Fiber biodurability and biopersistence: historical toxicological perspective of synthetic vitreous fibers (SVFs), the long fiber paradigm, and implications for advanced materials.

Extensive toxicology studies of synthetic vitreous fibers (SVFs) demonstrated that fiber dimension, durability/dissolution, and biopersistence are critical factors for risk of fibrogenesis and carcinogenesis. Lessons learned from the SVF experience provide useful context for predicting hazards and risk of nano-enabled advanced materials. This review provides (1) a historical toxicological overview of animal and toxicology studies of SVFs, (2) key findings that long durable fibers pose a risk of fibrogenic and tumorigenic responses and not short fibers or long soluble fibers, (3) and test methods for biodurability and biopersistence and associated predictive thresholds for fibrosis or tumors, and (4) recommendations for testing of advanced materials. Generally, SVFs (fiber lengths >20 µm) with fiber dissolution rates greater than 100 ng/cm/hr (glass fibers in pH 7 and stone fibers in pH 4.5) and fiber clearance less than WT 40 or 50 days were not associated with fibrosis or tumors. Long biodurable and biopersistent fibers exceeding these fiber dissolution and clearance thresholds may pose a risk of fibrosis and cancer. Fiber length-, durability-, and biopersistent-dependent factors that influence pathogenicity of mineral fibers are also expected to affect the biological effects of high aspect ratio nanomaterials (HARN). Only with studies aimed to correlate durability, biopersistence, and biological outcomes will it be determined whether similar or different fiber dissolution and half-life thresholds, which exempt carcinogenicity classification of SVFs, can also apply to HARNs.