The ability of the lung to mechanically remove inhaled deposited particles is an important mammalian defense mechanism that can be evaluated in the laboratory. Experiments that measure clearance kinetics have been performed by various investigators using human as well as large and small laboratory animals as subjects. Several agents have been shown to significantly alter clearance phenomena in the lung. This paper describes quantitative clearance experiments that used radioactive tracer particles to assess lung damages after exposure of rats to ozone, a photochemical air-pollutant gas. Radioactively labeled tracer microspheres were inhaled by groups of 30 rats prior to exposure to ozone. Ozone levels studied were 0.4, 0.8, and 1.0 ppm and all exposures were 4 hr in length. These exposures caused a delay in the early (0-50 hr postdeposition) clearance and an acceleration in the late (50-300 hr postdeposition) clearance rate of the tracer particles. Dose response curves show that clearance was affected more by the higher concentrations of ozone.

Monodisperse radioactive polystyrene latex microspheres have been prepared by attachment of chromium-51 to latex spheres. Labeling was by emulsion polymerization of chromium acetylacetonate dissolved in styrene monomer using commercially available microspheres. This was accomplished with two successive polymerization steps: radiation excitation and radical polymerization initiated with potassium persulfate. After attachment of the label the particle suspension was purified by repeated centrifugation wash cycles to remove labile radioactivity. Results indicate a radioactive binding yield of greater than 80% to the particles. The stability of the label was tested in in vivo and in vitro leaching studies. In these tests, the activity leaching rate was estimated to be less than 0.2% per day.

The effects of a 4 hour exposure to a model sulfur pollutant atmosphere on the clearance of inhaled insoluble tracer particles from the lungs of rats has been studied. The pollutant combination consisted of 5 ppm of sulfur dioxide gas and 1.5 mg/m3 of sulfate aerosol at 80-85% relative humidity. The exposure atmosphere was aged for 30 minutes upstream of the exposure chamber in an aging line in order to provide for gas/ particle interactions such as those occurring in industrial and environmental atmosphere. Results indicate that the sulfur pollutant atmosphere did not produce a statistically significant alteration in early (nasopharyngeal and tracheobronchial) or late (parenchymal) clearance rates such as those which have been identified in this laboratory following exposure to ozone-containing atmospheres.

Several inhaled atmospheres were tested for effects on the rat respiratory defense system. Materials studied included ozone and aerosols of ammonium sulfate, ferric sulfate, and sulfuric acid; relative humidity was also a controlled experimental variable. Each sulfate was studied alone as a submicrometer aerosol at a concentration of 3.5 mg/m3 in air and combined with ozone at 0.8 ppm. Results were compared with those for sham-exposed animals and for rats exposed to ozone alone. Air pollutant exposures, inside stainless steel chambers, were one time only for 4 h. The end points for evaluation of effects were measurements of early and late rates of clearance of radiolabeled insoluble tracer particles. Tracer particles were inhaled before air pollutant exposures and particle clearance was followed for about 2 wk. Ozone alone slowed the early (0-50 h after exposure) particle clearance and stimulated clearance during the later phase (2-17 d). High humidity usually amplified these effects of ozone as well as many of the other atmospheres studied. Sulfate aerosols alone tended to produce relatively small effects on early or late clearance. Combinations of ozone and aerosols resulted in effects that were similar to those of ozone alone. The data do not support the hypotheses that sulfate aerosols synergize with ozone in altering respiratory tract clearance, sulfuric acid being a probable exception. These data alone cannot be used to predict the overall health effects of the materials studied.