THORAXJNL143800 1..6

Background Environmental tobacco smoke (ETS) has been reported as a significant risk factor for childhood asthma. Among adults, personal smoking is a major cause of respiratory symptoms and diseases. The effects of these exposures on the prevalence of asthma and wheeze among teenagers are less well known. Objective The aim was to study the independent and combined effects of ETS and personal smoking on the prevalence of asthma and wheeze in teenagers. Methods A longitudinal study of asthma and allergic diseases in schoolchildren has been in progress in Northern Sweden since 1996. All children aged 7e8 years in three municipalities were invited and 3430 (97%) participants have been followed by annual questionnaires. At the age 16e17 years, 82% of the initial participants took part in the 2005 survey. Results Prevalence of physician-diagnosed asthma, ever wheeze and current wheeze was significantly higher among those exposed to maternal ETS and among daily smokers. In multivariate analyses, maternal ETS was a significant risk factor for physician-diagnosed asthma and ever wheeze (OR 1.3e1.5) and personal daily smoking for current wheeze (OR 2.0). ORs for asthma and ever wheeze were highest among daily smokers who were also exposed to maternal ETS with ORs of 1.7 and 2.5, respectively. A significant doseeresponse association between number of cigarettes/day and the prevalence of wheeze was also found. Conclusions Both ETS and personal smoking were significantly related to asthma and wheeze in teenagers. Maternal ETS exposure was associated with lifetime symptoms, but daily smoking among the teenagers was more strongly related to current symptoms. INTRODUCTION Exposure to environmental tobacco smoke (ETS) in early life, especially that from the mother, and maternal smoking during pregnancy are known risk factors for respiratory symptoms and asthma among children. However, less is known of the association between ETS and respiratory symptoms and asthma in teenagers. Some studies found an increased risk for asthma and respiratory symptoms among young adults exposed to ETS in childhood 6 or in utero while others have not found such associations. Smoking is a major cause of respiratory symptoms, chronic bronchitis and chronic obstructive pulmonary disease (COPD) among adults. 10 The association between smoking and asthma among adults is not as strong or consistent. While crosssectional studies have primarily found relationships between asthma and ex-smoking or ever smoking, several longitudinal studies have found significant associations between current smoking and the onset of asthma. 13 14 Among teenagers, smoking is reported to be a risk factor for asthma and wheeze in both cross-sectional and longitudinal studies. 7 8 16e19 Few studies have reported on the independent and combined effects of ETS and personal smoking on respiratory health among teenagers. In a French cross-sectional study, a higher risk of wheeze and asthma was found among smokers exposed to ETS compared to the non-exposed. Furthermore, in utero ETS exposure increases the risk of incident asthma among smoking teenagers. Within the Obstructive Lung Disease in Northern Sweden (OLIN) studies, a prospective cohort study about asthma and allergies among school children has been in progress since 1996. This study previously demonstrated associations between maternal smoking and prevalence of asthma at age 7e8 years and increased incidence of asthma during the following 2 years. Parental smoking also increased the risk of smoking initiation during adolescence. The present study aimed to explore the independent and combined effects of ETS and personal smoking on the prevalence of asthma and wheeze among teenagers. MATERIAL AND METHODS Study population The OLIN paediatric study I is a longitudinal study about asthma, rhinitis, eczema and allergic sensitisation among school children in Northern Sweden that started in 1996. Its aims and methods have been described previously. The parents of all 3525 children aged 7 and 8 years who enrolled in the first and second grades in three municipalities in Northern Sweden were invited to complete a questionnaire. Ninety-seven per cent (n1⁄43430) participated, thus forming a cohort that has been followed by annual questionnaires. The study population in this paper consisted of 2805 children (50.5% boys) who participated in 1996 and again in 2005 at the age of 16e17 years (82% of the original 1996 participants). The study was approved by the Ethics Committee at Umeå University, Sweden. Questionnaire The questionnaire included the International Study of Asthma and Allergies in Childhood (ISAAC) core < An additional table is published online only. To view this file, please visit the journal online (http://thorax.bmj.com). The OLIN-studies, Sunderby Central Hospital of Norrbotten, Luleå, Sweden Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, Umeå, Sweden Correspondence to Linnéa Hedman, The OLIN Study Group, Sunderby Central Hospital of Norrbotten, Robertsviksgatan 9, S-971 89 Luleå, Sweden; linnea.hedman@nll.se Received 27 May 2010 Accepted 9 September 2010 Hedman L, Bjerg A, Sundberg S, et al. Thorax (2010). doi:10.1136/thx.2010.143800 1 of 6 Asthma Thorax Online First, published on November 3, 2010 as 10.1136/thx.2010.143800 Copyright Article author (or their employer) 2010. Produced by BMJ Publishing Group Ltd (& BTS) under licence. on Jne 4, 2022 by gest. P rocted by coright. httphorax.bm jcom / T hrax: frst pulished as 10.1136.2010.143800 on 3 N ovem er 210. D ow nladed fom questionnaire with additional questions about symptoms, physician diagnosis of asthma and allergic diseases, parental smoking habits and other possible risk factors. Between ages 7e8 and 11e12 years, the questionnaire was completed by the parents. From 12 to 13 years of age and onwards, the teenagers completed the questionnaire at school and questions regarding personal tobacco use were added. 24 Definitions Definitions that were based on the 2005 questionnaire: Ever wheeze: ‘Have You ever had wheezing or whistling in the chest at any time in the past?’ Current wheeze: ‘Yes’ to any of the following: ‘Have You had wheezing or whistling in the chest in the last 12 months?’, ‘In the last 12 months, has Your chest sounded wheezy during or after exercise?’, ‘In the last 12 months, have You had wheezing or whistling in the chest without having a cold?’ or those reporting more than one attack in the question ‘How many attacks of wheezing have You had in the last 12 months?’ Physician-diagnosed asthma: ‘Have You been diagnosed by a physician as having asthma?’ Current asthma: Physician-diagnosed asthma, and either wheeze or use of asthma medication during the last 12 months. Personal smoking habits were assessed by one question that was completed by all participants: ‘Do you smoke?’, and two follow-up questions directed to smokers: ‘How often do you smoke?’ and ‘How much do you smoke?’ 1. Occasional smokers: Smoking on weekends or at parties. 2. Daily smokers: Smoking ‘Daily ’ or ‘Almost daily ’. Definitions that were based on data from 1996 to 2005: In utero exposure: Mother smoked during pregnancy ETS at age 7e8 (16e17) years: Mother (father) smoked in 1996 (2005). Continuous ETS: Mother (father) smoked at all surveys from 1996 to 2005. Family history of asthma: Mother, father or sibling had asthma as reported in 1996. Current place of residence: Living in a house, an apartment or both, as reported in 2004. House dampness: A report of past or present dampness at home in any of the questionnaires from 1996 to 2005. Data management and statistical analysis For the lifetime outcomes (ever wheeze and physician-diagnosed asthma) and the current status (current wheeze and current asthma at 16e17 years) missing answers to individual questions (<1%) were coded as negative responses. Missing answers to questions about ETS exposure (4e7%) and personal smoking (<1%) were assigned the response given in the previous questionnaire. Analyses were made using the computer software programme Statistical Package for Social Sciences (SPSS Version 16.0; SPSS Inc, Chicago, Illinois, USA). For assessment of differences in prevalence, c-tests were used and a p value <0.05 was considered statistically significant. The effect of ETS and smoking on asthma and wheeze was analysed by multiple logistic regression analyses and expressed as ORs with 95% CIs. All multivariate analyses included the covariates sex, family history of asthma, current place of residence, house dampness and low birth weight (<2500 g) because each is a known risk factor for asthma or related to personal smoking in this cohort. 20 21 Because maternal ETS and personal smoking are correlated, their effect was analysed in different multivariate models. In all models, the ETS variables were tested one at a time in separate analyses. Model A included maternal ETS and personal daily smoking, while Model B included maternal ETS and number of cigarettes smoked per day among the teenagers. Models C and D are presented in table S1 in the online supplement. In model C, only ETS variables were included and tested one at a time in separate analyses. Model D included four mutually exclusive exposure categories based on maternal ETS exposure and personal daily smoking. In a fifth model, the effect of maternal ETS exposure on asthma and wheeze was analysed among the non-smokers. RESULTS At age 7e8 years, the study population and those lost to followup reported almost identical prevalence of asthma and wheeze. However, the prevalence of ETS exposure at age 7e8 years was higher among those lost to follow-up compared to the study population; 40.0% vs 28.9% (p<0.001) were exposed from their mothers and 27.2% vs 21.4% (p<0.01) from their fathers. Prevalence of asthma and wheeze Among the 7e8-year olds, all asthma and wheeze variables were significantly more common among