Characteristics and health consequences of intermittent smoking: Long-term follow-up among Finnish adult twins

The definition of a smoker as someone who smokes daily has been challenged. No consensus exists regarding whether intermittent smoking represents transition toward daily smoking or cessation or whether intermittent smokers consistently maintain their low tobacco use frequency. Although abundant evidence supports the adverse health consequences of daily smoking, less evidence is available on intermittent smoking. We examined characteristics and health consequences of intermittent cigarette smoking among Finnish adult twins. We used longitudinal data of 21,340 persons with smoking status from questionnaires in 1975 and 1981 and data on lung cancer incidence from 1982 to 2004 from the Finnish Cancer Registry. We identified 641 consistent intermittent smokers comprising 3% of the study population. Consistent intermittent smokers had higher education, less use of other tobacco products, healthier lifestyles, and partly more favorable mental health profiles compared with lifetime regular smokers. However, in terms of other lifestyle factors, intermittent smokers compared mostly unfavorably with never-smokers, despite being better educated. Intermittent smoking showed substantial heritability. There were 213 incident lung cancer cases among all study subjects; only one case was found among the intermittent smokers. The sex- and age-adjusted hazard ratios of lung cancer were not significantly elevated for the intermittent smokers, but they were increased more than 10-fold for all other smokers. Although the present study did not find evidence of elevated lung cancer risk among intermittent smokers, compared with never-smokers, further studies should investigate other health consequences of intermittent smoking, such as cardiovascular and nonmalignant pulmonary outcomes.