In the absence of adequate data exclusively from studies of inhaled particles in people, the results of inhalation studies using laboratory animals are necessary to estimate particle retention in exposed people. To make accurate projections from animal studies and the limited human data, it is necessary to consider species similarities and differences in lung retention and accumulation patterns for inhaled materials. This paper reviews species similarities and differences in pulmonary retention and clearance for inhaled particles, with emphasis on animal species most commonly used in inhalation toxicology research (rats, guinea pigs, dogs, and nonhuman primates). Simulation models for these four species and for humans were used to compare projected lung burdens which would be accumulated during chronic inhalation exposures. These simulation models project an eightfold difference among these species in the lung concentration of particles per gram of lung after a 2-y chronic inhalation exposure to the same aerosol for 8 h d-1, 5 d wk-1. The largest lung accumulation would occur in guinea pigs, the smallest in rats. To reach the same target lung concentration of particles in the lungs of both animals would therefore require about an eightfold difference in air concentration of the exposure material. These comparisons are useful for selecting appropriate laboratory animal species to study as surrogates for humans, for setting aerosol concentrations to use in inhalation studies, and for making approximations of lung burdens that would result from defined exposure scenarios.

We investigated the modifying effects of preexisting, bleomycin-induced pulmonary fibrosis on the deposition, retention, and biological effects of inhaled 239PuO2 in the rat. Among rats exposed to similar airborne concentrations of 239PuO2, initial lung burdens of 239Pu per kilogram body mass were similar whether or not pulmonary fibrosis was present. However, clearance of 239Pu from the lungs was significantly decreased in the rats with preexisting pulmonary fibrosis. The incidence of lung lesions (epithelial hyperplasia, diffuse macrophage increases and aggregation, and loose and dense connective tissue) was significantly greater among rats with preexisting pulmonary fibrosis than among the exposed controls. Rats with preexisting fibrosis had shorter life spans than 239PuO2-exposed control rats. When groups of rats with similar alpha doses to the lungs were compared, the incidences of neoplastic lesions in the lung, the times to death of rats with lung neoplasms, and the risk of lung tumors per unit of alpha dose to the lungs in rats with or without pulmonary fibrosis were similar. The results of this study suggest that humans with uncomplicated pulmonary fibrosis may not be more sensitive to the carcinogenic effects of inhaled 239PuO2 than are individuals with normal lungs, assuming that the total alpha doses to the lungs are similar.

Chronic inhalation of insoluble particles of low toxicity that produce substantial lung burdens of particles, or inhalation of particles that are highly toxic to the lung, can impair clearance. This report describes model calculations of accumulations in lung of inhaled low-toxicity diesel exhaust soot and high-toxicity Ga2O3 particles. Lung burdens of diesel soot were measured periodically during a 24-mo exposure to inhaled diesel exhaust at soot concentrations of 0, 0.35, 3.5, and 7 mg m-3, 7 h d-1, 5 d wk-1. Lung burdens of Ga2O3 were measured for 1 y after a 4-wk exposure to 23 mg Ga2O3 m-3, 2 h d-1, 5 d wk-1. Lung burdens of Ga2O3 were measured for 1 y both studies using inhaled radiolabeled tracer particles. Simulation models fit the observed lung burdens of diesel soot in rats exposed to the 3.5- and 7-mg m-3 concentrations of soot only if it was assumed that clearance remained normal for several months, then virtually stopped. Impaired clearance from high-toxicity particles occurred early after accumulations of a low burden, but that from low-toxicity particles was evident only after months of exposure, when high burdens had accumulated in lung. The impairment in clearances of Ga2O3 particles and radiolabeled tracers was similar, but the impairment in clearance of diesel soot and radiolabeled tracers differed in magnitude. This might have been related to differences in particle size and composition between the tracers and diesel soot. Particle clearance impairment should be considered both in the design of chronic exposures of laboratory animals to inhaled particles and in extrapolating the results to people.

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