Robert J. Hancox, Andrew R. Gray, Richie Poulton, Malcolm R. Sears

Life-course persistent asthma and tobacco smoking are risk factors for irreversible airflow obstruction. It is often assumed that smoking and asthma have additive or multiplicative effects on the risk for airflow obstruction, but this has not been demonstrated in prospective studies of children with persistent asthma. To investigate the effects of smoking and asthma on the development of airflow obstruction in a population-based birth-cohort followed to age 38 years. Reports of childhood asthma from ages 9, 11, and 13 and self-reports of adult asthma at ages 32 and 38 years were used to define childhood-onset persistent (n=91), late-onset (n=93), asthma in remission (n=85), and non-asthmatic (n=572) phenotypes. Cumulative tobacco smoking histories and spirometry were obtained at ages 18, 21, 26, 32, and 38 years. Analyses were by generalised estimating equations (GEEs) adjusting for childhood spirometry, body mass index, age, and sex. Smoking history and childhood-onset persistent asthma were both associated with lower FEV1/FVC ratios. Associations between smoking and FEV1/FVC ratios were different between asthma phenotypes (interaction p<0.001). Smoking was associated with lower pre- and post-bronchodilator FEV1/FVC ratios among non-asthmatics and those with late-onset or remittent asthma, but smoking was not associated with lower FEV1/FVC ratios among those with childhood-onset persistent asthma. Childhood-onset persistent asthma is associated with airflow obstruction by mid adult life, but this does not appear to be made worse by tobacco smoking. We found no evidence that smoking and childhood-persistent asthma have additive or multiplicative effects on airflow obstruction.