Impact of declining exposure to secondhand tobacco smoke in public places to decreasing smoking-related cancer mortality in the US population
Introduction
Secondhand tobacco smoke (SHS), the combination of the side-stream smoke emitted from the burning end of a tobacco product and the mainstream smoke exhaled by the smoker, contains >6000 chemicals and is carcinogenic to humans (International Agency for Research on Cancer (IARC), 2012). The 2006 Surgeon General's Report on the Health Consequences of Involuntary Exposure to Tobacco Smoke concluded evidence is sufficient to infer that SHS exposure causes lung cancer (Moritsugu, 2007). Other health effects causally linked to SHS exposure include ischemic heart disease and stroke in adults, and lower respiratory tract infections and impaired lung function in children (Alberg et al., 2014). For many other diseases, there is suggestive evidence for causality (Moritsugu, 2007).
Exposure to SHS can be assessed by interviews or by measuring the levels of cotinine (a nicotine by-product in the body) in biological samples. According to self-reported information and serum cotinine concentrations, exposure to SHS in the United States has declined intensely during the past three decades (Disparities in secondhand smoke exposure–United States, 2008). This phenomenon has been attributed to downward trends in smoking rates and intensity, and to increases in the number of states with smoke-free policies in public places. As of June 30, 2016, a total of 29 US states and the District of Columbia have enacted statewide bans on smoking in all public places and workplaces, including bars, restaurants and private worksites (CDC (Centers for Disease Control and Prevention)).
Cancer death rates in the US, including those that arise at the most common sites (i.e. lung and bronchus, colon and rectum, breast or prostate), have also fallen during the last decades, with an estimated overall decrease of 23% from 1990 to 2012 (Howlader et al., 2016). This decline has been attributed to reductions in tobacco consumption, as well as to better surveillance and treatment options. Little is known, however, about the potential contribution of reductions in SHS exposure to the decline of cancer mortality among nonsmokers. Most studies on the health benefits of smoke-free policies have focused on acute cardiovascular and respiratory effects (Tan & Glantz, 2012; Lin et al., 2013), and on birth outcomes (Peelen et al., 2016). Moreover, a recently published Cochrane review reported evidence of reduced cardiovascular and respiratory mortality after introduction of national smoke-free bans (Frazer et al., 2016). Recently, though, an ecological study within EU countries reported no statistically significant changes in lung cancer mortality trends after the introduction of smoke-free legislation (Lopez-Campos et al., 2017). Although SHS is known to be carcinogenic (International Agency for Research on Cancer (IARC), 2012), the long latency period that is required for most cancers to develop makes it difficult to link changes in SHS exposure to changes in cancer rates.
The objective of this study was to evaluate the hypothesis that population changes in the distribution of SHS exposure in public places explain changes in smoking-related cancer mortality over time in two samples of the non-smoking US population, the U.S. National Health and Nutrition Examination Survey (NHANES) 1988–1994 and 1999–2004. For this purpose, we used individual information on serum cotinine and made adjustments for SHS exposure at home, so that our analyses reflect SHS exposure not coming from private settings. Moreover, to differentiate between occupational SHS exposure and SHS exposure from other public places, we also evaluated self-reported occupational SHS exposure time (hours/day). To address the study objectives, we implemented a causal inference mediation approach (Lange & Hansen, 2011) using study period as a major determinant of cancer mortality and changes in SHS exposure as a potential mediator of the relation between period and cancer mortality (see Supplemental Fig. S1). This method allows us to estimate the proportion of the decline in smoking-related cancer mortality rates among nonsmokers recruited in 1988–1994 and 1999–2004 that can be independently attributed to changes in SHS exposure.
Section snippets
Study population
NHANES is a program of studies designed to assess the health and nutritional status of the general population in the United States. The survey uses a complex multistage sampling design to obtain representative samples of the non-institutionalized US population (Centers for Disease Control and Prevention (CDC)), and it is unique in that it combines interviews and physical examinations. Blood and urine specimens are obtained from NHANES participants who give consent for their specimens to be used
Results
The age, sex and race-adjusted smoking-related cancer mortality rates were 249.1 and 158.4 deaths per 100,000 person-year in 1988–1994 and 1999–2004, respectively (Table 1). The corresponding all-cause cancer mortality rates were 401.3 and 315.3 deaths per 100,000 person-years (data not shown). Smoking-related cancer mortality rates decreased in all subgroups evaluated except in obese participants and in participants exposed to SHS at home.
Fig. 1 shows the progressive reduction in
Discussion
Smoking-related cancer mortality rates among non-smoking US adults decreased about 36% from 1988 to 1994 to 1999–2004. After accounting for potential confounders and SHS exposure at home, around 46% of the decrease in cancer mortality could be attributed to declines in SHS exposure in public places based on statistically significant serum cotinine estimates. Our results are unique in that they present the first evidence of the important contribution that smoking bans may have had in the cancer
Author contributions
MTP, ANA and EGE conceived the study. MTP and AJ performed the statistical analyses. EGE drafted the initial document. All authors reviewed the manuscript for important intellectual content. All authors had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis. MTP is the guarantor of this study.
Funding
This work was supported by the Strategic Action for Research in Health Sciences (CP12/03080; PI15/00071); CIBERESP and CIBEROBN. The Strategic Action for Research in Health Sciences, CIBEROBN and CIBERESP are initiatives from the Carlos the third National Health Institutes in Madrid and the Spanish Ministry of Economy and Competitiveness and are co-funded with European Funds for Regional Development (FEDER).
Competing interests
None to declare.
Consent and approval
NHANES study protocols were approved by the institutional review board of the National Center for Health Statistics, and written informed consent was obtained from all participants.
Data sharing
Anonymized, non-identifiable participant level cross sectional survey data are publicly available at https://wwwn.cdc.gov/nchs/nhanes/. The statistical code and reduced dataset restricted to the study population and variables used for this analysis is available upon request to the corresponding author.
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