Prospective Study of the Effect of Exposure to Other Smokers in High School Tutor Groups on the Risk of Incident Smoking in Adolescence

Andrew Molyneux1 , Sarah Lewis1, Marilyn Antoniak1, William Browne2, Ann McNeill3, Christine Godfrey4, Richard Madeley5 and John Britton6

1 Division of Respiratory Medicine, University of Nottingham, City Hospital, Nottingham, United Kingdom.
2 Division of Mathematical Sciences, University of Nottingham, University Park, Nottingham, United Kingdom.
3 Freelance consultant, Beckenham, Kent, United Kingdom.
4 Centre for Health Economics, University of York, York, United Kingdom.
5 Division of Epidemiology and Public Health, University of Nottingham, University Hospital, Nottingham, United Kingdom.
6 Division of Epidemiology and Public Health, University of Nottingham, City Hospital, Nottingham, United Kingdom.

Received for publication October 11, 2002; accepted for publication July 29, 2003.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Adolescent smokers tend to have peers who smoke, but it is unclear whether this arises from self-selection of smoking peers or whether this is a causal effect on the uptake of smoking (incident smoking). The authors used school tutor group current smoking prevalence, an unbiased measure of peer smoking in high schools in the United Kingdom, to estimate the independent effect of peer smoking on incident smoking among high school students. In a prospective cohort study of children aged 13–15 years (grades 9 and 10) in 10 high schools in Nottinghamshire, United Kingdom, smoking behavior was surveyed in 2,881 students in 2000, and the survey was repeated in 2,109 students (73%) in 2001. There were 267 incident smokers (15%) among the 1,766 nonsmokers in 2000. The adjusted odds of incident smoking were significantly higher in girls, in students with parents or siblings who smoke, and in relation to school tutor group current smoking prevalence in 2000 (relative odds for highest relative to the lowest quartile of prevalence = 1.78, 95% confidence interval: 1.20, 2.64). This tutor group effect was independent of having a best friend who smoked in the 2001 study. Incident smoking is therefore increased among students exposed to other students who smoke, and preventing smoking at school may reduce adolescent smoking.

adolescent; incidence; schools; smoking; students

Abbreviations: Abbreviation: MIMAS, Manchester Information and Associated Services.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Cigarette smoking is the single greatest preventable cause of premature mortality and morbidity in the developed world (1, 2). Most adult smokers begin to smoke during their early teenage years, while at high school (3, 4), and despite widespread knowledge of the harmful effects of smoking, smoking remains common in adolescence (57). Many adolescent smokers are addicted to nicotine from cigarettes (8), and having become addicted, the majority will continue to smoke for several decades (3, 4). It is therefore important to identify avoidable risk factors for incident smoking in high school students.

At the age of 11 years, children in the United Kingdom move from elementary (primary) to secondary education. In their new and much larger high (secondary) schools, children are mixed in "tutor groups" of 20–30 students with whom they attend the majority of their lessons during the early years of high school. In Nottinghamshire, as in many areas of the United Kingdom, it is usual policy to mix children from different elementary (primary) schools and of different abilities in school tutor groups to promote integration in the new high school. Tutor group membership is therefore a major driver of new peer group relationships, rather than a reflection of peer groupings established in elementary school or local friendships. We have measured the prevalence of current smoking within these tutor groups, based on individual reports of personal smoking from all tutor group members. Tutor group smoking prevalence in this study thus represents an objective, unbiased marker of exposure to peer smoking in adolescence.

We have recently reported evidence that the prevalence of ever smoking in the school tutor group that a child joins when commencing high school at age 11 years is associated with the subsequent risk of starting smoking (9), but this evidence was derived from data on retrospective recall of age at starting smoking and may therefore have been biased. We now report independent confirmation of this effect in a prospective cohort study of high school students aged 13–15 years.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We surveyed pupils aged 13–16 years in years (grade) 9–11 in 10 secondary schools selected at random from a local education authority list of all 50 state (public) secondary schools in Nottinghamshire, United Kingdom, after first excluding four schools due to be inspected during the study period and two that did not provide information on the number of pupils attending the school. Head teachers of these schools were approached and asked for permission to carry out the survey, which was approved by the Nottingham City Hospital Ethics Committee.

From January to March of 2000, each pupil in years (grade) 9–11 was asked to complete a questionnaire adapted from the Office for National Statistics’ Social Survey Division 1998 Teenage Smoking Attitudes Questionnaire (10). The questionnaire was designed for confidential self-completion, under examination conditions with teacher supervision, during a single Personal, Social, and Health Education period (typically lasting 30–40 minutes), and elicited the name, school tutor group, and home postal code of each pupil. Questions on current and past smoking ascertained whether they had never smoked, had tried smoking once, used to smoke but did not now, sometimes smoke but no more than one cigarette per week (occasional smokers), or smoked at least one cigarette per week (regular smokers) (11). Supervising teachers were asked to confirm that each pupil’s name and tutor group had been correctly completed and to ask any absentees to complete a questionnaire on a subsequent occasion.

One year later, we collected the same data and also information on the best friend’s smoking for as many students as possible in years 10 and 11 (students now aged 14–16 years) in the same high schools as part of a second cross-sectional study (9), our aim being to study a cohort of individuals who were representative of high school students of this age rather than a select cohort.

Regular and occasional smokers were categorized as current smokers. Parental smoking was categorized as neither parent smokes, one parent smokes, or both parents smoke; sibling smoking was categorized as nonsmoking sibling(s), smoking sibling(s), or no siblings (respondents answering "no" to the question, "Do you have any brothers or sisters?"); and best friend’s smoking was categorized by the student’s response to the question, "Does your best friend smoke?" ("yes," "no," or "don’t know"). Social deprivation was estimated for each respondent from the Townsend Material Deprivation Index, an area measure based upon 1991 census data (the most recent available) on unemployment, overcrowding, and car and house ownership, expressed as a standardized residual relative to the mean for England and Wales (negative values reflecting lesser and positive values greater degrees of deprivation), and available for each census enumeration district in England and Wales from Manchester Information and Associated Services (MIMAS) (Manchester Computing, University of Manchester, Manchester, United Kingdom) (12). Each respondent’s home postal code was converted to a census enumeration district code using the "PC2ED" utility (based on the 1998 Central Postcode Directory, MIMAS, http://census.ac.uk/cdu/Datasets/Lookup_tables/Postal/Postcode_Enumeration_District_Directory.htm#3), and this code was used to obtain the Townsend Material Deprivation Index from the MIMAS database.

We used each student’s current smoking status and tutor group reported in the 2000 survey to calculate the proportion of current smokers in each school tutor group in 2000, and we linked the 2000 and 2001 data sets (linking each individual’s data using school and tutor group in 2000 with at least two of surname, first name, date of birth, gender, or home postal code) to identify all students who became current smokers (incident smokers) between the two surveys. We analyzed incident smoking in relation to sex, parental smoking, sibling smoking, social deprivation (in quartiles), school year (grade), year 2000 tutor group current smoking prevalence (in quartiles), and best friend’s smoking in 2001 in univariate and multiple logistic regression using Intercooled Stata 7.0 (Stata Corporation, College Station, Texas). We have previously shown that the effect of tutor group smoking was stronger in children not exposed to parental smoking (9), and we therefore looked for any interactions between tutor group smoking and parental or sibling smoking. To examine for potential clustering, we also used a multilevel logistic regression model, using the penalized quasi-likelihood method in MLwiN software (Institute of Education, University of London, London, United Kingdom), fitting random effects for the different schools. Having found that the effects of the schools were negligible and consequently that the school level variance was very small and the fixed-effect coefficients and standard errors were approximately the same as in the logistic regression model, we used multiple logistic regression for all analyses.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In the survey in 2000, we obtained data on 2,881 students aged 13–15 years, and we were able to restudy 2,109 (73 percent) of these (52 percent boys; mean age at follow-up: 14.8 (standard deviation, 0.69) years) in 2001. Using information from school rolls, we have since determined that, of the 2,881 students initially surveyed, approximately 449 (16 percent) were lost to follow-up because of absence from school at the time of the second survey, and 48 (2 percent) had moved to another school; in addition, completed questionnaires from 45 (2 percent) students were lost by one school prior to collection by the research team. School roll data were incomplete for the remaining 232 (8 percent) students lost to follow-up. Compared with participants in the 2001 study, those lost to follow-up were older, more socioeconomically deprived, more likely to be current smokers, and more likely to have parents and siblings who smoked (table 1).


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TABLE 1. Baseline characteristics of students surveyed, by follow-up status, United Kingdom, 2000
 
The prevalence of current smoking among participants in 2001 increased from 16 percent in 2000 to 26 percent in 2001, with 267 (15 percent) incident smokers identified among the 1,766 nonsmokers in 2000. In univariate analysis, incident smoking was significantly related to female sex, parental and sibling smoking, and current smoking prevalence of the tutor group but not to school year (grade) or to the Townsend Material Deprivation Index. In multivariate analysis, these univariate associations remained independently significant (table 2), the relative odds of incident smoking being independently increased in girls, in students with parents or siblings who smoked, and in direct relation to the current smoking prevalence of the school tutor group (adjusted odds ratio for the highest relative to the lowest quartile of prevalence = 1.78, 95 percent confidence interval: 1.20, 2.65). There were no significant interactions between tutor group smoking and parental or sibling smoking. The addition of best friend’s smoking to the multivariate model did not change the effect of current smoking prevalence by the tutor group (adjusted odds ratio for the highest relative to the lowest quartile of prevalence = 1.76, 95 percent confidence interval: 1.13, 2.74), although the odds of incident smoking in girls were reduced and nonsignificant (table 3).


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TABLE 2. Independent risk factors for incident smoking, United Kingdom, 2000
 

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TABLE 3. Independent risk factors for incident smoking adjusted for best friend’s smoking, United Kingdom, 2000
 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This study shows for the first time in a prospective cohort that the prevalence of smoking in school tutor groups has an independent effect on the personal risk of incident smoking in adolescents. This finding therefore provides prospective confirmation of our previously reported cross-sectional study based on personal recall of age at starting smoking (9). Our study also confirms the well-recognized influence of parental, sibling, and best friend’s smoking on the risk of incident smoking in young people (13, 14).

We have shown previously that our sample is similar to the national population of teenagers attending high school in the United Kingdom in terms of socioeconomic status, educational achievement, and the pattern of prevalent smoking (9). Our findings are thus likely to be representative of and generalizable to this wider population. Our data were obtained using questions adapted from those used in national surveys of adolescent smoking behavior in the United Kingdom, to which adolescents have been shown to answer honestly (5). Approximately one fourth of participants in 2000 did not return questionnaires in 2001, predominantly because of absence from school. Data on these nonresponders indicate that they were more likely to be exposed to parents and siblings who smoked. Given that these factors were associated with incident smoking, this loss to follow-up is therefore likely to have led to an underestimate of incident smoking and also of the effect of these factors upon incident smoking in our study. In accordance with the original agreements on cooperation with the schools, we were unable to pursue nonresponders any further. Absence from school is a particular difficulty in restudying students in schools where attendance, particularly approaching leaving age, is not high.

Peer group smoking is a recognized correlate of individual smoking in adolescents, but previous studies have been unable to determine whether this is a causal influence or an effect of a tendency for smokers to aggregate in social groups (1517) or for adolescent smokers to systematically overreport smoking among their friends (17). When children enter high school in the United Kingdom, they join new tutor groups that are selected to promote mixing of children from different geographic areas and of different academic abilities; preexisting friendship groups are normally separated in this process. Students usually remain in these tutor groups throughout their school career and attend registration and the majority of their lessons in them. Our finding that the prevalence of current smoking in tutor groups is a significant independent determinant of incident smoking therefore suggests a direct influence of peer group smoking on an individual’s risk of smoking rather than an effect of peer group selection.

Best friend’s smoking is a particularly important component of peer group smoking exposure. Since the main objective of the first survey was to identify smokers as part of a project designed to develop cessation services for teenage smokers, our initial questionnaire was relatively brief and did not include information on best friend’s smoking. However, we did collect this information in our second survey and, as expected from the existing literature (18), we found that incident smokers were much more likely to have a best friend who smoked. However, while best friend’s smoking did account for some of the increased risk of incident smoking in girls, it did not account for the tutor group effect, indicating again that unselected peer group smoking behavior has an important effect on the personal risk of incident smoking.

Our previous cross-sectional study showed that prevalence of ever smoking in tutor groups, based on personal retrospective recall of age at starting smoking, was an important determinant of incident ever smoking in the first year of high school (9). The findings of the present study provide prospective confirmation of this, showing a similar magnitude of effect in older adolescents (although the odds ratios are not directly comparable), using an objective marker of exposure to peer smoking that is likely to be less biased than personal recall, and basing incident smoking and tutor group smoking prevalence upon current rather than ever smoking. Incident current smoking in this age group is particularly important, as it is a strong predictor of lifelong smoking, since 80 percent of current smokers aged 15 years continue smoking into young adulthood, of whom half will still be smoking at the age of 60 years (4, 19).

The etiology and natural history of smoking in adolescence are complex, and factors other than exposure to smokers in school tutor groups, at home, and in best friends who smoke are likely to be important. In particular, we have not measured exposure to other smokers (such as peers who are not in the same tutor group), the effect of conforming or nonconforming attitudes toward smoking, the ease of access to cigarettes, or the effect of smoking policies either at school or at home; previous studies have suggested that smoke-free policies are protective (2022) and that the perception of smoking in the environment may be more influential than the actual prevalence of smoking (23). Although some of these exposures may confound the relation between others’ smoking and incident smoking, some are also likely to be on the causal pathway, particularly conforming to social norms, attitudes toward smoking, and access to cigarettes. We thus postulate that exposure to other smokers may affect nonsmokers through their being more likely to frequent smoking places, by being among smokers, by making smoking appear "normal," and by making cigarettes more easily accessible.

In summary, we have studied a sizeable, representative cohort of adolescents attending high school in the United Kingdom, and we used an objective measure of peer smoking to prospectively study incident smoking. Although there was differential loss to follow-up, in our study this is likely, if anything, to have led to an underestimate of the effect of exposure to other smokers. Our findings indicate that exposure to others’ smoking has a major influence on incident smoking, and they support the concept of smoking in adolescence as a communicable disorder (24). Our findings also indicate that passive smoke exposure, in addition to the direct effects on health, has an additional dimension of risk in young people by virtue of increasing the likelihood that they too become smokers. Given that strong and well-enforced, smoke-free policies at school, in the home, and at work have been shown to be associated with a reduced risk of smoking in young people (2022), we conclude that such policies in school may not only help to reduce smoking in established smokers but also help to prevent incident smoking in young people. Similar conclusions may apply to encouraging smoking cessation among smoking parents and older siblings, particularly as parental smoking cessation may reduce incident smoking and increase cessation in adolescents (25). Overall, therefore, our data suggest that limiting exposure to other smokers is a potentially important measure in preventing incident smoking and, hence, smoking-related disease.


    NOTES
 
Correspondence to Dr. Andrew Molyneux, Division of Respiratory Medicine, University of Nottingham, City Hospital, Nottingham NG5 1PB, United Kingdom (e-mail: andrew.molyneux{at}nottingham.ac.uk). Back


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 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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