Occasional smoking is a risk factor for myocardial infarction in the population-based Tromsø Study, 2001-2021.

Aims: While the association between daily smoking and myocardial infarction (MI) risk is well-established, little is known concerning the relationship between occasional smoking and MI risk. In this Norwegian study, we aimed to investigate the risk of first-time MI among occasional smokers over a 20-year follow-up period.

Methods: In this prospective cohort study, data on smoking habits and relevant risk factors were collected from 15,617 participants enrolled in the fifth and/or sixth survey of the Tromsø Study. Participants were followed up for the first MI from 2001 to 2021. Smoking, age, marriage/cohabitant status, body mass index, hypertension, cholesterol, alcohol consumption, education, physical activity and sex were included as covariates in Cox proportional hazard models.

Results: At baseline, 9.2% participants were occasional smokers. Over the follow-up period, 1297 participants experienced their first MI. Adjusted for covariates, we observed a 41% increased MI risk among occasional smokers compared to never smokers (hazard ratio (HR) 1.41, 95% confidence interval (CI) 1.11 to 1.80). We noted a dose-response relationship in the hazards of smoking, categorized as never, former, occasional, and daily. Compared with never smokers, the relative risk of MI in occasional smokers was twice as high in women (HR 2.08, 95% CI 1.43 to 3.04) and 11% higher in men (HR 1.11, 95% CI 0.81 to 1.52).

Conclusion: This study highlights an elevated risk of MI among occasional smokers, with higher relative risk in women. The result emphasizes that there is no safe threshold for smoking, underlining the importance of smoking cessation.