Background: Heated tobacco product (HTP) use in Japan has rapidly increased. Despite this rapid spread, little is known about the health effects of HTP use. We conducted a longitudinal cohort study to investigate the change in smoking habits following the spread of HTP use and its effect on forced expiratory volume in 1 second (FEV1) decline. Methods: Participants consisted of a resident population (n = 2,612; mean age, 67.7 years) with FEV1 measurement in 2012–2014 and 2018–2019, and a worksite population (n = 722; mean age 49.3 years) without FEV1 data. Participants were categorized as combustible cigarette-only smokers, HTP-only users, dual users, past smokers, and never smokers. The association between smoking group and the change in smoking consumption over a mean 5.6 years was examined. Differences in annual FEV1 change between smoking groups were examined in the resident population. Results: Prevalence of HTP-only and dual users in 2018–2019 was 0.8% and 0.6% in the resident population, and 5.0% and 1.9% in the worksite population, respectively. The overall number of tobacco products smoked=used increased in dual users compared to baseline, but not in others. Annual FEV1 decline in dual users tended to be greater than that in cigarette-only smokers (16; 95% confidence interval, −34 to 2 mL=year after full adjustment). Participants switching to HTP-only use 1.7 years before had a similar FEV1 decline as cigarette-only smokers. Conclusions: HTP use, including dual use, is prevalent even in a rural region of Japan. Dual users appear to smoke=use tobacco products more and have a greater FEV1 decline. Tobacco policy should consider dual use as high-risk.
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