Does lung retention of inhaled particles depend on their geometric diameter?

Experiments with a bolus technique suggest that retained fractions in the a irways are dependent on the geometric diameter of the particles. This view has been adopted by the International Commission on Radiological Protection (ICRP) in its new human respiratory tract model (HRTM). The aim of the pre sent study was to test this view by the use of an inhalation technique, in which particles with an aerodynamic diameter of about 6 mu m are inhaled ex tremely (0.05 l/s) and as a result, the particles are deposited mainly in s mall ciliated airways. Nine healthy subjects inhaled on one occasion monodi sperse In-111-labelled polystyrene particles (geometric diameter 6.05 mu m, aerodynamic diameter 6.2 mu m) and on another occasion manodisperse In-111 -labelled Teflon particles (geometric diameter 4.47 mu m, aerodynamic diame ter 6.5 mu m). Both particles were inhaled at 0.045 L/s and radioactivity i n the body was measured after 0, 24, 48, and 72 hours as well as after 1, 2 , and,for some subjects, also 3 weeks. The retention in the lungs at 24 hou rs was slightly lower for the Teflon particles (47%) than for the polystyre ne particles (51%). From earlier experimental data with different particle sites as well as from predictions with theoretical lung models, this differ ence is reasonably explained by the somewhat larger aerodynamic diameter of the Teflon particles. Clearance of the 2 particle types between 1 day and 2 weeks was similar within each individual as well as in the whole group. T he differences between the clearance of 4.5 mu m and 6 mu m geometric diame ter particles observed in the present experiment are significantly differen t (P < .01) from the differences seen in earlier shallow bolus experiments as well as from the differences for such particles calculated with the HRTM , i.e., our experiment does not support the hypothesis that the fraction re tained after 1 day is dependent on the geometric diameter in the size range studied.