Evaluation of Particle Clearance and Retention Kinetics in the Lungs of U.S. Coal Miners

Abstract

Rodent studies are frequently used to assess risk in humans, yet it is not known whether the overloading of lung clearance, as observed in rodents, occurs in humans, or whether overloading is related to particle-related lung diseases in humans. The objective of this study is to develop a biologically based mathematical model to describe the retention and clearance of respirable coal mine dust in the lungs of humans. A human dosimetric lung model was developed that includes alveolar, interstitial, and hilar lymph-node compartments. The model describes the particle mass transfer kinetics among these compartments and clearance via the tracheobronchi. The model was calibrated using data in U.S. coal miners, including individual working lifetime exposure histories and lung and lymph-node particle burdens. The model fit to the human data was evaluated using a least-squared error criterion. The end-of-life lung dust burdens of all coal miners in this study were substantially greater than expected from a simple, linear first-order model with effective clearance, yet their lung and lymph-node dust burdens were lower than expected from the rodent-based overload model, particularly at higher exposures. The best fitting model included a predominant first-order interstitial compartment, in which the particles are essentially sequestered (with very slow clearance to the lymph nodes), and a first-order alveolar clearance compartment with either no dose-dependent decline (overloading) or much less than expected from the rodent studies. These findings are consistent with the findings from magnetopneumography studies of clearance in retired miners and from studies of particle retention patterns in rodents and primates. This human dosimetric lung model is useful for evaluating the kinetic differences of particle retention in humans and rodents, and for evaluating the lung closes in humans given different exposure scenarios.