Recent inhalation toxicology studies suggest that when laboratory rats are exposed to inorganic particles to the point that lung overload occurs, both benign and malignant tumours may develop. The significance of these results for estimating human risk, however, is unclear.; Originally published as a special issue of the journal "Inhalation Toxicology", the contributions to this volume are derived from a symposium held in March, 1995, by the Environmental Medical Service of the Massachusetts Institute of Technology. Topics addressed include epidemiology, the pathology of lung tumours, maximum tolerated doses in inhalation toxicology studies, risk assessment, the extrapolation of results from animal studies to humans, current modelling techniques and regulatory concerns.