Physiological and histological alterations in the bronchial mucociliary clearance system of rabbits following intermittent oral or nasal inhalation of sulfuric acid mist.

Rabbits were exposed to submicometer sulfuric acid mist (H2SO4) for 1 h/d, 5 d/w for 4 wk, during which time mucociliary clearance was monitored by external in vivo measurements of tagged tracer aerosol retention. One group was exposed orally to 250 micrograms/m3, another to the same concentration via the nose, and a third to 500 micrograms/m3 also via nasal breathing. Clearance was accelerated on specific individual days during the course of the acid exposures, especially at 500 micrograms/m3. In all series, clearance was significantly faster, compared to preexposure controls, during a 2-wk follow-up period after acid exposures had ceased. At the end of this period, the rabbits were sacrificed, and histological sections were obtained from the tracheobronchial tree. Significantly increased epithelial thickness of small conducting airways, compared to sham exposure controls, occurred in rabbits exposed orally at 250 micrograms/m3 or nasally at 500 micrograms/m3, and additionally the lumen of the smallest airways of the former group was narrower than control. The number of airways containing epithelial secretory cells was also significantly greater in these acid exposure groups compared to sham controls. The only change in the rabbits exposed nasally at 250 micrograms/m3 was a significant increase in the number of airways with epithelial secretory cells in the smallest airway classification. The histological alterations provide a basis for observed changes in clearance, and are similar to those found in chronic bronchitis in humans and experimental animals. Differences in site and degree of histological response and degree of physiological change between the two groups exposed to identical acid concentrations appear to have been due to differences in exposure mode, with resultant effects on breathing pattern, aerosol size distribution, and concentration penetrating beyond the upper respiratory tract to specific lung sites.