An experimental study of clearance of inhaled particles from the human nose.

Retention in the extrathoracic airways, and clearance by nose blowing, of monodisperse indium-111-labeled polystyrene particles were followed for at least 2 days after inhalation by healthy volunteers. Nine volunteers inhaled 3-µm aerodynamic diameter particles while sitting at rest, whereas subgroups of 3 or 4 inhaled 1.5-µm or 6-µm particles at rest, and 3-µm or 6-µm particles while performing light exercise. Retention of the initial extrathoracic deposit (IETD) in the extrathoracic airways was described by 4 components: on average 19% IETD cleared by nose blowing; 15% was swallowed before the first measurement, a few minutes after inhalation; 21% cleared by mucociliary action between the first measurement and about an hour later; and 45% subsequently cleared by mucociliary action. Geometric mean times in which 50% and 90% of IETD cleared were 2.5 and 22 hours. The geometric mean retention fractions at 24 and 48 hours were 7% and 2.4% IETD, respectively. No clear trends were found between parameters describing retention and any related to deposition (e.g., particle size). However, the fraction cleared by nose blowing was related to the frequency of nose blowing and therefore appears to be a characteristic of the individual.

Effect of particle size on slow particle clearance from the bronchial tree.

The Human Respiratory Tract Model of the International Commission on Radiological Protection assumes that a fraction of particles deposited in the bronchial tree clears slowly, this fraction decreasing with increasing particle geometric diameter. To test this assumption, volunteers inhaled 5-microm aerodynamic diameter 111In-polystyrene and 198Au-gold particles simultaneously, as a 'bolus' at the end of each breath to minimize alveolar deposition. Because of the different densities (1.05 versus 19.3 g cm3), geometric diameters were about 5 and 1.2 microm, respectively, and corresponding predicted slowly cleared fractions were about 10% and 50%. However, lung retention of the 2 particles was similar in each subject. Retention at 24 hours, as a percentage of initial lung deposit (mean +/- SD) was 34 +/- 12 for polystyrene and 31 +/- 11 for the gold particles.

Further study of the effect of particle size on slow particle clearance from the bronchial tree.

The ICRP human respiratory tract model (HRTM) assumes that the 'slow-cleared fraction' fs in the bronchial (BB) and bronchiolar (bb) regions decreases with increasing particle geometric diameter dp. To test this assumption, five volunteers inhaled 8-microm aerodynamic diameter, dae, (111)In-polystyrene (PSL) and (198)Au particles simultaneously. The particles were administered at the end of each breath to maximise deposition in BB and bb. Because of the difference in densities (1.05 vs. 19.3 g cm(-3)), corresponding dp values are approximately 8 and 1.8 microm, and fs values are approximately 2 and 50%, respectively. However, lung retention was somewhat greater for PSL than for gold, contrary to the HRTM prediction. Other recent human experimental studies found no difference in retention for particles with the same dae but different dp values. It is concluded that the HRTM model of particle clearance from BB and bb needs revision.

Implications of human nasal clearance studies for the interpretation of nose blow and bioassay sample measurements.

The Smith-Etherington extrathoracic (ET) clearance model has been developed to describe clearance from the human nasal passage as observed for 1.5, 3 and 6 microm monodisperse particles inhaled by participants while sitting or at light exercise. Model parameter values have been determined for 22 sets of nasal clearance data from which typical nasal clearance parameter values have been derived. In this investigation, the faecal excretion of 5 microm activity median aerodynamic diameter (AMAD) Type S 239Pu was calculated for three sets of model parameter values, i.e. typical values and those determined for two experiments that respectively exhibited particularly rapid and slow clearance. The study has shown that a greater fraction of material deposited in ET is cleared to the GI tract, and that this clearance takes place over a longer period than assumed by the Human Respiratory Tract Model for Radiological Protection. The study has also shown that deliberate nose blow sampling might potentially be developed to determine the order of magnitude of intakes.

A feasibility study for transportable 241Am-in-lung and 241Am-in-nose-blow monitoring systems for use following a weapons accident.

In a nuclear weapon accident involving fire or conventional explosion, most of the radiation dose received by people in the immediate vicinity would result from inhalation of 239Pu. This is accompanied by the nuclide 241Am, which is much easier to determine by external counting because of the 60 keV gamma ray emission. In the event of an accident, a priority would be to identify any people who have had intakes of 239Pu which were so large that decorporation therapy should be considered. Direct measurement of lung content provides the most rapid and convenient method for assessing intakes by inhalation. A transportable system has been considered as this could be deployed close to the site of the accident and would allow rapid measurements to be made. The feasibility of a transportable 241Am-in-nose-blow and nasal swab measurement system has also been considered. This would be used to help select people for 241Am-in-lung measurements.