Asthma is a common chronic airway disease worldwide in children and adults. 1 , 2 The prevalence of asthma is increasing in industrialised countries, whereas a rural upbringing has been shown to protect from allergies and asthma. 3 China has developed from a traditional agricultural society into an industrialised country with dense populations in megacities and the accompanying challenges of air pollution. These rapidly changing lifestyle and environmental factors could affect the incidence and prevalence of asthma. Estimates of the prevalence of asthma in China from previous epidemiological studies 4 , 5 ranged from 1·2% to 5·8% in the surveyed territories, but the studies did not include systematic lung function testing.