Pulmonary retention and clearance of particles.

Research in pulmonary retention and clearance of particles intensified in the fifties in connection with interests in pneumoconiosis and in inhalation of radioactive particles, and more recently with the increased interest in the effects of environmental particles. The studies enhanced our understanding of clearance mechanisms, the various clearance pathways, the different clearance capacities of various species and the importance of other factors affecting lung clearance. Based on research in recent years, the historical concept of inert and fibrogenic particles was abandoned. It seems that particles even at surprisingly low concentrations may have negative health effects and that ultrafine particles have higher than expected toxicity when compared to similar particles of a larger size.