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

To evaluate the safety of rock wool (RW) fibers, we examined the biopersistence of RW fibers in the lungs of rats, based on the changes of fiber number and fiber size in the length and width, in a nose-only inhalation exposure study. Twenty male Fischer 344 rats (6 to 10 wk old) were exposed to RW fibers at a fiber concentration of 70.6 (20.4) fiber/m(3) and a dispersion density of 30.4 (6.6) mg/m(3) [arithmetic mean (SD)] continuously for 3 h daily for 5 consecutive days. Five rats each were sacrificed shortly after exposure ended (baseline group) and at 1, 2, and 4 wk after exposure, and their lung tissues were ashed by a low temperature plasma-asher. The numbers and sizes of fibers in the ash samples were determined using a phase contrast microscope and a computed image analyzer. The fiber numbers in the lungs at 4 wk after exposure had significantly decreased from the baseline value, i. e. shortly after exposure (p<0.05). The half-lives of RW fibers calculated using the one-compartment model were 32 d for total fibers and 10 d for fibers longer than 20 microm in length. Fiber number was 53.6% of the baseline at 4 wk after exposure (baseline group=100%). Likewise, fiber sizes had significantly decreased at 4 wk after exposure (p<0.05), probably because fibers had been dissolved in body fluid, phagocytosed by alveolar macrophages or discharged from the body by mucociliary movement. In future studies, it will be necessary to examine the carcinogenicity of RW fibers through long-term inhalation studies.