A radioactive tracer technique to determine in vivo the number of fibers in the lungs of rats following their administration by intratracheal instillation.

A radioactive tracer technique is described which enables the total number of fibers present in the lungs of rats to be estimated following administration of the fibers by intratracheal instillation. The glass fiber used in the study was irradiated with thermal neutrons to induce radioactive 24Na. A suspension of the radioactive fiber was administered to eight rats by intratracheal instillation and to two additional rats by intraesophageal instillation. The 24Na radioactivity in the rats was counted in vivo at 24 and 48 hr after administration, after which they were killed. The amounts of fiber in the lungs, in the gastrointestinal tracts, and excreted in feces were estimated radiometrically. On average 93% of the administered fiber was accounted for. The lungs were digested with sodium hypochlorite solution and aliquots of the resulting digest filtered through membrane filters which were clear for examination by phase-contrast optical microscopy (PCOM). The numbers of fibers in the lungs, estimated by PCOM, were well correlated with the in vivo counting rates at 48 hr, indicating that the latter can be used to provide an accurate index of the number of fibers retained in the lung at that time.