Inhalation pathways for plutonium

A number of workers (Garland and Nicholson 1991: Nicholson 1993 :) have noted that large particles greater than 20 um diameter are more easily resuspended from a surface than smaller particles. The least resuspendable size fraction of soils are the less than 1 um diameter particles (Gillette & Walker 1977). The major limiting hazard from the resuspension of plutonium is via an inhalation pathway, i.e. breathing in this material va the nose or mouth (Nicholson 1992).

Particles up to 100 um in diameter are generally thought of as being inhalable with the less than 4 um diameter size fraction defined as the respirable fraction (Lippman & Harris 1962). Inhaled particles deposit to either the nasopharangeal regions, the tracheo-bronchial region or the pulmonary or alveolar regions of the body (Geiss 1993). The fraction of inhaled particles which is retained in the respiratory system and the depth to which the particles penetrate before deposition is sensitively related to particle size (Brown et al 1950). Inhaled particles less than 0.5 um aerodynamic diameter may be carried deep into the lung to irradiate sensitive alveolar tissue (Burkart 1989). Particles greater than 5 um deposit out of the inhaled air stream via impaction or they are intercepted by mucus layers within the nasopharangeal cavity

Mucociliary clearance from the nasopharangeal cavity at the back of the nose transfers the deposited material into the gastrointestinal tract where it is eventually excreted from the body. Transfer factors for plutonium and americium across the gut wall for primates are very small, 10^-3 and 10^-4 respectively (Ham et al 1994). There is little work, however, on changes in speciation of plutonium or americium which may affect the solubility and hence bioavailability of inhaled actinides as they pass down the gastrointestinal tract.

The resuspension and inhalation of particles greater than about 5 um aerodynamic diameter are radiologically less damaging to lung tissue than particles with an aerodynamic size range less than 1 um because this size fraction is less respirable (Geiss 1993).

The figure below show the probability of deposition in various parts of the lung relative to particle size

Figure 1A

Size-dependent deposition probabilities of particulate radioactivity in the different compartments of the human lung (ICRP 1975)

Resuspension Case Study:

In recent years a considerable amount of interest has focused on the inhalation dose from example, the resuspension of plutonium around nuclear plants and old weapons test sites located in Australia (Johnston et al 1976 ;Fry 1983 ;Langer 1983 ;Nicholson & Fulker 1994) via wind-stress. A number of resuspension studies have focused on the inhalation dose of plutonium and americium to Aboriginal tribes-people from single-shot nuclear explosions and radionuclide dispersal experiments conducted between 1955 and 1963 at the desert environment test site in Maralinga W. Australia (Johnston et al 1993).

These authors found an average K of 10^-10 with a three orders of magnitude increase in the resuspension factor during dust storms with average wind speeds in excess of 10 m s^-1 In an earlier study based on the immediate environs of this site, Haywood & Smith (1992) determined the total annual effective dose equivalents from a range of radionuclides including caesium and strontium to the semi-traditional lifestyle of an Aboriginal average population of adult, child and infant. They found the most limiting dose of 470 mSv to the 10 year old child group with the principal pathway being inhalation. This relatively high annual effective dose equivalent is roughly 15 times higher than a similar group some 25 km north east of the test site see also (Johnston et al 1992 ;Stradling et al 1992). The findings of many of these studies, however, may not be strictly applicable to the more temperate latitudes of the northern hemisphere where ground and soil conditions are substantially wetter during the year with average rainfalls of 1100-1300 mm y^-1 (Geiss 1993 ;Playford et al 1992).

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