Behavior of rock wool in lungs after exposure by nasal inhalation in rats

To evaluate the safety of rock wool (RW fibers), we examined the biopersistence of a RW sample in the lungs of rats, based on the changes of fiber number and fiber size in terms of length and width, by a nose-only inhalation exposure study. Twenty male Fischer 344 rats (6-10 weeks old) were exposed to RW fibers at a concentration of 70 (21) fiber/m(3) and 30 (6.6) mg/m(3), arithmetic mean (geometric standard deviation), continuously for 3 h daily for five consecutive days. Five rats each were sacrificed shortly and at 1, 2, and 4 weeks after exposure, and their lung tissues were ashed by a low-temperature plasma-asher. Then, the numbers and sizes of fibers in the ashed samples were determined using phase-contrast microscope and computed image analyzer. The fiber numbers in the lungs 4 weeks after exposure significantly decreased from the baseline value, i.e., shortly after exposure (P < 0.05). The half-lives of RW fibers calculated from the one-compartment model were 32 days for total fibers and 10 days for fibers longer than 20 mum. The decrease of fiber number was 53.6% by 4 weeks after exposure (baseline group = 100%). Likewise, fiber sizes significantly decreased by 4 weeks after exposure (P < 0.05), probably because fibers were dissolved in body fluid, ingested by alveolar macrophages or discharged to outside of the body by mucociliary movement. In future studies, it is necessary to examine the long-term persistence of RW fibers in the lungs.