If inhaled irritants lead to airway obstruction, the distribution of ventilation will be altered so that the inspired gas is diverted principally through unobstructed airways. Continued exposure to the irritant will thus establish a vicious cycle in which the irritant will principally be distributed to the healthy airways. In addition, radial diffusion of inspired gas wherever the velocity profile at the front between inspired and alveolar gas is other than blunt will enrich the fast-moving central core with the particulates so that alveolar penetration is deeper than would otherwise be the case.
Peter T. Macklem, William E. Hogg, James Brunton. Peripheral Airway Obstruction and Particulate Deposition in the Lung,. Arch Intern Med. 1973;131(1):93–97. doi:10.1001/archinte.1973.00320070089010