Small airways (<2 mm in diameter) are probably involved across almost all asthma severities and they show proportionally more structural and functional abnormalities with increasing asthma severity. The structural and functional alterations of the epithelium, extracellular matrix and airway smooth muscle in small airways of people with asthma have been described over many years using in vitro studies, animal models or imaging and modelling methods. The purpose of this review was to provide an overview of these observations and to outline several potential pathophysiological mechanisms regarding the role of small airways in asthma.,
European Respiratory Review

van den Bosch, W.B. (Wytse B.), James, A.L. (Alan L.), & Tiddens, H. (2021). Structure and function of small airways in asthma patients revisited. European Respiratory Review (Vol. 30, pp. 1–18). doi:10.1183/16000617.0186-2020