We read with interest the study of Smith and colleagues,1 because besides their counterintuitive result of thinner airways in COPD, another important result is their smaller airway lumen areas of the whole bronchial tree. Their Table E4 shows that adjusted values of areas for confounding factors are significantly reduced from trachea to the sixth airway generation in COPD.

Based on theoretical arguments, the whole human bronchial tree anatomy can be described by only two factors: the tracheal cross-sectional area and the homothety factor (diameter of daughter bronchus over diameter of parental bronchus), which describes the reduction of calibre at each subsequent generation.2 The airway sections are highly variable from person to person, and gender and height explain only a minor part of this variance,2 but it justifies the statistical approach of Table E4. One may hypothesise that pre-existing lung anatomy constitutes a risk...