Synthetic vitreous fibers: A review of toxicology research and its impact on hazard classification

Because the inhalation of asbestos, a naturally occurring, inorganic fibrou s material, is associated with ling fibrosis and thoracic cancers, concerns have been raised about the possible health effects of synthetic vitreous f ibers (SVFs). SVFs include a very broad variety of inorganic fibrous materi al with an amorphous molecular structure. Traditionally, SVFs have been div ided into three subcategories based on composition: fiberglass, mineral woo l (rock, stone, and slag wools), and refractory ceramic fiber. For more tha n 50 years, the toxicologic potential of SVFs has been researched extensive ly using human epidemiology and a variety of laboratory studies. Here we re view the research and its impact on hazard classification and regulation of SVFs. Large, ongoing epidemiology studies of SVF manufacturing workers have provi ded very little evidence of harmful effects in humans. Several decades of r esearch using rodents exposed by inhalation have confirmed that SVF pulmona ry effects are determined gy the "Three D's", fiber dose (lung), dimension, and durability. Lung dose over time is determined by fiber deposition and biopersistence in the lung. Deposition is inversely related to fiber diamet er. Biopersistence is directly related to fiber length and inversely relate d to fiber dissolution and fragmentation rates. Inhaled short fibers are cl eared from the lung relatively quickly by mobile phagocytic cells, but lon fibers persist until they dissolve or fragment. In contrast to asbestos, mo st of the SVFs tested in rodent inhalation studies cleared rapidly from the lung (were nonbiopersistent) and were innocuous. However, several relative ly biopersistent SVFs induced chronic inflammation, lung scarring (fibrosis ), and thoracic neoplasms. Thus, biopersistence of fibers is now generally recognized as a key determinant of the toxicologic potential of SVFs. IN vi tro dissolution of fibers in simulated extracellular fluid correlates fairl y well with fiber biopersistence in the lung and pulmonary toxicity but sev eral exceptions suggest that biopersistence involves more than dissolution rate. Research demonstrating the relationship between biopersistence and SVF toxi city has provided a scientific basis for hazard classification and regulati on of SVFs. For a nonhazardous classification legislation recently passed b y the European Union requires a respirable insulation wool to have a low lu ng-biopersistence or be noncarcinogenic in laboratory rats. U.S. fiberglass and mineral wool industries and the Occupational Health and Safety Adminis tration (OSHA) have formed a voluntary Health and Safety Partnership Progra m (HSPP) that include: a voluntary permissible exposure level (PEL) in the workplace of 1 fiber/cc, an respirator protection program fors specified ta sks, continued workplace air monitoring, and where possible, the developmen t of fiber formulations that do not persist in the lung. RCF manufacturers have implemented a Product Stewardship Program that includes: a recommended exposure guideline of 0.5 fibers/cc; a 5-year workplace air monitoring pro gram; and research into the development of high-temperature-resistant, bios oluble fibers.