We have previously shown that 1-hr exposures to 0.5 microns sulfuric acid (H2SO4) mist at 100 and 1000 micrograms/m3 produced transient alterations of bronchial mucociliary clearance of monodispersed 7.6 and 4.2 microns mass median aerodynamic diameter gamma-tagged ferric oxide (Fe2O3) in healthy nonsmoking humans in a dose-dependent manner. To determine the role, if any, of the length of exposure, 10 healthy volunteers were exposed to 100 micrograms/m3 H2SO4 for 1 hr and 2 hr on separate occasions, 1 week apart, with measurements of their mucociliary clearance of 5.2 microns Fe2O3 particles inhaled both before and after the inhalation of the H2SO4. Their rate of bronchial mucociliary clearance was markedly reduced for both Fe2O3 aerosols, with slower clearance of the aerosol inhaled after the H2SO4 exposure. For the tagged Fe2O3 aerosol inhaled after exposure for 2 hr at 100 micrograms/m3 H2SO4, the tracheobronchial clearance halftime, (T50), tripled from control, and the reduced rate of clearance was still evident 3 hr after the end of exposure. The 1-hr 100 micrograms/m3 H2SO4 exposure doubled T50 from control, and the reduced rate of clearance lasted for about 2 hr after the end of exposure. These results indicate that the effect of doubling the length of exposure was as great or greater than an order of magnitude increase in the concentration of H2SO4.