Comparative Dosimetry of Radon in Mines and Homes (1991) / Chapter Skim
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Breathing, Deposition, and Clearance
Breathing, Deposition, and Clearance
Pages 137-165
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From page 137... ...
This chapter summarizes aerosol deposition and clearance mechanisms and discusses how both of these influence the amount of alpha energy from radon progeny delivered to target sites. The chapter also discusses approaches that can be used to describe the amount and distribution of doses and, finally, some factors that are known to influence the amount and distribution of retained aerosols.
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From page 138... ...
Since the rates and pathways of clearance are determined by the sites of aerosol deposition, it is necessary to study factors such as exercise and disease that strongly influence the distribution of the retained aerosol in the lungs. DEPOSITION OF RADON PROGENY: GENERAL PRINCIPLES Deposition is the process that determines what fraction of the inspired particles is caught in the respiratory tract and, thus, fails to exit with expired air.
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From page 139... ...
However, as particle size becomes very small, diffusion may become an important mechanism even in the upper airways. Gravitational Gravity accelerates falling bodies downward, and terminal settling velocity is reached when viscous resistive forces of the air are equal and opposite in direction to gravitational forces.
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From page 140... ...
Generally, inertial impaction is an important deposition mechanism for particles with aerodynamic diameters larger than 2 Am. Thus, it is probably unimportant for radon progeny in indoor air.
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From page 141... ...
The extent to which charge influences deposition of radon progeny is largely unknown. Other forces acting to affect deposition such as acoustic forces, magnetic forces, or thermal forces, are normally not significant in the respiratory tract.
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From page 142... ...
Since the MMAD is expressed as an aerodynamic diameter, it describes how the aerosol behaves in the respired air and can be used to estimate where and by what processes the aerosol deposits in the respiratory tract. The GSD denotes the spread of particle sizes.
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From page 143... ...
, and has recently been reviewed (Morrow, 1986~. The overall effect borne out in these studies is that, with increasing hygroscopicity and relative humidity, deposition fraction as a function of particle size is shifted to smaller particle sizes.
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From page 144... ...
A significant change in the effective anatomy of the respiratory tract occurs when there is a switch between nose and mouth breathing. There are inter- and intraspecies differences in lung morphometry; even within the same individual, the dimensions of the respiratory tract vary with changing lung volume, with aging, and with pathological processes.
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From page 145... ...
In contrast, particles deposited in the conciliated compartments have much longer residence times; there, small differences in in viva solubility can have great significance. The speed of mucous flow can be affected by factors influencing either the cilia or the amount and quality of the mucus.
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From page 146... ...
More studies are needed to elucidate the best methodology to model mucociliary clearance and to understand its role and importance in patients with pulmonary disease. Clearly, it is also essential to understand the initial deposition pattern of an inhaled aerosol in order to assess the importance of mucociliary clearance on the disappearance of the aerosol from the lungs.
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From page 147... ...
Furthermore, along with the inspiratory volumetric flow rate, airway anatomy specifies the local linear velocity of the airstream and, thus, whether the flow is laminar or turbulent. There are inter- and intraspecies differences in lung morphometry (Soon" et al., 1979; Schlesinger and McFadden, 1981; Phalen, 1984; Nikiforov and Schlesinger, 1985~; even within the same individual the dimensions of the respiratory tract vary with changing lung volume, with aging, and with pathological processes.
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From page 148... ...
Within one person, a decrease in lung volume from 4,800 to 2,400 ml not only increases deposition but also causes the major site of particle deposition to shift from the lung periphery to the central airways (Agnew, 1984~. At low lung volumes, central airways have smaller cross-sectional areas and, thus, higher linear velocities.
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From page 149... ...
Inertial impaction is more important in central airways, where linear velocities are high; diffusion and settling are more important in peripheral airways and alveoli, where residence times are longer and distances are smaller. Second, minute volume (respiratory frequency x tidal volume)
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From page 150... ...
The combination of a small cross section for airflow, sharp curves, and interior nasal hairs helps to maximize particle impaction. Abundant evidence indicates that significant fractions of inhaled particles and gases can be deposited in the nose and pharynx.
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From page 151... ...
The nose also has a major role as a collector of inhaled aerosol particles. A small cross section for airflow and the resulting high linear velocities, sharp curves, and interior nasal hairs all help to promote particle impaction.
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From page 152... ...
, there may be increases in aerosol deposition that may be associated with flow limitation (Smaldone and Messina, 1985~. One factor that determines where radon progeny are deposited in the lungs is the distribution of ventilation.
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From page 153... ...
. Scintigraphic analysis of particle deposition shows that deposition is greater in the central airways of patients with airway obstruction than in airways of normal patients (Lourenco et al., 1972; Ramanna et al., 1975; Taplin et al., 1977; Itch et al., 19811.
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From page 154... ...
One common result in all three diseases is that the presence of detectable pulmonary disease always results in less uniform patterns of particle retention throughout the lungs. This pattern could not be explained by differences in breathing pattern (Sweeney et al., 1983b, 19861.
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From page 155... ...
With the progressive loss of supportive parenchymal tissue as emphysema progresses, the decreasing airway caliber would favor enhanced airway deposition and increased overall heterogeneity of deposition. Airway changes caused by physical obstruction of the airways because of mucus plugging (as in animals with chronic bronchitis)
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From page 156... ...
A compelling conclusion is that ethnicity is not a major factor influencing exposure-dose relationships for inhaled particles.
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From page 157... ...
CONCLUSIONS Determination of the distribution of inhaled radon daughters within the respiratory tract is one dimension of the more general problem of determining exposure-dose and dose-response relations. It is likely that some of the variability in response among different individuals and various animal species may result from differences in the concentration of radon progeny at the site of action as well as variations in the inherent responsiveness of specific active tissues.
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From page 158... ...
1957. Aerosol deposition in the human respiratory tract.
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From page 159... ...
1977. The size of soluble aerosol particles as a tunction of the humidity of the air: Application to the human respiratory tract.
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1982. Biological variability of particle deposition in the human respiratory tract during controlled and spontaneous mouth-breathing.
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1969. The effect of particle size on the regional deposition of inhaled aerosols in the human respiratory tract.
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1985. Ambient sulfate aerosol deposition in man: Modeling the influence of hygroscopicity.
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1975. Radioaerosol lung imaging in chronic obstructive pulmonary disease.
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1982. Breathing patterns influence aerosol deposition sites in excised dog lungs.
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From page 165... ...
1971. Inhaled Particles III.
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