Clearance from the pulmonary region

To calculate the dose to the lung from inhalation of insoluble particulate activity of long radioactive half-life, it is necessary to estimate the clearance from the P region. ICRP (1979) assumed a clearance half-life of 500 d. Booker et al. (1967) found half-lives in the range 150-300 d in subjects who had inhaled polystyrene particles labelled with 51Cr, but the radioactive half-life of this isotope (27 d) restricted the duration of the measurements to 100 d. In more recent 

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Lung Clearance Class (fast, F medium, M slow, S)—A classification scheme for inhaled material according to its rate of clearance from the pulmonary region of the lungs to the blood and the gastrointestinal tract. 

Fig. 17. Biological model recommended for describing the uptake and retention of cerium by humans after inhalation or ingestion. Numbers in parentheses give the fractions of the material in the originating compartments which are cleared to the indicated sites of deposition. Clearance from the pulmonary region results from competition between mechanical clearances to the lymph nodes and gastrointestinal tract and absorption of soluble material into the systemic circulation. The fractions included in parentheses by the pulmonary compartment indicate the distribution of material subject to the two clearance rates however, these amounts will not be cleared in this manner if the material is previously absorbed into blood. Transfer rate constants or functions, S(t), are given in fractions per unit time. Dashed lines indicate clearance pathways which exist but occur at such slow rates as to be considered insignificant compared to radioactive decay of the cerium isotopes.

For the control of internal doses, annual limits of intake (ALI) and derived air concentrations (DAC) have been determined. ALIs and DACs in EPA guidance and the USNRC and DOE regulations are based on the recommendations of the ICRP (ICRP 1979). Values of the ALIs and DACs for uranium isotopes are presented in Table 7-1. These values are for soluble. Class D (Days) material, which has a half-time for clearance from the pulmonary region of the lung of less than 10 days. Values of ALIs and DACs for Class W (Weeks) and Class Y (Years) uranium are available in Appendix B to 10 CFR 20 (USNRC 1993f). 

There are also significant species differences in respiratory tract deposition and clearance of inhaled particles (Thomas, 1972). Data from a number of studies are summarized in Table 14 to assist in evaluating the several factors that influence the retention of inhaled materials. In studies with rats, Syrian hamsters, Chinese hamsters, and mice, the l44Ce was generally lost at a more rapid rate from the pulmonary region than was noted for the dog. A major factor in this difference may be differences in initial deposition sites and perhaps... 

Morris, K.J., Khanna, P., Batchelor, A.L. (1990). Long-term clearance of inhaled U02 particles from the pulmonary region of the rat. Health Phys. 58 477-85. 

Significant differences exist among species in the rate of removal of inhaled particulate matter. These differences are perhaps most evident for long-term clearance of PM from the pulmonary region (Fig. 11a) and translocation to lung-associated lymph nodes (see Fig. lib). The basis for the marked difference in... 

The polystyrene particles and the 1-IHD labelled tar were removed from the TB region with similar clearance half-lives of about 2 h (Fig. 7.5). The pulmonary removal of polystyrene was too slow to measure with the 123I label, but 1-IHD was removed from the P region with a half-life of 18 h, and this was attributed to absorption into the bloodstream. 

Pulmonary clearance of tantalum dust following insufflation by humans was dependent upon particle size a 1-pm powder was removed from the alveolar regions with a clearance half-time of 2.1 years, while 5-pm and 10- xm powders were removed with a half-time of 333 days (Morrow et al. 1976). Following the accidental exposure of a human to Ta and Ta via inhalation at a nuclear reactor test site, 93% of the activity was eliminated entirely in the feces within 7 days (Sill etal. 1969) the remaining radioactivity was slowly eliminated at a rate of 0.05% per day, but no radioactivity was detected in the urine. In another... 

Figure 11 Schematic rendering of the clearance of relatively insoluble particles from (a) pulmonary region and (b) accumulation in lung-associated lymph nodes in several species following a single brief inhalation exposure. (From Ref. 32.)...

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