Different associations are observed between the socio-economic status (SES) indicators education and income, and non-daily smoker transitions. Studies should determine carefully which SES indicator to use when studying the SES gradient in tobacco smoking, because income may be less stable over time than education in adulthood. Wang et al. have, in their study 1, provided an informative analysis of factors associated with short-term transitions of non-daily smokers based on a representative sample of the US population. Tobacco smoking is still a major health problem in economically developed countries, despite a decline in smoking prevalence in most of the countries over a long time-period. Socio-economic status (SES) is an increasingly relevant issue in smoking epidemiology in these countries due to the well-known SES gradient in smoking and the increase in the SES gradient during recent decades 2, 3. During several decades, SES has been defined in three main dimensions, which include education, occupation and income. Wang et al. do not include the SES indicator occupation, although this has long been regarded as a central 4, 5 but not exclusively important 6 indicator of SES, but they include education and income as SES indicators. Just as in other studies already referenced by the authors, the majority of non-daily smokers in the study remain non-daily smokers over time. The transitions of the non-daily smokers to daily smokers and former smokers, respectively, in the study partly reflect processes of smoking initiation and smoking cessation. Tobacco smoking, smoking initiation and smoking cessation are complex phenomena which include psychological, psychosocial and social factors as well as biological and other aspects of dependence to a varying extent across different individuals and social groups 7-10. Smoking cessation is a dynamic process which is initiated by a decision to quit, leading eventually to a long period of maintained abstinence 11. In Table 2 in the Wang et al. [1] study, the results show that the highest educational category of college graduates has a significantly lower transition rate from non-daily to daily smoker and also a significantly higher transition rate from non-daily to former smoker compared to all other lower educational categories, which is as expected, in the sense that categories with higher education levels have been found to be more likely to quit at follow-up 10, 12. In contrast, the associations between income and non-daily smoker transitions show that the category with lowest income has a significantly higher transition from non-daily to daily smoker than some of the categories with higher income, while no statistically significant differences can be observed between the lowest income category and the other income categories for the transition of non-daily to former smoker. The observed differences between educational achievement and income as SES indicators, respectively, and their associations with non-daily smoker transitions have several implications. First, while the educational level of an individual is, predominantly, decided early in adolescent or adult life, individual income may vary more throughout adult life. Consequently, studies of the SES gradient in smoking should consider thoroughly which SES indicator to use and for what purpose. Secondly, following from the first point, income as an SES indicator may reflect a less stable aspect of SES among adults and throughout adulthood than education, because for an individual it may vary over time more than education. This means that low income assessed at a given point in time may reflect a shorter or longer period of increased economic and psychosocial stress, which may affect the propensity to increase smoking intensity by going from non-daily to daily smoker. In contrast, no reverse association is observed for the transition from non-daily to former smoker, probably because the latter transition is much less connected with a high intensity of economic stress and probably psychosocial stress. Thirdly, the hypotheses following from the second observation in the second point call for further longitudinal studies, preferably using panel data as in the current paper, with further income and stress items regarding factors which may determine tobacco smoking, smoking initiation and smoking cessation 13. None.
Read full abstract