Deposition of Ultrafine Particles in Human Tracheobronchial Airways

Abstract

Ultrafine particles smaller than 200 nm in diameter are produced by combustion, radioactive decay and gas-to-particle conversion in the urban air. Inhalation of ultrafine particles in both outdoor and indoor environments is a health concern. Exposure to radioactive radon progeny in the size range of 1-200 nm causes lung cancer in uranium miners and may have similar health effects in people exposed indoors. In the ambient air, ultrafine particles have a higher number concentration and may be more toxic than larger particles. Ultrafine particles deposit in the respiratory tract primarily by diffusion with increased deposition of smaller particles. Total lung deposition of ultrafine particles measured in human volunteers confirms diffusion as the mechanism (Schiller etal., 1988). Higher deposition in the extrathoracic region has been determined in human nasal/oral casts and in human volunteers (Cheng etal., 1993, in press). Deposition in the human tracheobronchial airways has been measured in an adult cast for particle sizes between 40 and 200 nm (Cohen etal., 1990). Their data indicated higher deposition in the tracheobronchial airway than predicted by diffusional deposition in a tube, assuming a fully developed flow profile. The purpose of our study was to determine the tracheobronchial deposition in wider size range of particles using airway casts of adults and children. The deposition data were compared with theoretical predictions of both parabolic and plug flows.