In a human study, Forster et al (Foster et al, 1989) examined clearance of radiolabeled 57Co3O4 by external counting of lung burden and urine and faecal collections over 230 days. The fast clearance during the first few days was interpreted as tracheobronchial clearance which was followed by a slow clearance phase. The investigators reported that the measured pulmonary retention half-times in their subjects ranged between 150 and 250 days. Taking into account the urinary and faecal excretion, which they associated with dissolution and AM-mediated (mechanical) clearance, respectively, they concluded that about ¾ of the pulmonary clearance was due to dissolution. Morrow (Morrow 1992) calculated 231 day as the half-time for dissolution and 640 day half-time for AM-mediated clearance. According to the criteria mentioned above, Co3O4 is a poorly soluble particle.