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For radiation protection purposes, the International Commission on Radiological Protection (ICRP) Task Group proposes to apportion radiation risk within the respiratory tract according to the tumor mortality rates observed in the different anatomical regions. This approach requires that doses absorbed by extrathoracic tissues must be considered, in addition to those in the lung. For the extrathoracic region, the tissues at highest potential risk are the pharyngeal parts of the nasopharynx and oropharynx and a part of the larynx. In the lung, all tissues are potentially at risk, and it is necessary to consider doses absorbed by bronchial tissues, the lung parenchyma, and lymph nodes. This paper outlines the methods proposed by the Task Group to evaluate the heterogeneous doses absorbed by sensitive cells in these tissues from radioactive decays of alpha-emitters. The objective is to evaluate doses to broad regions of the respiratory tract, where the regions are defined to reflect substantial differences in potential risk when taking into account deposition and clearance behavior. The Task Group proposes to represent the respiratory tract by three generic regions: an extrathoracic region and two thoracic regions, one clearing fast and one slowly. The models of aerosol deposition and clearance applied for each region are outlined. To illustrate the use of the model, doses are evaluated for the key cases of short-lived radionuclides and long-lived insoluble alpha-emitters and are discussed with regard to current ICRP recommendations. [ABSTRACT FROM AUTHOR]

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 clearance was periodically evaluated throughout 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 clearances 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. [ABSTRACT FROM AUTHOR]

A simple bioassay model for predicting the organ burdens of 241Am from excretion rates is presented for inhalation exposures. The model uses three compartments representing lung, liver and skeleton. The model was developed using data from studies in laboratory animals of inhaled or injected 241Am and was validated for people by comparison to cases of accidental inhalation exposures to 241Am. The data for people have a large amount of variability but indicate that the retention half-time of 241Am in liver is ∼2 yr and in skeleton is ∼30 yr. These parameters can be used in the model to estimate body and organ burdens from excretion rates after inhalation of 241Am or the model can be fitted to an individual's measured excretion rates. [ABSTRACT FROM AUTHOR]

Potential health effects resulting from the accidental exposure of people to beryllium metal are of concern. To investigate the effects of relatively low levels of beryllium metal on lung clearance, we simuitaneously exposed rats to beryllium metal and radioactive tracer particles. Exposure to beryllium metal aerosol to achieve estimated lung burdens of 9 or 52 μg significantly retarded clearance up to 365 days after exposure compared to controls, whereas lung burdens of 1.5 or 2 μg had no significant effect on clearance. Groups of rats were sacrificed at 8,16,40,90,210 and 365 days after exposure for bronchoalveolar lavage. The total numbers of cells, incidence of neutrophils, the levels of total protein, and the enzymes lactate dehydrogenase and β-glucuronidase were generally elevated in lavage fluids from groups of rats that also had impaired lung clearance. This study serves to further define the levels of beryllium metal required to retard lung clearance and induce accompanying pathological responses in the lungs of rats. [ABSTRACT FROM PUBLISHER]

Respiratory tract dosimetry models for inhaled radioactivity are currently being reviewed and revised by a task group of the U.S. National Council on Radiation Protection. The revised models are likely to place increased emphasis on the nasal and tracheobronchial airways because studies with laboratory animals and people have shown cancers in these regions to be associated with inhaled radioactivity or other toxic substances. A morphometric model of the tracheobronchial airways that describes a typical airway path for the respiratory tract and a clearance model based upon mucus flow velocity in conducting airways are also being evaluated. This combination of models appears to offer the greatest flexibility for dosimetry calculations applicable to people of different body size and health status. Doses to all epithelial cells lining the conducting airways will be calculated, instead of focusing exclusively on the basal cells. [ABSTRACT FROM PUBLISHER]

Lifespan studies of Beagle dogs that inhaled beta-emitting radionuclides in a relatively insoluble form have been conducted at the Lovelace Inhalation Toxicology Research Institute. These studies were designed to determine the types and locations of radiation-induced tumors as well as the influence of dose protraction on the incidence of such tumors. Although not all these studies are complete, data on organs and cells at risk are available. Histologic tumor types of the lung indicate that numerous cell types are probably at risk after inhalation of beta-emitting radionuclides. Tumors of blood vessels, connective tissue, and pulmonary epithelium have been seen. The numerous cell types involved in tumors of the pulmonary epithelium make it difficult to designate one cell type at risk. The tumors do originate, however, in the terminal airway and alveolar portions of the lung, not the larger airways. Organs other than lung developed tumors through direct irradiation of the tissues. These are tracheobronchial lymph nodes and heart. Neither of these organs has been considered targets for radionuclide-induced tumors. Results from these studies indicate that attention to dose effect relationships in these non-pulmonary tissues may be warranted. [ABSTRACT FROM PUBLISHER]

Alterations in Particle Accumulation and Clearance in Lungs of Rats Chronically Exposed to Diesel Exhaust. WOLFF, R. K., HENDERSON, R. F., SNIPES, M. B., GRIFFITH, W. C., MAUDERLY, J. L., CUDDIHY, R. G., AND MCCLELLAN, R. 0. , 154–166. F344 rats were chronically exposed to diesel exhaust at target soot concentrations of 0 (control, C), 0.35 (low, L), 3.5 (medium, M), and 7.0 (high, H) mg/m Accumulated lung burdens of diesel soot were measured after 6, 12, 18, and 24 months of exposure. Parallel measurements of particle deposition and clearance were made to provide insight into the mechanisms of particle accumulation in lungs. The fractional deposition of inhaled GaO particles after 6, 12, 18, and 24 months of exposure and of inhaled Cs-fused aluminosilicate particles after 24 months were similar for all groups. Progressive increases in lung burdens of soot particles were observed in M and H exposed rats, reaching levels of 11.5 ± 0.5 and 20.5 ± 0.8 mg/lung (SE), respectively, after 24 months. Rats in the L group had smaller relative increases in lung burden, reaching levels of 0.60 ± 0.02 mg/lung after 24 months. Tracheal mucociliary clearance measurements, using Tc-macroaggitated albumin deposited in the trachea, showed no changes at anytime. There were statistically significant increases inclearance half-times of inhaled radiolabeled particles of GaO as early as 6 months at the H level and 18 months at the M level; no significant changes were seen at the L level. Rats inhaled fused aluminosilicate particles labeled with Cs after 24 months of diesel exhaust exposure to measure long-term components of pulmonary clearance. The long-term clearance half-times were 79 ± 5, 81 ± 5, 264 ± 50, and 240 ± 50 days (± SE) for the C, L, M, and H groups, respectively. Differences were significant between the C and both the M and H exposure groups (p <0.01). Lung burdens of diesel soot were more than expected at the H and M levels and were also associated with impaired particle clearance while smaller responses were observed in both burdens and clearance at the L level. [ABSTRACT FROM PUBLISHER]