Results

Table 1 shows the sociodemographic characteristics of study subjects and highlights their ethnic diversity. A total of 104 countries of birth were reported by subjects for themselves and their parents. Almost half of the fathers (45.7%) and 25.4% of the mothers smoked. Children lost to follow up differed from those who remained in the study in several respects: they were slightly older, fewer lived in two-parent families, fewer were born in Canada, and fewer mothers were employed (table 1).

PATTERNS OF SMOKING ONSET

At baseline, 17.1% (95 % confidence interval (CI)) = 16.7 to 19.2) of girls and 25.1% (95% CI = 22.3 to 27.9) of boys had ever smoked; 15.5% (95% CI = 13.8 to 17.2) of all children were past smokers, 3.1% (95% CI = 2.3 to 3.9) were currently trying smoking, and 2.6% (95% CI = 1.9 to 3.3) were experimental/regular smokers. A year later, 25.7% of girls (95% CI = 22.8 to 28.5) and 34.6% of boys (95% CI = 31.6 to 37.7) had ever smoked; 19.1% of all children (95% CI = 17.3 to 20.9) were past smokers, 4.5% (95% CI = 3.6 to 5.5) were trying, and 6.5% (95% CI = 5.4 to 7.6) were experimental/regular smokers. Although a lower proportion of girls had ever smoked at baseline, the patterns of change over a year were similar among boys and girls.

Baseline smoking status was predictive of smoking status a year later (table 2). Most never-smokers (82.4%) remained never-smokers. Of particular interest, 50.0% of children trying smoking at baseline did not progress, while 50.0% continued to try or progressed to experimental/regular smoking. Also, there was a marked pattern of progression to experimental/regular smoking by past smoking experience. Only 2.6% of never-smokers reported regular/experimental smoking a year later; 16.3% of past smokers, 32.1% of those trying, and 38.3% of experimental/regular smokers at baseline reported experimental/regular smoking a year later. The patterns of progression were similar among boys and girls.

It is notable that the data in table 2 are reported exactly as the children responded in the questionnaires, and therefore provide information on misclassification related to self-reports of smoking status among young children. Of particular interest, 88 of 385 ever-smokers at baseline (22.8%), reported never having smoked in the one-year follow up. These subjects are misclassified either at baseline (that is, they reported having smoked at baseline when in fact they had never smoked) or at the one-year follow up (they had “forgotten” about earlier smoking experiences that they had correctly reported a year earlier). Comparison of the characteristics of these “misclassified” children with those of other ever-smokers, suggested that they were slightly younger and fewer were of Canadian family origin (data not shown). For the analyses reported here, these children are considered to have been ever-smokers at baseline who did not continue to smoke at the one-year follow up.

PREDICTORS OF SMOKING INITIATION

Over the one-year follow up, 14.7% of girls and 20.8% of boys who reported never smoking at baseline, initiated smoking. Table 3shows the proportion of subjects who initiated smoking according to selected potential predictors. Table 4 shows that the independent predictors of initiation retained in the multivariate model included age, gender, friends who smoke, smoking by parents and siblings, and high fat/“junk food” consumption. None of the terms to test for interactions between each independent predictor and gender were significant, and an age × grade interaction term was not significant.

PREDICTORS OF CONTINUED SMOKING

At the one-year follow up, 32.0% of girls and 31.4% of boys who had ever smoked reported continued smoking. Table 5 shows that although age and friends who smoke were important predictors of continued smoking in both boys and girls, other predictors of continued smoking differed by gender. Boys who were current smokers at baseline were 2.6 times more likely to continue smoking than boys who were past smokers at baseline. In the model for girls, ever-smokers who were overweight were 3.5 times more likely to continue smoking than ever-smokers who were not overweight.

The variable “school” was included in the final models for smoking initiation and for continued smoking, to control for possible clustering related to homogeneity of students within schools. School was not significant in any model. Similarly, for the 363 students included in both the 1993 and 1994 cohorts, a variable to control for possible dependence between observations of the same subject, was not significant in any model. Finally, year of cohort (1993 or 1994) was not significant in the final models.

Discussion

This study is one of the first to examine the process of smoking onset longitudinally in young children in multiethnic, low-income, inner-city neighbourhoods. One of the more important findings is that one in six children aged 9–12 years who had never smoked, experimented with cigarettes over the one-year follow up. One in three children who had already tried smoking reported continued smoking. These data suggest that age 9–12 years is a critical period in the smoking onset process, at least in low-income, inner-city neighbourhoods, and that there is considerable need for prevention programmes for these age groups in these milieus.

Few studies have systematically differentiated predictors of smoking initiation from predictors of continued smoking among young children, from such a wide range of sociodemographic, anthropometric, behavioural, and psychosocial characteristics. Disentangling which factors are more important at each of these early stages of onset, could lead to more effective prevention programmes by identifying specific sub-groups to target with specific prevention messages. In particular, never-smokers and ever-smokers in this age range could be two distinct target groups, in need of different prevention programmes. Our results support the notion that, although age and smoking by friends are consistently important, the predictors of initiation and continuation do differ somewhat. Furthermore, there are important differences in the predictors of continued smoking between boys and girls. Programme planners will need to consider whether or not these differences warrant tailoring prevention programmes to specific subgroups.

There is considerable controversy over the relative contribution of parental, sibling, and peer smoking to the smoking onset process. Many studies substantiate the strong influence of peer smoking1 5 35-44 evident as early as age 9.43 Parental smoking is thought to be a weak predictor of onset.6 It might lead children to try smoking, but be unrelated to smoking at a later age.43 Sibling smoking is positively associated with younger siblings beginning to smoke, and like parental smoking, might be more influential in the early stages of cigarette use.1 Family members and peers could exert influence in similar ways—through providing easy access to cigarettes, through communicating information about the physical and psychological effects of use, through modelling or demonstrating how to use cigarettes, and through conveying attitudes and social norms favourable to smoking.11 Peer and sibling smoking could also exert influence through the need for social acceptance from peers as well as through direct pressure to smoke.5

The results reported here substantiate the importance of peer smoking in both smoking initiation and continued smoking—never-smokers with friends who smoke were more than twice as likely as never-smokers without friends who smoke, to initiate smoking. Similarly, ever-smokers with friends who smoke were approximately twice as likely to continue smoking. Indeed, most psychosocial smoking prevention programmes focus on refusal skills training on the premise that such training will have a suppressive effect on onset by enabling non-using adolescents to refuse offers of cigarettes from peers.45

In addition, our results support previous findings that smoking by family members is a strong determinant of initiation, although it appears to be less important to continued smoking. In fact, the strength of the association between family members smoking and smoking initiation was similar to that of friends smoking. Although further research is needed to determine which aspects of sibling and parental use are most important in influencing children’s smoking,11 this study provides compelling evidence for including elements to address familial influences in prevention programmes. More specifically, these results support the notion that it will not be sufficient to provide refusal skills training to children without taking the family environment into account. Intervention programmes must address the issue of exposure to smoking in the home. Minimally, prevention efforts should inform parents of the potential of their own smoking behaviour to influence the smoking behaviour of their offspring. More intense prevention efforts could include, in addition to parental education, cessation programmes for family members who smoke. Cessation programmes tailored to parents who smoke should emphasise the possible role modelling aspect of their behaviour, in smoking and possibly in their efforts to quit smoking.

A finding of considerable interest is that, whereas weight status was not associated with smoking initiation, overweight girls who had tried smoking were more than three times more likely to continue smoking than girls who were not overweight. Several studies have identified subgroups of “weight control smokers” among adult females,46-48 and there is evidence from both cross-sectional and longitudinal studies that adolescent girls use smoking for weight control.49-51 The results reported here suggest that the association between smoking and weight status is evident even in pre-adolescent girls. Further research is needed to increase our understanding of how this association contributes to the smoking onset process in young girls, and whether or not smoking prevention programmes targeted at young girls should incorporate and emphasise healthy weight issues.

Interestingly, frequent consumption of high fat/“junk food” consumption was an independent predictor of smoking initiation, although it was unimportant to continued smoking in either boys or girls. This could indicate early clustering of unhealthy lifestyle behaviours.

Although there is considerable evidence of class-based differences in the prevalence of smoking, none of our indicators of social class including parents’ education, household income, or parents’ employment status, was associated with smoking onset. It is possible that missing data on parents’ education and household income obscured the association. Alternatively, our results do support those of a recent longitudinal study of Danish children52, which found no associations between either household income or parents’ education and later smoking as adults.

Finally, cultural factors and ethnicity were not important in this study. Some authors have argued that ethnic differences disappear once socioeconomic status is taken into account1. Our results support the notion that once familial and peer smoking are taken into account, cultural factors and ethnicity are not influential in the smoking onset process.

LIMITATIONS

Because the study schools do not represent a random sample of schools and because of possible selection bias caused by subjects lost to follow up, the generalisability of results might be limited. Also the number of potential predictors of smoking onset studied was restricted because of time limitations related to administration of the questionnaire in-class. Future research should examine whether other variables reported to be associated with smoking in young people1 11 53-67 are independent predictors of smoking initiation and continuation in this population.

Finally, this study did not use objective measures to validate self-reported smoking status of students. Nor did it use the bogus pipeline method which has been reported to enhance the validity of self-reports of smoking among the young.68 However biochemical measures are not perfect indicators of tobacco use and controversy exists over the efficacy of the pipeline method.69 In addition the reliability of self-reports of smoking by adolescents has generally been quite high (88–100%), with most inconsistencies occurring among subjects who use tobacco infrequently.69