Predicting the in vitro dissolution rate constant of mineral wool fibers from fiber composition.

OBJECTIVE: We developed predictive formulae for the in vitro dissolution rate constant kdis of acid-soluble synthetic vitreous fibers (SVF), paralleling our earlier work with glass wools, which are typically more soluble at neutral pH. Developing simple models for predicting the kdis of a fiber can allow prediction of in vivo behavior, aid fiber developers, and potentially reduce in vivo testing. METHODS: The kdis of several acid-soluble SVF were determined using high simulant fluid flow/fiber surface area (F/A) conditions via a single-fiber measurement system. Four fluids were employed, varying in base composition and citrate levels. Equations predicting the kdis were derived from fiber chemistry and dissolution measurements for two of the fluids. RESULTS: Testing of several fibers showed a ∼10× increase in the kdis when citrate was included in the simulant solution. Data from tests with Stefaniak's citrate-free Phagoloysosmal Simulant Fluid (PSF) yielded kdis values aligned with expectations from in vivo results, unlike results from citrate-containing modified Gamble's solution. Predictive equations relating fiber chemistry to kdis showed reasonable agreement between the measured and predicted values. CONCLUSIONS: Citrate inclusion in the solution under high F/A conditions significantly increased the measured kdis. This resulted in more biorelevant data being obtained using the PSF fluid with the high F/A method used. The developed predictive equations, sufficient for fiber development work, require refinement before a recommending their use in place of in vivo biopersistence testing. Significant fit improvements are possible through additional measurements under these experimental conditions.

The role of fiber durability/biopersistence of silica-based synthetic vitreous fibers and their influence on toxicology.

This work summarizes what is known about the role of fiber durability/biopersistence of silica-based synthetic vitreous fibers (SVFs) and their influence on toxicology. The article describes the key processes leading from exposure to biological effect, including exposure, pulmonary deposition, clearance by various mechanisms, accumulation in the lung, and finally possible biological effects. The dose-dimension-durability paradigm is used to explain the key determinants of SVF toxicology. In particular, the key role played by the durability/biopersistence of long (>20microm) fibers is highlighted. Relevant literature on the prediction of in-vitro dissolution rates from chemical composition is summarized. Data from in-vitro and in-vivo durability/biopersistence tests show that these measures are highly correlated for long fibers. Both durability and biopersistence are correlated with the outcome of chronic inhalation bioassays. A schematic approach is presented for the design and testing of new SVFs with lower biopersistence.

Quantitative risk assessment of durable glass fibers.

This article presents a quantitative risk assessment for the theoretical lifetime cancer risk from the manufacture and use of relatively durable synthetic glass fibers. More specifically, we estimate levels of exposure to respirable fibers or fiberlike structures of E-glass and C-glass that, assuming a working lifetime exposure, pose a theoretical lifetime cancer risk of not more than 1 per 100,000. For comparability with other risk assessments we define these levels as nonsignificant exposures. Nonsignificant exposure levels are estimated from (a) the Institute of Occupational Medicine (IOM) chronic rat inhalation bioassay of durable E-glass microfibers, and (b) the Research Consulting Company (RCC) chronic inhalation bioassay of durable refractory ceramic fibers (RCF). Best estimates of nonsignificant E-glass exposure exceed 0.05-0.13 fibers (or shards) per cubic centimeter (cm3) when calculated from the multistage nonthreshold model. Best estimates of nonsignificant C-glass exposure exceed 0.27-0.6 fibers/cm3. Estimates of nonsignificant exposure increase markedly for E- and C-glass when non-linear models are applied and rapidly exceed 1 fiber/cm3. Controlling durable fiber exposures to an 8-h time-weighted average of 0.05 fibers/cm3 will assure that the additional theoretical lifetime risk from working lifetime exposures to these durable fibers or shards is kept below the 1 per 100,000 level. Measured airborne exposures to respirable, durable glass fibers (or shards) in glass fiber manufacturing and fabrication operations were compared with the nonsignificant exposure estimates described. Sampling results for B-sized respirable E-glass fibers at facilities that manufacture or fabricate small-diameter continuous-filament products, from those that manufacture respirable E-glass shards from PERG (process to efficiently recycle glass), from milled fiber operations, and from respirable C-glass shards from Flakeglass operations indicate very low median exposures of 0, 0.0002, 0.007, 0.008, and 0.0025 fibers (or shards)/cm3, respectively using the NIOSH 7400 Method ("B" rules). Durable glass fiber exposures for various applications must be well characterized to ensure that they are kept below nonsignificant levels (e.g., 0.05 fibers/cm3) as defined in this risk assessment.

Fiber glass and rock/slag wool exposure of professional and do-it-yourself installers.

The fiber glass (FG) and rock/slag wool (RSW) manufacturers have developed a Health and Safety Partnership Program (HSPP) with the participation and oversight of the Occupational Safety and Health Administration (OSHA). Among its many provisions the HSPP includes the continuing study of FG and RSW workplace concentrations in manufacturing facilities operated by FG/RSW producers and among their customers and end users. This analysis estimates the probable cumulative lifetime exposure (fiber-months/cubic centimeter [f-months/cc]) to those who install FG and RSW insulation in residential, commercial, and industrial buildings in Canada and the United States. Both professional and do-it-yourself (DIY) cohorts are studied and the estimated working lifetime exposures are compared with benchmark values derived from an analysis of the epidemiological studies of FG and RSW manufacturing cohorts. The key finding of this analysis is that both of these end-user cohorts are likely to have substantially lower cumulative lifetime exposures than the manufacturing cohorts. As the most recent updates of the epidemiological studies concluded that there was no significant increase in respiratory system cancer among the manufacturing cohorts, there is likely to be even less risk for the installer cohorts. This analysis also underscores the wisdom of stewardship activities in the HSPP, particularly those directed at measuring and controlling exposure.