Respiratory epithelium exposed to sulfur dioxide--functional and ultrastructural alterations

The value of morphological investigations of airway mucosa should be compared to a functional method when estimating the toxicity of airborne pollutants. In 34 guinea pig tracheas, mucociliary activity was measured using a modified light beam reflex method before and following exposure to sulfur dioxide for 30 minutes in concentrations ranging between 7.5 and 37.5 mg/m3. Exposure to air served as a control. Simultaneously, specimens were taken for light and electron microscopy. Mucociliary activity decreased from 8.4 +/- 2.9 Hz (control exposure) to 4.0 +/- 2.9 Hz following exposure to 7.5 mg/m3, to 3.4 +/- 2.7 Hz at 15 mg/m3 sulfur dioxide, to 1.8 +/- 2.2 Hz at 22.5 mg/m3 sulfur dioxide, to 1.5 +/- 1.8 Hz at 30 mg/m3 sulfur dioxide, and to 2.0 +/- 1.2 Hz at 37.5 mg/m3 sulfur dioxide, respectively (P < .01). Despite a 56% decrease in mucociliary activity, only minor morphological alterations were observed following exposure to 7.5 mg/m3 sulfur dioxide. However, following exposure to 15 mg/m3 sulfur dioxide or higher, structural alterations of respiratory mucosa such as epithelial sloughing, intracellular edema and mitochondrial swelling, widened intercellular space, and ciliary cytoplasmic extrusions were found. Thus measurement of mucociliary activity proved to be a more sensitive indicator of airway toxicity than structural investigations alone.