Synthetic vitreous fibers: a review toxicology, epidemiology and regulations.

This review addresses the characteristics which differentiate synthetic vitreous fibers (SVFs, e.g., fiber glass, stonewool, slagwool, refractory ceramic fibers, etc.), how these influence the potential biopersistence and toxicity, the most recent epidemiological results and the integration of these findings into the health and safety regulations in Europe and the United States. Also presented is the historical basis for the European classification directive. The use and equivalence of the chronic inhalation toxicology and chronic intraperitoneal injection studies in laboratory rodents for evaluation of fiber toxicology is assessed as well as the impact of dose selection and design on the validity of the study. While synthetic vitreous fibers can span a wide range of chemistries, recognition and understanding of the importance of biopersistence (ability to persist in the lung) in fiber toxicity has led to the development of more and more biosoluble fibers (that break down rapidly in the lung). Still, the epidemiological data available which are largely based upon the use of fibers in past decades, indicate that the SVF do not present a human health risk at current exposure levels. The animal toxicology and biopersistence data provide a coherent basis for understanding and evaluating the parameters which affect SVF toxicity. The current regulations are based upon an extensive knowledge base of chronic studies in laboratory rodents which confirm the relationship between chronic adverse effects and the biopersistence of the longer fibers that can not be fully phagocytised and efficiently cleared from the lung. The amorphous structure of synthetic vitreous fibers facilitates designing fibers in use today with low biopersistence. Both the epidemiological data and the animal studies database provide strong assurance that there is little if any health risk associated with the use of SVFs of low biopersistence. IARC (2001) reclassified these fibers from Category 2b to Category 3 (with RCF and special purpose fibers remaining in 2b) an event which has not been common in the history of these monographs.