The effects of one hour exposures of healthy nonsmoking human volunteers to submicrometer H2SO4 droplets via nasal mask on tracheobronchial mucociliary particle clearance were studied using two different sized monodisperse gamma-tagged Fe2O3 test aerosols. The larger sized Fe2O3 aerosol, 7.5 micrometers AMAD, was deposited primarily in the larger bronchial airways, while the smaller 4 micrometers AMAD aerosol had a much greater fraction deposited in the smaller and more distal conductive airways. Thoracic retention of the Fe2O3 aerosols as a function of time after a brief inhalation was measured with external collimated radiation detectors. At the highest H2SO4 exposure, 1,000 micrograms/m3, there was a pronounced transient slowing of bronchial mucociliary clearance of both the 7.5 and 4 micrometers Fe2O3. On the other hand, at the lowest H2SO4 concentration, 100 micrograms/m3, there was a marked acceleration of the clearance of the 7.5 micrometers Fe2O3, but a slowing of the clearance of the 4 micrometers Fe2O3. Thus, submicrometer H2SO4, which deposits primarily in the distal airways, can slow mucociliary clearance in those airways. In the larger airways, where its deposition is minimal, the H2SO4 can, at the same time, accelerate mucus transport.