The 1994 ICRP66 Human Respiratory Tract Dosimetry Model as a Tool for Predicting Lung Burdens from Exposures to Environmental Aerosols

Abstract

The International Commission on Radiological Protection's human respiratory tract dosimetry model was used to predict average particle deposition and retention patterns for two example trimodal (fine, intermodal, and coarse modes) environmental aerosols (Phoenix, Arizona, and Philadelphia, Pennsylvania). Deposited dose metrics are presented as mass (and number) of particles normalized to either respiratory tract region surface area (square centimeters) or mass (grams) of epithelium. Deposition metrics ranged over several orders of magnitude, with extrathoracic > tracheobronchial > alveolar-interstitial. Dissolution-absorption half-times for fine, intermodal, and coarse particles were defined as 10, 100, and 1000 days, respectively, to allow modeling of chronic exposures. Default values for particle physical clearance parameters were used. Retained dose for the alveolar-interstitial region is presented as the steady-state mass of particles (micrograms)/gram epithelium. Modeling results indicated similar retention patterns for the fine particles, but substantially different patterns for the intermodal and coarse particles. Intermodal and coarse particles dominated the Phoenix aerosol, resulting in predictions that long-term retained lung burdens would be about four times higher in individuals chronically exposed in Phoenix versus Philadelphia. This modeling approach improves the understanding of relationships between exposures to environmental aerosols and deposition/retention patterns in the human respiratory tract. Modeling demonstrated significant thoracic deposition of environmental aerosol particles larger than those collected in a PM_2.5 sampler. This result supports the conclusion that using the PM₁₀ aerosol fraction as an exposure index should be a good indicator of potential health effects. Therefore, aerosol sampling should retain PM₁₀ sampling in order to include the entire respirable size range and provide adequate information for predicting deposition and retained dose metrics for environmental aerosols. Ventilatory activity patterns are also necessary to characterize total personal exposure, and the dissolution—absorption of environmental aerosol particles must be determined to allow accurate modeling of their long-term retention in the lung.