Man-made mineral fibers (MMMF): human exposures and health risk assessment.

MMMF are made by spraying or extruding molten glass, furnace slag, or mineral rock. Health concerns are based on the morphological and toxicological similarities between MMMF and asbestos, and the well-documented evidence that asbestos fibers can cause lung fibrosis (asbestosis), bronchial cancer, and mesothelioma in humans. Epidemiological evidence for human disease from inhalation exposures to fibrous glass is largely negative. Some positive associations have been reported for slag and rockwools. Most of the toxicological evidence for MMMF toxicity in laboratory animals is based on non-physiological exposures such as intratracheal instillation or intraperitoneal injection of fiber suspensions. The risks for lung fibrosis, lung cancer, and mesothelioma for industrial exposures to most fibrous glass products are either low or negligible for a variety of reasons. First, most commercial fibrous glass products have mean fiber diameters of approximately 7.5 microns, which results in mean aerodynamic diameters greater than 22 microns. Thus, most glass fibers, even if dispersed into the air, do not penetrate into the lung to any great extent. Second, the small fraction of smaller diameter fibers which do penetrate into the lungs are not persistent within the lungs for most fibrous glass products, due to mechanical breakage into shorter lengths and dissolution. Dissolution is most rapid for the smaller diameters (less than 0.1 micron) capable of producing mesothelioma. The greater hazards for slag and rockwools, in comparison to conventional fibrous glass, appear to be related to their smaller diameters and greater durability within the lungs.