Lung cancer is the second most commonly diagnosed cancer in the United States and the leading cause of cancer-related death [30, 31], contributing to an estimated 158,080 deaths in 2016 alone [32]. Although many studies show significant variation in nonmetropolitan-metropolitan differences in lung cancer incidence on a national scale [19], we report a consistent higher incidence of lung cancer in nonmetropolitan Utah counties than metropolitan counties. This pattern is opposite the trend seen in other SEER sites across the United States, where lung cancer incidence is higher in metropolitan regions than nonmetropolitan regions. After controlling for smoking, we find that nonmetropolitan areas in Utah have a significantly higher rate of distant lung cancers, suggesting that nonmetropolitan region of residence is a contributing factor to late stage cancer at diagnosis and may influence mortality.
All metropolitan Utah counties have predicted indoor radon levels between 2 and 4 pCi/L, but there is greater variability estimated for nonmetropolitan counties, which have some of the highest radon levels in the state. When we compare lung cancer rates in moderate radon regions, we see that the rate of lung cancer in nonmetropolitan, moderate radon and metropolitan, moderate radon counties is nearly identical. In contrast, overall incidence in high radon, nonmetropolitan counties is significantly higher than moderate radon, nonmetropolitan counties after adjustment for smoking. These results suggest that radon is a significant contributor to higher lung cancer incidence in high radon, nonmetropolitan Utah counties even after smoking is taking into consideration. Since the incidence rate among men is significantly higher in high radon counties than moderate radon counties after adjustment for smoking, radon may be a significant contributor to the burden of lung cancer among males residing in nonmetropolitan counties.
Utah has the lowest smoking prevalence and the lowest number of lung cancer cases attributed to smoking than any other state in the United States [33, 34], but we find that the prevalence of smoking in nonmetropolitan Utah counties from 2006 to 2010 of 18.2% is similar to the current national average of 15.1% [13]. The nonmetropolitan counties with the highest predicted indoor radon levels also have the highest smoking prevalence, meaning that residents of these counties may be at risk for dual exposure to radon and tobacco. In occupationally exposed populations, we find a higher absolute increase in risk per unit of radon in smokers than non-smokers, but larger relative risk per unit of radon in nonsmokers [10]. Although occupational radon exposure differs in dose and duration from residential radon exposure, these findings support a complex relationship between smoking status and radon that should be further explored in this low-smoking population. These regional differences in smoking prevalence should be taken into account when planning community-based interventions and epidemiologic research in geographically diverse states.
In addition to environmental risk factors for disease, populations in nonmetropolitan Utah counties display patterns of socioeconomic deprivation seen nationwide in similar nonmetropolitan regions [18, 20]. Utah’s nonmetropolitan counties consistently have a lower percent of residents with a bachelor’s degree and a higher percent of families living under the federal poverty level than metropolitan counties. Less education and poverty are risk factors for poorer cancer survival, and may impact nonmetropolitan populations differently than metropolitan populations [20]. In our study, the greatest social and economic disparities are seen between moderate radon, metropolitan counties and high radon, nonmetropolitan counties.
Since lung cancer prevention efforts largely focus on reducing smoking or other tobacco-use behaviors, additional funding should be dedicated towards reducing radon exposure as a significant risk factor. National educational efforts related to increasing awareness about radon and preventing exposure are limited. Out of the 65 state and territory cancer prevention plans created between 2005 and 2011, only 27, including Utah, mentioned activities related to radon and lung cancer prevention [35]. Utah’s 2016 to 2020 Comprehensive Cancer Prevention and Control Plan identifies radon as a priority, with an emphasis on testing homes for radon [16].
Despite these past efforts, awareness about the role of radon in lung cancer and the need for testing in Utah is low. Only 51.6% of the respondents in Utah’s 2013 Behavioral Risk Factor Surveillance System (BRFSS) could correctly identify lung cancer as a health risk of radon exposure, and only 20% of BRFSS respondents report testing their homes for radon [36]. When asked about reasons why they have not tested their home for radon, 34% say that they had not thought about it, 14% think they are not at risk, and 13% say that they do not know about radon [36]. These results demonstrate a need for radon awareness and mitigation activities directed towards the public.
This ecologic study is limited in the conclusions it can make about the dual roles of geographic area and radon in lung cancer in Utah. Ecologic studies are able to make group-level inferences that may not be true on the individual level [37]. However, we are able to identify county-level associations of radon and lung cancer in Utah. We are not able to examine how social and economic characteristics of the Utah population are associated with radon exposure and lung cancer incidence. We are not able to control for county-level air pollution, which is a potential confounder of the association between lung cancer and nonmetropolitan and metropolitan area of residence. Since the majority of traffic and other sources of pollution are in the metropolitan areas, we expect that air pollution will increase the rate of lung cancer cases in metropolitan counties, which is the opposite of what we report. Because of this, we do not expect air pollution to be a confounder in this study, but we will examine air pollution in future studies. We also have a relatively small number of counties (n = 19) since Utah is a relatively small state in terms of population. We are unable to adjust for calendar year, which may be an important confounder as it is a surrogate measure of lung cancer trends by county. Although we adjusted for smoking on the county-level, we may have some residual confounding from smoking from the study design and method of adjustment for smoking.
Despite these limitations, we are able to identify the age-adjusted lung cancer incidence rate for metropolitan and nonmetropolitan counties in Utah over a 20 year period, and examine differences in lung cancer incidence rates by radon level in nonmetropolitan counties. While county level estimates can be viewed as a weakness relative to individual or household exposure, a county-level assessment is still valuable in generating possible explanations for population trends in lung cancer incidence. Our study suggests that radon is a serious risk factor for lung cancer in the low smoking population of Utah, especially within nonmetropolitan counties.
Future research should assess patterns of radon testing, barriers to radon mitigation, and develop interventions and public health programs to improve awareness about radon and access to radon mitigation procedures. Previous interventions that provided tailored information and guidance to households and in primary care settings are effective at increasing awareness [38,39,40]. Differences in access to lung cancer screening and delays in treatment by geographic area, and the cumulative impact of smoking, radon exposure, and socioeconomic risk factors on lung cancer incidence and survival among nonmetropolitan residents in should be explored in future studies.