Bidirectionality in the relationship between asthma and smoking in adolescents: a population-based cohort study.

Purpose: Several cross-sectional studies have showed higher smoking rates among adolescents with asthma, but hardly any study has investigated this relation longitudinally. This study examines whether these cross-sectional results are caused by higher smoking onset among adolescents with asthma, or by the development of asthma after smoking onset.

Methods: This was a longitudinal study (22 months) among 7,426 Dutch adolescents (mean age at baseline = 12.9 years). Asthma was assessed with self-report questionnaires of the International Study of Asthma and Allergies in Childhood (ISAAC) and the student questionnaire of the American College of Allergy, Asthma and Immunology. Adolescents' smoking and parental smoking were also assessed by adolescent-report. All analyses were controlled for age, gender, education, ethnicity, and parental smoking.

Results: In baseline non-smokers, adolescents with current diagnosed asthma and with more severe asthma had an increased risk to become regular smokers. Among girls and adolescents with a smoking mother, having asthma symptoms was a risk factor for starting experimental smoking. In contrast, among boys and adolescents with a non-smoking mother, having asthma symptoms was protective for experimental smoking. With regard to the effect of smoking on asthma, adolescent smoking predicted a higher incidence of asthma symptoms. In addition, smoking predicted increased symptom severity scores, and this effect was stronger in girls. Adolescent smoking was a stronger predictor for the development of asthma symptoms when the mother smoked.

Conclusions: The relationship between asthma and smoking in adolescence seems to be bidirectional, with relations in both directions being stronger in females and when the mother smokes.