the validity of a proposed dermal cancer slope factor for Benzo[a]pyrene

In 2014, the United States Environmental Protection Agency (EPA) proposed a Dermal Slope Factor (DSF) for benzo[a]pyrene (BaP) of 0.006 (μg/day)^-1 (USEPA 2014a). It would make cancer risk estimates associated with soil contact 100 times greater than those from soil ingestion and would predict that a large fraction of skin Basal Cell Carcinomas (BCCs) and Squamous Cell Carcinomas (SCCs) worldwide are caused by low level dermal exposures to PAHs, such as BaP. This is not logical given that sunlight (ultraviolet radiation (UV)) exposure is the generally recognized cause of BCCs and SCCs. This paper critically evaluates the proposed DSF. First, a reality check is performed using EPA standard risk assessment methods and comparing the results to actual BCC and SCC rates in the U.S. population. Then, the biological plausibility of the mechanism by which PAHs might cause human skin cancer is evaluated by exploring the generally recognized etiology of human skin cancer and comparing the genetic mutation signatures of rodent skin tumors caused by PAH exposures to those of human skin cancers. It is concluded that scientific flaws resulted in a proposed DSF value that greatly overestimates the skin cancer risk for humans dermally exposed to BaP in soil.

Compilation and spatio-temporal analysis of publicly available total solar and UV irradiance data in the contiguous United States

Skin cancer is the most common type of cancer in the United States, the majority of which is caused by overexposure to ultraviolet (UV) irradiance, which is one component of sunlight. National Environmental Public Health Tracking Program at CDC has collaborated with partners to develop and disseminate county-level daily UV irradiance (2005-2015) and total solar irradiance (1991-2012) data for the contiguous United States. UV irradiance dataset was derived from the Ozone Monitoring Instrument (OMI), and solar irradiance was extracted from National Solar Radiation Data Base (NSRDB) and SolarAnywhere data. Firstly, we produced daily population-weighted UV and solar irradiance datasets at the county level. Then the spatial distributions and long-term trends of UV irradiance, solar irradiance and the ratio of UV irradiance to solar irradiance were analyzed. The national average values across all years are 4300 Wh/m2, 2700 J/m2 and 130 mW/m2 for global horizontal irradiance (GHI), erythemally weighted daily dose of UV irradiance (EDD) and erythemally weighted UV irradiance at local solar noon time (EDR), respectively. Solar, UV irradiances and the ratio of UV to solar irradiance all increased toward the South and in some areas with high altitude, suggesting that using solar irradiance as indicator of UV irradiance in studies covering large geographic regions may bias the true pattern of UV exposure. National annual average daily solar and UV irradiances increased significantly over the years by about 0.3% and 0.5% per year, respectively. Both datasets are available to the public through CDC's Tracking network. The UV irradiance dataset is currently the only publicly-available, spatially-resolved, and long-term UV irradiance dataset covering the contiguous United States. These datasets help us understand the spatial distributions and temporal trends of solar and UV irradiances, and allow for improved characterization of UV and sunlight exposure in future studies.

Minimal and maximal incidence rates of skin cancer in Caucasians estimated by use of sigmoidal UV dose-incidence curves

BACKGROUND

Sigmoidal (S-shaped) dose-cancer incidence relationships are often observed in animal bioassays for carcinogenicity. Ultraviolet (UV) radiation is an established skin carcinogen. The aim of this study is to examine if S-shaped curves describe the relationship between solar UV doses and skin cancer incidences, and if such relationships can be used to estimate threshold levels of non-carcinogenic UV exposure, as well as maximal incidence rates.

METHODS

We studied the incidence rate-annual erythema-effective UV dose relationship for squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and cutaneous melanoma (CM) among different Caucasian populations in Europe, Australia and New Zealand.

RESULTS

Our analysis indicates that S-shaped associations describe the data well (P < 0.0001). The age-adjusted incidence rates for cases expected to be due to other causes than solar UV exposure (at zero UV dose) were found to be around 0.6, 9.7 and 4.0 per 100,000 for women in 1997-2007 for SCC, BCC and CM, respectively, and around 1.2, 14.3 and 2.6 per 100,000 for men. The analysis indicates that SCC, BCC and CM have maximal incidence of 361 ± 24, 1544 ± 49 and 36 ± 4 per 100,000 for women, and 592 ± 35, 2204 ± 109 and 50 ± 4 per 100,000 for men.

CONCLUSIONS

Between 89 and 95% of the annual CM cases, around 99.8% SCC and 99.4% BCC cases are caused by solar UV exposure. The analysis did not identify any "safe" UV dose below which the risk for skin cancer was absent. Avoidance of UV radiation has a potential to reduce the incidence of skin cancer in fair-skinned population.

Skin cancer incidence is highly associated with ultraviolet-B radiation history

Recently, the increased amount of ultraviolet-B (UV-B) exposure due to ozone depletion has been found to be associated with increased incidence of skin cancer across the world. The quantification of individual, regional, and historical UV exposure directly affects establishment of the association between skin cancer and UV exposure, but accurate assessment and measurement have been challenging for decades. As a sequence, cumulative studies using different metrics reported conflicting results on whether UV radiation, including sunburns, early childhood sun exposure, and chronic exposure, increases melanoma risk. This paper aims to establish the relationship between UV-B and melanoma incidence across the continental U.S. using an ecological approach that incorporate more accurate UV-B exposure measured by the National Aeronautical and Space Administration Nimbus-7 total ozone mapping spectrometer, and the United State Department of Agriculture ground-based network. Using statistical linear mixed models, we found strong positive associations between the skin cancer and the past UV exposure or the past cumulative 3-year UV exposure 3 or 4 years ago. UV has regional distributions and its regional effects on the skin cancer incidence are still significant after adjusting the effect of UV exposure. Research findings yield deepened understanding of spatiotemporal distribution of melanoma incidence rates and a greater appreciation for the complexity and heterogeneity of melanoma risk factors especially the UV-B exposure at different temporal and spatial scales.