Product mix and time since cessation among Korean former smokers using non-combusted nicotine products: a KNHANES analysis with implications for lung cancer risk comparisons

BackgroundObservational comparisons of former smokers who use non-combusted nicotine products with former smokers who quit without them require that two quantities be measured precisely: which product is being used, and how long ago cigarette smoking stopped. Neither quantity is recorded by the National Health Insurance Service (NHIS) screening instrument used in a recent Korean cohort study of post-cessation e-cigarette use and lung cancer risk. We characterized both quantities in a contemporaneous, nationally representative survey of the same population. MethodsWe analyzed the public-release microdata of the Korea National Health and Nutrition Examination Survey (KNHANES), 2018 to 2023, restricted to adults aged 19 years and older. Former smokers were identified by smoking status, and cessation duration was taken from the item recording months since the last cigarette. Former smokers currently using a heated tobacco product (HTP) or an e-cigarette (EC) were compared with former smokers using neither. KNHANES 2018 asked a generic e-cigarette question and, separately, a checklist naming HTP brands, allowing the two product classes to be separated. Distributions were compared with rank-based methods, the age-duration relationship with Theil-Sen regression, and residual imbalance by restricting the comparison group to respondents age-matched to within two years. ResultsThe 2018 analytic sample comprised 1,348 former smokers, of whom 43 currently used HTP or EC and 1,305 used neither. Among the product-using former smokers, 58% reported HTP use without e-cigarette use, 21% reported both, and 21% reported e-cigarette use without HTP use; 79% reported any HTP use. Median cessation duration was 0.7 years (IQR 0.25 to 1.5) among product users and 12.0 years (IQR 5.0 to 20.0) among those using neither (Kolmogorov- Smirnov D = 0.76, P < 0.001), with the product user having quit more recently in 92% of cross-group pairs. The separation persisted within the short-term (<5 year) stratum (D = 0.34, P < 0.001; 73% of pairs) and after age matching, where the residual gap was 9.3 years. Cessation duration rose with age among those using no product (Theil-Sen slope +0.30 years per year) but was flat among product users (-0.01). Restricting to the screening-eligible stratum used in the cohorts high-risk analysis did not attenuate the imbalance: among those aged 50 to 80, median cessation among no-product quitters rose to 15.5 years (n = 858), and adding a 20 pack-year criterion left 421 no-product quitters with a median of 11.0 years against three HTP/EC users who had quit 0.25, 1.0 and 2.0 years earlier, despite closely matched cumulative exposure (mean 37.6 vs 37.7 pack-years). The overall contrast reproduced in every wave from 2018 to 2023, with an age-matched residual of 9 to 11 years. ConclusionsIn a nationally representative survey of the same population and the same calendar year as the NHIS screening cohort analyzed by Kim et al., Korean former smokers using non-combusted nicotine products differed from other former smokers in two respects that bear directly on how such comparisons should be read. First, they were predominantly HTP users: 79% reported any HTP use, and only 21% reported e-cigarette use without HTP use. Second, they had stopped smoking approximately a decade more recently, a difference that survived stratification at five years and exact age matching. Neither quantity is recorded in the NHIS screening instrument. Cohort estimates comparing post-cessation product users with other quitters should therefore be interpreted with caution if they do not precisely characterize product composition and to time since cessation, and future studies should measure both directly.