Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the ‘quiet zone’, in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to st...

Recently, there has been renewed interest in the importance of the small airways (internal perimeter v6 mm) in asthma [1]. This was initiated largely by the observation that the use of the inhaled corticosteroid beclomethasone as a small-particle formulation, with greater penetration to the peripheral lung, results in similar improvements in symptoms and lung function at half the delivered dose...