Biopersistence of rock wool in lungs after short-term inhalation in rats

To evaluate the safety of rock wool (RW), an asbestos substitute, we examined the biopersistence of RW fibers in rat lungs based on the changes of fiber number and fiber size (length and diameter) by a nose-only inhalation exposure study. Twenty-four male Fischer 344 rats were exposed to RW fibers at a concentration of 30 mg/m(3) continuously for 3 h daily for 5 consecutive days. Six rats each were sacrificed shortly and at 1, 2, and 4 wk after exposure, and their lung tissues were ashed by a low-temperature plasma asher. Then the fiber numbers and fiber sizes in lungs were determined using a phase-contrast microscope and computed image analyzer. During the study period, the arithmetic mean (SD) values of fiber and weight concentrations were 78.5 (35.7) fibers/cm(3), and 29.9 (28.3) mg/m(3), respectively. The fiber number in lungs 4 wk after exposure significantly decreased from the baseline value (shortly after exposure) (p < .05). The half-life of fibers calculated from the approximate curve was 28 days for all fibers and 16 days for fibers with L > 20 microm, and the rate of decrease in fiber number was 46.3% at 4 wk after exposure (shortly-after group = 100%). Likewise, both length and diameter significantly decreased at 4 wk after exposure (p < .05), probably because fibers were phagosytosed and digested by alveolar macrophages, discharged to outside of the body by mucociliary movement, or dissolved by body fluid. It will be necessary in the future to further confirm the safety of RW fibers by assessing the biopersistence of fibers in the lungs and their pathological effects in our ongoing study performed in accordance with the guidelines established in the "Methods for Determination of Hazardous Properties for Human Health of Man Made Mineral Fibers" (EC protocol).