Background:
Stressful life events associated with forced migration increase vulnerability for adverse health consequences. Intermediate factors, such as smoking, can shed light on the mechanisms underlying this process. We investigated if and how forced migration from Ceded Karelia due to the World War II is associated with subsequent cigarette smoking within the nationwide Finnish Adult Twin Cohort.

Methods:
Total of 12,933 were born before 1945 and included in this study. The exposure (forced migration due to war, categorized as 'no', 'once',' 2+') used following measures: the municipality of birth and whether the respondent had moved municipality ever and if so, for what reason (10 options, including war). Of the respondents born in Ceded Karelia 83% replied that they had had to move due to war, with the corresponding percentages varying from 5% to 20% in other provinces. Detailed smoking questions were used to create two smoking status variables: 1) 'ever smoker' versus 'never smoker' and 2) 'current smoker' versus 'non-current smoker'. Logistic regression with correction for sampling of twins as twin pairs was used for analyses.

Results:
Those who had experienced forced migration due to World War II showed higher likelihood of being ever smoker (age, sex, education, and birth area adjusted Odds Ratio =1.70, 95% Confidence Interval=1.43-2.02; p< 0.001) than those without forced migration experience. Among 1155 individuals born in Ceded Karelia increased likelihood for current smoking was found (adjusted Odds Ratio=1.73; 95% Confidence Interval=1.08-2.78; p=0.023) in those with migration experience.

Conclusions:
Forced migration experience as a stressful life event seems to increase vulnerability for use of addictive substances, such as cigarette smoking. A further analysis shall explore whether cigarette smoking is a mediator between forced migration and mortality by the end of 2016.