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 increa
sed interest in the effects of environmental particles. The studies en
hanced our understanding of clearance mechanisms, the various clearanc
e pathways, the different clearance capacities of various species and
the importance of other factors affecting lung clearance. Based on res
earch 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.