Generation and characterization of respirable metallic oxide aerosols for pulmonary clearance studies in calves.

An aerosol generating system capable of producing inert, insoluble, non-toxic, respirable, submicronic particles suitable for deposition, retention and clearance studies in the lungs of calves was designed and tested. The machine generated high volume outputs of respirable aerosols by nebulization of an aqueous solution of metallic ions, drying the droplet aerosol, thermal degradation in the presence of oxygen to form insoluble oxides, and subsequent cooling of the aerosol. Aqueous solutions of ferric nitrate, ferric chloride, ferric citrate, ferric oxalate, zinc acetate, lead acetate, cobalt nitrate, cobalt chloride and cobalt acetate were examined, and the resultant aerosols characterized by particle size distribution, shape, aerosol mass concentration, and production of toxic by-products. Iron oxide and cobalt oxide aerosols were judged suitable for inhalation studies in calves, and cobalt oxide was selected for pulmonary clearance studies due to the low background content of cobalt in lung tissue. The cobalt oxide aerosol selected had a geometric mean diameter (GMD) of 0.54 to 0.65 micron with a dimensionless geometric standard deviation (GSD) of 1.41 to 1.71, a mass aerosol concentration of 124 to 335 mg/m3, and a NO2 contamination of 5 to 33 ppm. The cobalt oxide particles were measurably polydisperse, but virtually all particles were smaller than 2.0 microns making the aerosol totally respirable.