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.

After a release of radionuclides, accidental or otherwise, there will be an urgent need to identify members of the general public who have received a significant intake of radioactive material, sufficient to require medical treatment or further investigation. A large number of people could be contaminated in such an incident. For gamma-ray emitting radionuclides this screening could be carried out using gamma camera medical imaging systems, such as those that are present in many large UK hospital sites. By making a number of simple reversible changes such as removal of collimators, these cameras could be employed as useful additional screening instruments as well as an aid in contamination control. A study was carried out to investigate which systems were present in sufficient number to offer wide scale coverage of UK population centres. Nine gamma cameras (eight dual head and one single head) were assessed using point source and bottle mannequin (BOMAB) phantom measurements so that a mathematical model could be developed for use with the MCNPX Monte Carlo radiation transport code. The gamma camera models were assessed for practical seated and supine geometries to give calibration factors for a list of target radionuclides that could be released in a radiological incident. The minimum detectable activities (MDAs) that were achieved for a five minute measurement demonstrated that these systems are sufficiently sensitive to be used for screening of the general public and are comparable to other body monitoring facilities. While gamma cameras have on-board software that are designed for imaging and provide for a gamma-ray energy range suitable for radionuclides for diagnostic imaging (such as Tc), they are not as versatile as custom-built body monitoring systems.

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.

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.

The part played by individual monitoring within the context of the overall response to incidents involving the malevolent use of radiation or radioactive material is discussed. The main objectives of an individual monitoring programme are outlined, and types of malevolent use scenario briefly described. Some major challenges facing those with responsibilities for planning the monitoring response to such an incident are identified and discussed. These include the need for rapid selection and prioritisation of people for individual monitoring by means of an effective triage system; the need for rapid initiation of individual monitoring; problems associated with monitoring large numbers of people; the particular difficulties associated with incidents involving pure-beta and alpha-emitting radionuclides; the need for techniques that can provide retrospective estimates of external radiation exposures rapidly and the need for rapid interpretation of contamination monitoring data. The paper concludes with a brief review of assistance networks and relevant international projects planned or currently underway.