Tobacco remains a leading cause of preventable diseases and premature death worldwide, and it attributes to cancer and respiratory illness(GBD, 2019 Tobacco Collaborators, 2021). Countries around the world are continuously strengthening tobacco control regulations to protect current and future generations from smoking, with the expansion of smoke-free areas in public spaces being one of the most prominent policies (Perez-Warnisher et al., 2019). Article 8 of the Framework Convention on Tobacco Control (FCTC) urges its parties to actively implement effective legislative and administrative measures to minimize exposure to tobacco smoke in indoor workplaces, public transport, and other public places(World Health Organization, 2003). The implementation rate of this indicator is as high as 95 % (World Health Organization, 2024), suggesting a shared global interest in smoke-free public policies and indicating the effectiveness of such measures(Hiilamo and Glantz, 2022).

Smoke-free area expansion policies are favored not only by non-smokers but also by smokers, making them a pro-tobacco control measure(Ritchie et al., 2010). Expanding smoke-free areas has been shown to reduce the risk of mortality from cardiovascular diseases and respiratory conditions such as asthma caused by secondhand smoke (SHS)(Shetty et al., 2011). Studies in South Korea and Ireland found that improvements in indoor air quality due to smoke-free policies significantly reduced SHS related stress and cotinine levels among workers in bars and restaurants compared to pre-policy conditions(Kim et al., 2015; Mulcahy et al., 2005). In addition to expanding smoke-free zones, various tobacco control measures based on FCTC provisions have led to reductions in smoking rates and SHS exposure among non-smokers(Perez-Warnisher et al., 2019; Hiilamo and Glantz, 2022). Global smoking prevalence among adults dropped from 32.7 % in 2000 to 21.7 % in 2020, with further declines expected, while SHS exposure rates have similarly decreased(World Health Organization, 2004). In South Korea, adult smoking prevalence also declined steadily during this period, from 34.5 % in 2000 to 19.6 % in 2023(Korea Disease Control and Prevention Agency, 2024).

Korea has been one of the leading countries in successfully implementing FCTC-driven policies, particularly smoke-free indoor regulations(Cho, 2014). In 2010, local governments were empowered to regulate outdoor smoking, and by the following year, public spaces such as government buildings and schools were designated as fully smoke-free(Cho, 2014). By 2012, indoor smoking was banned in restaurants, bars, and cafes over 150 m2, and by 2015, the ban extended to all such venues regardless of size(Kang and Cho, 2020). The policy now also covers areas near public facilities, such as within 30 m of kindergartens and schools, expanding protection from indoor to surrounding environments. In addition, tobacco product advertising have been comprehensively banned, and smoking cessation services are actively provided to smokers, as part of Korea's ongoing efforts toward a tobacco endgame strategy (Kang et al., 2024).

However, new forms of nicotine products, such as e-cigarettes, heated tobacco products, and hookahs, have emerged, creating a growing global market that poses new threats, as seen in Korea with the rising use of liquid e-cigarettes among adolescents and women(Korea Disease Control and Prevention Agency, 2024; Jerzyński et al., 2021). While some earlier studies suggested that e-cigarettes may act as a gateway to conventional cigarette smoking(Martinelli et al., 2023), a recent systematic review indicates that the association between the two behaviors does not necessarily imply causation, and the existence of a true gateway effect remains uncertain (Begh et al., 2025). Nevertheless, the rapid rise in the use of novel nicotine products is alarming(Sreeramareddy et al., 2024). Additionally, exposure to secondhand vaping aerosols from e-cigarettes has been shown to significantly increase cotinine levels(Johnson et al., 2019). However, many users mistakenly perceive the risks of SHS exposure to be low, believing that nicotine is only released as vapor (Kwon and Park, 2020), which obscures the seriousness of the issue.

Common methods for assessing SHS exposure include surveys and biological indicators such as cotinine levels. Survey items often measure the presence of at least one smoker in the household, exposure time, and the number of regular smokers nearby(Avila-Tang et al., 2013). While self-reported measures of SHS exposure are convenient and effective, they can suffer from validity issues due to differences in participants’ perceptions of exposure(Prochaska et al., 2015). In contrast, cotinine levels serve as a reliable biological indicator of SHS exposure providing accurate information about nicotine exposure for both smokers and non-smokers (Prochaska et al., 2015). Notably, self-reported SHS exposure rates have shown a gradual decline over time, reflecting the impact of tobacco control policies; however, this trend may not fully capture actual exposure levels due to potential underreporting or unawareness of exposure sources(Sim and Park, 2021). Moreover, although cotinine provides an objective measure of exposure, few studies have investigated long-term trends in cotinine-measured SHS exposure across different smoking status groups.

This study aimed to assess whether the declining trend in self-reported SHS exposure among Korean adults is consistent with changes in objectively measured urinary cotinine levels. By analyzing trends across three smoking behavior groups—never smokers, former smokers, and current smokers—we sought to verify if actual SHS exposure is truly decreasing, and to identify hidden sources of SHS that may not be captured through self-reports. These findings help assess whether SHS exposure trends, based on biological markers, align with self-reported data, providing a clearer status of the present tobacco control landscape.