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Abstract
Background To examine the impact of the Scottish smoke-free legislation on social inequalities in secondhand smoke (SHS) exposure among primary school children.
Methods Comparison of nationally representative, cross-sectional, class-based surveys carried out in the same schools before and after legislation. Participants were 2532 primary school children (primary 7; aged around 11 y) surveyed in January 2006 (before legislation) and 2389 in January 2007 (after legislation). Outcome measures were salivary cotinine concentrations, self-reported family socioeconomic classification (family SEC) and family affluence scale (FAS).
Results After adjusting for number of smoking parents, mean cotinine concentration varied significantly across both family SEC and FAS groups, and increased significantly stepwise from high to low family SEC/FAS. Mean cotinine fell in all family SEC/FAS groups after legislation. The relative drop in mean cotinine was equal across all family SEC/FAS groups. Adding an interaction term between survey-year and family SEC/FAS to the model showed an increase in inequalities over time, but was only significant at the 93% level using FAS and 73% using family SEC.
Conclusion Inequalities in SHS exposure exist among 11-year-old children in Scotland. Smoke-free legislation has reduced exposure to SHS among all children. Although the greatest absolute reduction in cotinine is observed in the lowest SEC/FAS group, cotinine levels remain highest for this group and there is a suggestion of possible increases in inequalities, which may warrant longer-term monitoring.
- Child
- cotinine
- tobacco smoke pollution
- social class
- socioeconomic factors
- child health
- passive smoking
- social inequalities
- socio-economic
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- Child
- cotinine
- tobacco smoke pollution
- social class
- socioeconomic factors
- child health
- passive smoking
- social inequalities
- socio-economic
Introduction
Scotland's smoke-free legislation came into force in March 2006. It prohibits smoking in virtually all enclosed public and workplaces, including bars and restaurants1 and has had a dramatic impact on levels of secondhand smoke (SHS) exposure in Scotland. One year after its implementation there was a significant reduction in SHS exposure among both bar workers who were occupationally exposed and adults in the general population.2–4 Similar reductions have been observed in other jurisdictions where comprehensive smoke-free legislation has been introduced.5–7
However, there has been little research in to the impact of smoke-free legislation on children's exposure to SHS. To our knowledge, the only published study was carried out by the current authors.8 This found a 39% reduction in SHS exposure, as measured by salivary cotinine, among school children in Scotland after smoke-free legislation was introduced. However, the reduction was not equivalent for all groups of children. Instead, it was associated with both the number and gender of parents who smoked. The reduction in exposure was highest among children living in households where neither parent smoked or only the father smoked. The reduction in exposure among children living in households where only the mother smoked or where both parents smoked was considerably smaller and failed to reach statistical significance.
Socioeconomic status has been found to be a predictor of smoking status, with high smoking rates and low quit rates associated with many aspects of deprivation and disadvantage, including poor housing, low income, lone parenthood, unemployment and homelessness.9–11 Children's exposure to SHS is also known to vary according to the socioeconomic status of their families.12–14 In this paper, we assess whether socioeconomic differences in SHS exposure among 11-year-olds in their final year of primary school in Scotland have changed since the introduction of the Scottish smoke-free legislation and whether these differences persist, after taking parental smoking habits into account.
Methods
The changes in child exposure to environmental tobacco smoke (CHETS) study is part of a national evaluation of Scottish smoke-free legislation.15 Its main aim was to assess changes in SHS exposure in children after implementation of the legislation. The CHETS study obtained both objective (salivary cotinine) and self-reported measures of exposure to SHS. Only the main design features are described here. Full details can be found in Akhtar et al (2007).8
Two nationally representative class-based surveys of 11-year-old children in their final year of primary school in Scotland (primary 7) were conducted in the same schools 1 y apart: the first in January 2006 before the legislation came in to force and the second in January 2007 post-legislation. A repeat cross-sectional design was preferred over longitudinal in order to disentangle the effects of the smoke-free legislation from SHS exposure associated with behavioural changes linked to pupils' maturation.
The questionnaire included questions on pupil's own smoking status and that of their friends and ‘parent figures’ (defined as parents or step parents whom the children reported living with all or most of the time). Saliva samples were provided at the same time as questionnaire completion. Under supervision of trained researchers, pupils were asked to place a dental roll in their mouths without touching it and hold it between their cheek and gum for at least 3 min, to ensure the collection of a sufficient volume of saliva. Supervising researchers were non-smokers, and samples were stored in a smoke-free environment before analysis.
Ethics approval
Ethics approval was obtained from the University of Edinburgh School of Education ethics committee. Appropriate permissions to approach schools were received and pupils received opt-out parental consent letters before the survey and had the option to withdraw on the day of the survey.
Cotinine
Cotinine is a major metabolite of nicotine and is a sensitive indicator of the absorption of tobacco smoke and in non-smokers is a valid measure of recent (3–4 days) exposure to SHS.16 Concentration of salivary cotinine was determined using capillary gas chromatography with a specific nitrogen/phosphorous detector from a 100 μl sample.17 The assay had a detection limit of 0.1 ng/ml. Pupils who reported being current smokers or who had cotinine concentrations above 15 ng/ml, the accepted cut-off point for active smoking, were excluded.18 Log transformed cotinine values were used in these analyses as the distribution of cotinine values was positively skewed.
Family structure and parental smoking status
Pupils' family structure was determined using the question ‘Think of the home where you live all or most of the time now, tick all of the adults who live there’ (response options: Mother/Father/Stepmother (or father's partner)/Stepfather (or mother's partner)/Grandmother/Grandfather/I live in a foster home/Someone or somewhere else, please write it down). Parental figures' smoking status was determined using the question ‘Do any of the following people smoke? Father, Mother, Stepfather (or Mother's partner), Stepmother (or Father's partner)’ (response options: Smokes everyday/ Smokes sometimes/ Does not smoke/ Don't know/ Don't have or see this person). Parent figures were classified as smokers when described by their children as smoking ‘every day’ or ‘sometimes’. Using data on family structure and reported parental figures' smoking status, children were classified as living with ‘none’, ‘one (father figure only)’, ‘one (mother figure only)’ or ‘two’ smokers.
Socioeconomic status
There are well-known difficulties in collecting good quality, unbiased data on parental occupation from children.19 20 Therefore, two measures of socioeconomic status were used in this study, the first based on reported parental occupation and the second on family affluence.
Family socioeconomic status was determined by asking pupils ‘Does your father have a job?’(Yes/No/Don't know/Don't have or don't see father). ‘If YES, please say in what place he works (eg, hospital, bank, restaurant)’ and ‘Please write down exactly what job he does here (eg, teacher, bus driver)’. The same question was asked about mother. Reported parental occupations were coded according to the 1991 Registrar General's Occupational Groups21 and collapsed into four categories: 1: I professional occupations & II managerial and technical; 2: IIIN skilled non-manual & IIIM skilled manual; 3: IV partly skilled & V unskilled; 4: economically inactive. A family socioeconomic classification (family SEC) variable was derived using the parent (mother or father) with the higher occupational status. Where one parent's details were not given, the other parent's occupation was used instead. Family SECs will be referred to as SEC 1, SEC 2, SEC 3 and SEC 4. Children whose family SEC was categorised as ‘Army’, ‘inadequately described’ or ‘not stated’ were omitted from the analyses (see table 1).
Description of sample before and after smoke-free legislation in Scotland. Values are numbers (percentages) unless stated otherwise
The family affluence scale (FAS) is a measure of family material affluence that was developed for use with children as an alternative to occupation-based measures of socioeconomic status.20 The questions used to determine family affluence were: ‘Do you have your own bedroom for yourself’ (Yes/No); ‘Does your family own a car or truck?’ (No/Yes, one/Yes, two or more), ‘During the past 12 months, how many times did you travel away on holiday with your family?’ (Not at all/once/twice/more than twice) and ‘How many computers (PCs, Macs or laptops) does your family own?’ (None/One/Two/More than two). A FAS score was generated for each child. The scores for the sample were then split into tertiles corresponding to groups of children from families of low, medium and high affluence.22
Statistical analysis
Cotinine values below the limit of detection (0.1 ng/ml) were assigned an imputed value randomly sampled from the left tail of a truncated log normal distribution. Geometric mean cotinine concentrations and their 95% CIs are reported. As individual children within a school class may be more similar with respect to secondhand smoke exposure or other measures than randomly selected children all analyses reported here take account of the survey design (clustering and stratification).
Univariate analyses χ2 tests were used to test differences in proportions. Analysis of variance and linear regression analyses were used to assess the difference in mean cotinine concentration before and after legislation by family SEC and by family affluence. Cotinine values were calculated using model parameter coefficients for children of mean age adjusting for age and number of parents who smoke to investigate whether parental smoking mediates the relationship between family SEC/FAS and SHS exposure. Interaction terms between year and family SEC/family affluence were entered into the models to examine the change in socioeconomic inequalities of cotinine concentration between 2006 and 2007.
Response rates
Altogether, 116 (68%) of 170 schools approached agreed to take part in the study before the legislation and 111 of the original 116 schools also participated at follow-up in 2007 (65% of schools originally approached). Schools that declined to participate were not significantly different from participating schools with respect to denomination, urban/rural classification, school size and socioeconomic deprivation based on the proportion of pupils receiving free school meals. In each survey year, participating schools were also representative of Scottish schools with respect to these indicators.
In 2006, 2559 (86%) pupils from participating schools completed the questionnaire and 2424 (85%) pupils completed the questionnaire in 2007. We excluded individuals for whom information on gender was missing and primary 6 pupils who were taught in a composite primary 6/primary 7 class. Further details on response rates can be found in Akhtar et al (2007).8 The final data sets contained 2532 pupil questionnaires in 2006 and 2389 in 2007. Of pupils in the final dataset, 95% in each survey year (n=2403 in 2006; n=2270 in 2007) provided a saliva sample with a valid cotinine level.
Results
Sample characteristics
Table 1 details the characteristics of the samples. The mean age of pupils, proportion of boys and girls, and proportion of pupils living in each family structure and in each family SEC and family affluence group were not significantly different before and after legislation. We were able to assign a FAS score to 96.6% of pupils in 2006 and 94.5% in 2007, and meaningful family SEC (family SEC 1–4) to 79.1% in 2006 and 76.7% in 2007. Most pupils in both years were classified as non-smokers based on self-report and cotinine concentration equal to or below 15 ng/ml.
Associations between number of smokers and family SES/family affluence scale
In each survey year there was a significant association between family SEC and the number of parents in a household who smoke and between FAS and the number of parents in a household who smoke. Being in a lower family SEC/FAS group was associated with a higher proportion of children reporting that their mother only or two parents smoke (Pearson χ2; all p<0.001) (table 2).
Family socioeconomic classification (SEC) and family affluence scale (FAS) by number of parent figures in the household who smoke—before and after smoke-free legislation: Scotland
Change in exposure to SHS by family SEC/FAS
Table 3 gives model estimates for geometric mean cotinine concentration before and after legislation in each of the family SEC and family affluence groups. Mean cotinine concentration increased significantly stepwise from high to low family SEC/FAS. Once the number of parents who smoke was taken into account, differences in cotinine concentration between family SEC/FAS groups were attenuated but remained significant. After legislation mean cotinine concentration fell across all family SEC/FAS groups, with the greatest absolute reduction in the lowest family SEC/FAS group.
Geometric mean cotinine concentrations and 95% CIs in non-smoking children by family socioeconomic classification (SEC) and family affluence scale (FAS), before and after smoke-free legislation in Scotland
Linear models were created with geometric mean cotinine concentration as the outcome variable. The parameter estimates for family SEC/FAS, year and interaction terms between family SEC/FAS and survey year are shown in table 4. Interaction coefficients were positive and larger for low relative to high FAS and family SEC, particularly in the case of the model with FAS, suggesting that inequalities in cotinine concentration have increased between 2006 and 2007. In particular, the low FAS group has a significantly smaller reduction (−0.38) in mean cotinine concentration than the high FAS group (−0.61). However, overall, the addition of the interaction term is significant only at the 93% level using FAS and at the less convincing 72% level using family SEC. Similar parameter estimates are found when the model does not adjust for number of parents who smoke (not shown), with interaction term significant at the 93% level using FAS and 90% level using family SEC.
Linear regression with geometric mean cotinine concentration as outcome variable, adjusting for age, number of parents who smoke and interaction terms between year and family socioeconomic classification (SEC)* (model 1, n=3322) and year and family affluence scale (FAS) (model 2, n=4049)
Discussion
This study provides evidence that there are marked social inequalities in SHS exposure among 11-year-old children from different socioeconomic groups in Scotland. Consistent with other studies,12–14 mean cotinine concentration was higher among children from families of lower socioeconomic status whether this was measured using occupational status or the FAS. Following implementation of Scotland's smoke-free legislation, mean cotinine concentration fell across all family SEC and family affluence groups. Although the absolute reduction in mean cotinine concentration was highest in the lowest family SEC/FAS group, cotinine levels remained highest for this group, and relative changes over time suggest that inequalities that persist within the period of 1 y after the introduction of the smoke-free legislation may increase in the longer term.
Children in low family SEC/FAS groups were more likely to have a mother who smokes compared to children in high family SEC/FAS groups. Furthermore, cotinine concentrations that were unadjusted for parental smoking were larger than adjusted figures, which lends weight to the argument that the number of parents who smoke in a child's household is an important factor in exposure to SHS. Number of parents who smoke may partially mediate the relationship between socioeconomic status and SHS exposure. Alternatively, other factors not included in this study may influence both socioeconomic status and parental smoking. That the home is an important source of SHS exposure for many children23 and that children spend most of their time in their own homes,8 helps to emphasise that an important factor in reducing inequalities in secondhand smoke exposure among children lies in educating parents who smoke about the health benefits of keeping their homes smoke free. As mothers' smoking is related to higher cotinine concentrations among children,8 this factor may help to explain persistent socioeconomic differences in SHS exposure and provide a cue to future preventive actions.
After adjusting for the number of parents who smoke, social inequality in SHS exposure among children persisted. This may be due to a number of factors related to social inequality in the amount (frequency and duration) of parental smoking or differences in the location in which parents smoke. There is some evidence that indoor smoking is more prevalent among low income parents,24 25 and that smokers from lower social groups tend to smoke for longer durations (onset to cessation)26 and are more likely to be heavy smokers compared to higher social groups.27 Phillips et al also found that people in lower socioeconomic groups were less likely to have a total ban on smoking in their homes compared to people in higher socioeconomic groups.28
Inequalities in SHS exposure among children were seen both before and after legislation in the current research and there is a suggestion that inequalities post-legislation could be increasing. It may be that parents in high family SEC/high affluence families who do smoke, may smoke less heavily, find it easier to restrict their smoking in the home or find ways of making restrictions effective compared to parents in low family SEC/low affluence families. They may have more opportunities to smoke away from their children compared to low affluence families and it may be less socially acceptable within their circle of friends and family to smoke inside. In addition, dietary habits differ by socioeconomic group29 and it should be noted that cotinine from dietary sources may also contribute to mean cotinine measures. However, these contributions are likely to be very small.30 31
Therefore, it seems that children in lower SEC/affluence families are at particular risk of exposure, but especially if their parents smoke. Initiatives that address awareness and education about the benefits of smoke-free homes would be useful, especially if they are effective in empowering families in lower SES groups and smokers to reduce SHS exposure in their children.
Strengths of the study
This study is based on a large, nationally representative sample that permits population-level inference. An objective measure (salivary cotinine concentration) was used, which is a good indicator of secondhand smoke exposure. This study provides a reliable description of the smoking status of parent figures; for example, making the distinction between absent and lived with fathers. Basing the survey in schools may also have encouraged more honest reporting of parental smoking than if the survey had been carried out at home with parent figures present in the house.32
Limitations of the study
The school take-up at baseline was lower than expected given response rates in another national survey among this age group in Scotland that was carried out at the same time.33 This perhaps reflects the different content of the CHETS study and the requirement to collect saliva samples. However, we detected no systematic bias in the final sample of schools arising from non-participation.
This study included only children who attended school on the day of the survey in mainstream schools across mainland Scotland. Children who are truanting or excluded from school have a higher likelihood of participating in risky behaviours (such as drinking, drug misuse and smoking).34 They may also have higher secondhand smoke exposure. However, they represent a small proportion of children in this sample, and our results are based on non-smoking pupils; therefore, this small group of children would be unlikely to cause substantive change in the conclusions drawn. In future studies, as well as parental smoking, sibling smoking should be included as this may also vary by socioeconomic group and add to our understanding of the smaller relative decrease in SHS exposure among less affluent groups.
Future work
Further research is needed on prevalence and type of smoking restrictions in households with children and understanding the barriers and enablers to introducing home restrictions among parents of children and adolescents. The authors intend to examine the impact of the smoke-free legislation on reported restrictions within the home and how different levels of reported restrictions affect SHS exposure among children. A follow-up study is recommended to establish whether the weakly associated interactions detected are in fact an early warning of a longer-term widening of socioeconomic inequalities, possibly due to smoke-free legislation.
What is already known on this topic
Despite a significant reduction at population level in secondhand smoke (SHS) exposure after introduction of smoke-free legislation in Scotland, children with mothers only who smoke or both parents who smoke did not experience a similar reduction in SHS exposure. Exposure to SHS among children varies according to the social status of their parents.
What this study adds
The study confirms that social inequalities in secondhand smoke (SHS) exposure existed among 11-year-old children both before and after legislation, even after taking account of the number of smoking parents. The smoke-free legislation has reduced exposure to SHS among all children. However, cotinine levels remain highest among children from low family socioeconomic classification and family affluence scale groups and there is a suggestion of possible increases in inequalities that may warrant longer-term monitoring.
Acknowledgments
We thank ABS Labs, London, who analysed salivary cotinine; MVA Consultancy for managing the fieldwork; Scottish local education authorities for granting permission to approach schools under their authority; pupils and teachers of all participating schools; Winfried Van der Sluijs (CAHRU) for providing useful comments on drafts of this paper.
References
Footnotes
Funding NHS Health Scotland and the Scottish Government. The study design was developed in discussion with an advisory group that included SJH (NHS Health Scotland). SJH contributed to preparation of the manuscript, but this did not create any conflicts of interest.
Competing interests None.
Ethics approval This study was conducted with the approval of the School of Education Ethics Committee, University of Edinburgh.
Provenance and peer review Not commissioned; externally peer reviewed.