AbstractImmediately after a 20 min nose only exposure to51Cr labeled polystyrene latex microspheres (1.4 MMAD, geometric standard deviation = 1.3, 2 μg m−3), 23 of the measured radioactivity was within the trachealungs of the exposed rats, 37 was within the gastrointestinal tract, 10 was within the defurred head, and 30 was on the fur. One hour after deposition these percentages had changed to 20, 64, 5 and 11 respectively. As a result of this non‐pulmonary particle load transiting the esophagus, external thoracic radioactivity measurements did not accurately reflect excised lung measurements until 30 h post‐deposition. Consequently, it was necessary to combine excised lung ratioactivity data (0–30 h post deposition) with serial external thoracic measurements (30–934 h post‐deposition) to measure the clearance of these microspheres. Use of an annulus shaped detector sufficiently increased sensitivity to allow measurements to be made through at least 934 h post‐deposition. The lung radioactivity retention curve was biexponential with half times of 34.3 ± 2.3 and 963 ± 107 h. Simultaneous exposures to 5 CO2increased deposition by 40 but failed to reduce deposition inhomogeneity and
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