A Comparison of Mathematical Methods for the Determination of In Vitro Dissolution Constants for Glass Fibers

Biopersistence plays a significant role in determining the potential bioactivity of respirable fibers. In vivo biopersistence in the lung is frequently assessed by in vitro fiber dissolution studies using simulated biological solutions and flow-through techniques. The dissolution rate (k) of a fiber is typically determined by elemental analysis of the flow-through solution to measure the mass of material leached from the fibers over a given time. Various methods may be used to estimate the value of k from these results. The present study compared the in vitro dissolution characteristics of seven experimental glass fiber compositions to those obtained for four recognized fiber compositions (MMVF 10-glass fiber; MMVF 11-glass fiber; MMVF 21-rockwool fiber; crocidolite fiber). Fiber dissolution was examined over a 17-wk period using a flow-through system designed to simulate the conditions encountered by fibers in the extracellular environment of the lung. Mass loss and changes in fiber diameter were determined over time and were then used to calculate k using five different methods. Although the selected methodologies did not produce identical estimations of k for each fiber, the resulting ranking of fiber solubility for each method was consistent. The seven experimental glass fibers were found to have k values intermediate between those of MMVF 11 and MMVF 21.     

Safety Assessment of Continuous Glass Filaments Used in Eclipse

Eclipse is a cigarette that produces smoke by primarily heating, rather than burning, tobacco. The Eclipse heat source assembly employs a continuous filament glass mat jacket to insulate the heat source. The glass mat insulator is composed of continuous glass filaments and a binder. The purpose of this article is to address the potential toxicological significance of the continuous glass filaments under the conditions of intended use. Transfer data and the unique physical characteristics of the filaments demonstrate that significant exposure of the smoker will not occur. The available environmental survey data clearly demonstrate that Eclipse smokers are extremely unlikely to be exposed to continuous glass filaments at a level that represents a biologically significant increase over background exposure to glass fibers. The chemical composition of the continuous glass filaments used in Eclipse is generally similar to C-glass fiber compositions such as MMVF 11 that have failed to produce either tumors or fibrosis in chronic inhalation studies conducted in rats. In vitro dissolution data demonstrate that the continuous glass filaments used in Eclipse are more soluble than biologically active fibers such as rock wool (MMVF 21) or asbestos. However, the continuous glass filaments used in Eclipse were not as soluble in simulated extracellular lung fluid as representative C-glass fibers (MMVF 10 and MMVF 11). In brief, exposure of Eclipse smokers to continuous glass filaments is extremely unlikely to occur at a level that may be construed to be of biological significance.     

Safety assessment of continuous glass filaments used in eclipse

Eclipse is a cigarette that produces smoke by primarily heating, rather than burning, tobacco. The Eclipse heat source assembly employs a continuous filament glass mat jacket to insulate the heat source. The glass mat insulator is composed of continuous glass filaments and a binder. The purpose of this article is to address the potential toxicological significance of the continuous glass filaments under the conditions of intended use. Transfer data and the unique physical characteristics of the filaments demonstrate that significant exposure of the smoker will not occur. The available environmental survey data clearly demonstrate that Eclipse smokers are extremely unlikely to be exposed to continuous glass filaments at a level that represents a biologically significant increase over background exposure to glass fibers. The chemical composition of the continuous glass filaments used in Eclipse is generally similar to C-glass fiber compositions such as MMVF 11 that have failed to produce either tumors or fibrosis in chronic inhalation studies conducted in rats. In vitro dissolution data demonstrate that the continuous glass filaments used in Eclipse are more soluble than biologically active fibers such as rock wool (MMVF 21) or asbestos. However, the continuous glass filaments used in Eclipse were not as soluble in simulated extracellular lung fluid as representative C-glass fibers (MMVF 10 and MMVF 11). In brief, exposure of Eclipse smokers to continuous glass filaments is extremely unlikely to occur at a level that may be construed to be of biological significance.     

IN VITRO MEASUREMENT OF FIBER DISSOLUTION RATE RELEVANT TO BIOPERSISTENCE AT NEUTRAL pH: AN INTERLABORATORY ROUND ROBIN

Measurements are presented of the dissolution rates in neutral pH simulated lung (SLF) of several man-made vitreous fibers (MMVF) and crocidolite asbestos that were recently in chronic rodent inhalation studies. The measured dissolution rate depended strongly on the fiber composition. The MMVF tested dissolved from 30 times to nearly 1000 times faster than the crocidolite asbestos. Measurements were made in flow-through equipment in four different laboratories in North America and Europe. The standard deviations of the measured values for each fiber were typically between 30 and 50% of average value. It is believed that in order to be relevant to the dissolution of long fibers the extracellular fluid of the lung, the in vitro measurement must be performed at a rate high enough that corrosion products do not accumulate in sufficient concentration affect the dissolution rate.     

Modeling the retention and clearance of manmade vitreous fibers in the rat lung

A mathematical model describing the dissolution and disintegration of long fibers and the clearance of short fibers is developed. For short fiber clearance, the model is based on previous modeling of the retention and clearance of particles, and most model parameters are taken from that particulate model. In addition to modeling the disappearance of long fibers, the present study includes a quantitative measure of goodness of fit of the model to observed data. Data from chronic inhalation experiments with insulation glass wools (MMVF10 and MMVF11) and rockwool (MMVF21) were provided for this study. These data comprised lung burdens at 10 time points at each of 3 concentrations for each fiber in inhalation experiments lasting up to 104 wk. At the two higher concentrations, the model had to take into account the effects of lung burden on macrophage-mediated clearance. The modeling shows that the overload dependence appears remarkably similar to that for low-toxicity particles in that the critical volumetric lung burden is similar to that for low toxicity dust. The model describes overload as leading to alveolar sequestration of short fibers or particles, and the estimated rate of alveolar sequestration for MMVF10 was similar to that for particles, but the estimated rate was lower for the other two fibers. Two alternative hypotheses to describe the process of the disappearance of longer fibers were tested by assessing their effect on a quantitative measure of fit of model predictions to the lung-burden data. These tests indicated that (a) dissolution leading to disintegration of long fibers into shorter fibers gave a much better fit than the alternative assumption that dissolution would leave only nonfibrous residue and (b) the relative rates of disintegration of the fibers in the lung appear to be directly dependent on their rates of in vitro dissolution and their diameters.     

Evaluation of the Biopersistence of Commercial and Experimental Fibers Following Inhalation

Abstract

The biopersistence of three fibers was evaluated using an inhalation model. The fibers studied were man-made vitreous fiber (MMVF) 11, a commercially produced glass fiber; Fiber B, a relatively soluble glass fiber; and Fiber J, a synthetic stonewool with a high content of alkaline earth oxides. Fischer 344 male rats were exposed to a well-defined rat respirable aerosol (mean diameter of ≤1 μm) at a target concentration of 30 mg/m³, 6 h/day for 5 days. Following the end of the exposure period, subgroups were sacrificed at 1 h, 1 day, 5 days, 4 wk, 13 wk, 26 wk, and 52 wk. At sacrifice, the whole lung was removed, weighed, and immediately frozen at -20°C for subsequent digestion by low temperature plasma ashing. The number and bivariate size distribution of the fibers in the aerosol and lung were determined. At 1 h following the last exposure, the 3 fibers were found to have similar lung burdens ranging from 7.36-7.72 × 10⁶ fibers/lung with geometric mean diameters of 0.42-0.54 μm. The three fibers were found to be removed from the lung following the cessation of inhalation exposure with half-lives of 8-42 days, in addition, an important difference in removal was seen between the long fiber (>15 μm) and short fiber (<15 μm) fractions. The long fibers cleared more rapidly with a T_ψ. of 20, 5, and 7 days for MMVF 11, Fiber B, and Fiber J, respectively. The dissolution of the long fibers appeared to result in rapid breaking and disintegration with the formation of short fibers and particles within the first 24 h. The short fiber fraction had a longer Y_ψ of 46, 10, and 12 days for MMVF 11, Fiber B, and Fiber J, respectively. Short fibers have been reported to be phagocytized by macrophages and either cleared by ciliated mucous transport or eventually translocated to the bronchial-associated lymphoid tissue and lymph nodes. The pH of the phagolysosome within the macrophage has been reported to be <5. Acellular in vitro studies indicate a slower dissolution at this pH. The diameters of either the long or short fibers did not change significantly over time, supporting the in vitro observation of the formation of a leached layer with similar physical dimensions as the original fiber. The clearance half-times for the three fibers evaluated were considerably shorter than that reported for crocidolite asbestos, a known fiber carcinogen, suggesting that these fibers would not persist in the lung as has been shown to be in the case for crocidolite. These results demonstrate that the inhalation biopersistence model provides a sensitive means of evaluating the critical parameters of fiber biodurability and clearance in the lung. For the MMVF fibers tested, the longer fibers not only dissolve but also appear to break apart in the lung, thereby quickly removing the potentially carcinogenic fraction from the lung.     

DISSOLUTION OF FIBERS INHALED BY RATS

Abstract

This article provides an analysis and an interpretation of the results of an inhalation study of the durability of several man-made vitreous fiber (MMVF) insulation wool samples in rats. The study was sponsored by the Thermal Insulation Manufacturers Association and was conducted by the Research and Consulting Company in Geneva with fiber size measurements made by Schuller Mountain Technical Center in Denver, CO. In this study, rats were exposed for 5 days to 4 types of airborne, respirable-sized MMVF and to crocidolite asbestos fibers. The MMVF included two compositions of glass wool, and one each of rock and slag wool. After exposure, animals were sacrificed at intervals up to 18 mo, and the number, length, and diameter of a representative sample of fibers in their lungs were measured. These data show that the long fibers (>20 μm) are eliminated from the rats' lungs at a rate that is predicted from the dissolution rate measured in vitro for these fiber compositions. In particular, the long MMVF were nearly completely eliminated in several months, whereas the long crocidolite asbestos fibers remained in significant numbers at the end of the study. The number, length, and diameter distributions of fibers remaining in the rats' lungs agreed well with a computer simulation of fiber clearance. This simulation assumed that the long fibers dissolved at the rate measured for each fiber in vitro, and that the short fibers of every type were removed at the same rate as short crocidolite asbestos. The results are strong evidence that long MMVF are cleared by complete dissolution at the rate measured in vitro and that short fibers do not dissolve but are cleared by macrophage-mediated physical removal.     

Calibration study on subchronic inhalation toxicity of man-made vitreous fibers in rats

This 3-mo inhalation study investigated the biological effects of a special-purpose glass microfiber (E-glass microfiber), the stone wool fiber MMVF21, and a new high-temperature application fiber (calcium-magnesium-silicate fiber, CMS) in Wistar rats. Rats were exposed 6 h/day, 5 days/wk for 3 mo to fiber aerosol concentrations of approximately 15, 50, and 150 fibers/ml (fiber length >20 microm) for E-glass microfiber and MMVF21. For the CMS fiber only the highest exposure concentration was used. During a 3-mo postexposure period, recovery effects were studied. In the highest exposure concentration groups, gravimetric concentrations were 17.2 mg/m3 for E-glass microfiber, 37 mg/m3 for MMVF21, and 49.5 mg/m3 for the CMS fiber. After 3 mo of exposure, lung retention of fibers longer than 20 micro m per lung was 17 x 10(6) for E-glass microfiber, 5.7 x 10(6) for MMVF21, and 0.88 x 10(6) for CMS. After 3 mo of recovery the concentration of the long fiber fraction was decreased to 38.4%, 63.9%, and 3.0% compared to original lung burden for the E-glass microfiber, MMVF21, and CMS, respectively. Biological effects measured included inflammatory and proliferative potential, histopathology lesions, and the persistence of these effects over a recovery period of 3 mo. Generally, observed effects were higher for E-glass microfiber when compared to MMVF21. The following clear dose-dependent effects on E-glass microfiber and MMVF21 exposure were observed as main findings of the study: increase in lung weight, in measured biochemical parameters and polymorphonuclear leukocytes (PMN) in the bronchoalveolar lavage fluid (BALF), in cell proliferation (BrdU-response) of terminal bronchiolar epithelium, and in interstitial fibrosis. The values observed in the proliferation assay on the carcinogenic E-glass microfiber indicate that this assay has an important predictive value with regards to potential carcinogenicity. Surprisingly, for the biosoluble CMS fiber, fibrogenic potential was detected in this study. The results of the CMS exposure group indicate that effects may be dominated by the presence of nonfibrous particles and that fibrosis may not be a predictor of carcinogenic activity of fiber samples, if the fiber preparation contains a significant fraction of nonfibrous particles. In summary, this study demonstrates the importance of fiber dust contamination by granular components. For future subchronic studies a longer posttreatment observation period would be advisable.     

Calibration Study on Subchronic Inhalation Toxicity of Man-Made Vitreous Fibers in Rats

This 3-mo inhalation study investigated the biological effects of a special-purpose glass microfiber (E-glass microfiber), the stone wool fiber MMVF21, and a new high-temperature application fiber (calcium-magnesium-silicate fiber, CMS) in Wistar rats. Rats were exposed 6 h/day, 5 days/wk for 3 mo to fiber aerosol concentrations of approximately 15, 50, and 150 fibers/ml (fiber length >20 µm) for E-glass microfiber and MMVF21. For the CMS fiber only the highest exposure concentration was used. During a 3-mo postexposure period, recovery effects were studied. In the highest exposure concentration groups, gravimetric concentrations were 17.2 mg/m 3 for E-glass microfiber, 37 mg/m 3 for MMVF21, and 49.5 mg/m 3 for the CMS fiber. After 3 mo of exposure, lung retention of fibers longer than 20 µm per lung was 17 × 10 6 for E-glass microfiber, 5.7 × 10 6 for MMVF21, and 0.88 × 10 6 for CMS. After 3 mo of recovery the concentration of the long fiber fraction was decreased to 38.4%, 63.9%, and 3.0% compared to original lung burden for the E-glass microfiber, MMVF21, and CMS, respectively. Biological effects measured included inflammatory and proliferative potential, histopathology lesions, and the persistence of these effects over a recovery period of 3 mo. Generally, observed effects were higher for E-glass microfiber when compared to MMVF21. The following clear dose-dependent effects on E-glass microfiber and MMVF21 exposure were observed as main findings of the study: increase in lung weight, in measured biochemical parameters and polymorphonuclear leukocytes (PMN) in the bronchoalveolar lavage fluid (BALF), in cell proliferation (BrdU-response) of terminal bronchiolar epithelium, and in interstitial fibrosis. The values observed in the proliferation assay on the carcinogenic E-glass microfiber indicate that this assay has an important predictive value with regards to potential carcinogenicity. Surprisingly, for the biosoluble CMS fiber, fibrogenic potential was detected in this study. The results of the CMS exposure group indicate that effects may be dominated by the presence of nonfibrous particles and that fibrosis may not be a predictor of carcinogenic activity of fiber samples, if the fiber preparation contains a significant fraction of nonfibrous particles. In summary, this study demonstrates the importance of fiber dust contamination by granular components. For future subchronic studies a longer posttreatment observation period would be advisable.     

CHRONIC INHALATION STUDIES OF TWO TYPES OF STONE WOOL FIBERS IN RATS

A summary is given of the pathology results after long-term inhalation in rats of insulation wool representing the new biosoluble types. The pathology results are compared with previously conducted long-term inhalation study with MMVF21 (traditional stone wool). The biosoluble fiber MMVF34/HT (HT) is characterized by a relatively high content of aluminum and a relatively low content of silica compared to the older MMVF21. HT has a high in vitro dissolution rate at pH 4.5, and a relatively low dissolution rate at pH 7.5. Male Fischer 344 rats were exposed at one exposure level of 30 mg/m³ by nose-only inhalation of a well-characterized fiber test atmosphere. The fibers had been size selected to be largely rat respirable. The negative control group was exposed to filtered air. The exposure duration was 6 h/day, 5 days/wk for 104 wk, with a subsequent nonexposure period lasting until approximately 20% survival in the air control group. Interim sacrifices were performed at wk 13, 26, 52, 78, and 104 to monitor the progression of pulmonary change and fiber numbers. Effectively the main protocol for the previously conducted chronic study with MMVF21 was the same, except that there were three concentration levels (3, 16, and 30 mg/m³). In addition to the endpoints measured in the previous study, slides from both studies were evaluated for collagen deposition using a quantitative morphometric method. The results of the comparative study clearly showed a marked difference in the pulmonary pathogenicity of the MMVF21 and HT in terms of their fibrogenic potential. MMVF21 caused pulmonary fibrosis, but the HT fiber did not. The incidence of tumors for both the HT and the MMVF21 fiber was comparable to the control groups.     

Chronic Inhalation Toxicity of Size-Separated Glass Fibers in Fischer 344 Rats

This study was initiated to determine the chronic biologicaleffects in Fisher 344 rats of inhaled size-separated respirablefractions of fibrous glass (FG) having compositions representativeof common building insulation wools. Rats were exposed usingnose-only inhalation chambers, 6 hr/day, 5 days/week, for 24months to three concentrations (3, 16, and 30 mg/m³) of twodifferent compositions of FG (designated MMVF 10 and MMVF 11),or to filtered air (negative control). Fibrous glass findingswere compared to those from a concurrent inhalation study ofchrysotile asbestos and refractory ceramic fiber (RCF). TheFGs used in this study were size selected to be largely respirablein the rat and the aerosol generation technique did not alterthe dimensions of the fibers. Interim euthanizations took placeat 3- to 6-month intervals to monitor progression of pulmonarychanges. Fibers were recovered from digested lung tissue fordetermination of changes in fiber number and morphology. Inanimals exposed to 30 mg/m³ of MMVF 10 or MMVF 11, 4.2±0.9x10⁵and 6.4±3.1x10⁵ fibers/mg dry lung tissue, respectively,were recovered after 24 months of exposure. Exposure to chrysotileasbestos (10 mg/m³) and to a lesser extent RCF (30 mg/m³) resultedin pulmonary fibrosis as well as mesothelioma and significantincreases in lung tumors. FG exposure was associated with anonspecific inflammatory response (macrophage response) in thelungs that did not appear to progress after 6–12 monthsof exposure. These cellular changes are reversible and are similarto the effects observed after inhalation of an inert dust. Nolung fibrosis was observed in the FG-exposed animals. Further,FG exposure resulted in no mesotheliomas and no statisticallysignificant increase in lung tumor incidence when compared tothat of the negative control group. These findings, along withprevious inhalation studies, suggest that respirable fibrousglass does not represent a significant hazard for fibrotic orneoplastic lung disease in humans.     

ESTIMATING ROCK AND SLAG WOOL FIBER DISSOLUTION RATE FROM COMPOSITION

A method was tested for calculating the dissolution rate constant in the lung for a wide variety of synthetic vitreous silicate fibers from the oxide composition in weight percent. It is based upon expressing the logarithm of the dissolution rate as a linear function of the composition and using a different set of coefficients for different types of fibers. The method was applied to 29 fiber compositions including rock and slag fibers as well as refractory ceramic and special-purpose, thin E-glass fibers and borosilicate glass fibers for which in vivo measurements have been carried out. These fibers had dissolution rates that ranged over a factor of about 400, and the calculated dissolution rates agreed with the in vivo values typically within a factor of 4. The method presented here is similar to one developed previously for borosilicate glass fibers that was accurate to a factor of 1.25. The present coefficients work over a much broader range of composition than the borosilicate ones but with less accuracy. The dissolution rate constant of a fiber may be used to estimate whether disease would occur in animal inhalation or intraperitoneal injection studies of that fiber.     

Estimating rock and slag wool fiber dissolution rate from composition

A method was tested for calculating the dissolution rate constant in the lung for a wide variety of synthetic vitreous silicate fibers from the oxide composition in weight percent. It is based upon expressing the logarithm of the dissolution rate as a linear function of the composition and using a different set of coefficients for different types of fibers. The method was applied to 29 fiber compositions including rock and slag fibers as well as refractory ceramic and special-purpose, thin E-glass fibers and borosilicate glass fibers for which in vivo measurements have been carried out. These fibers had dissolution rates that ranged over a factor of about 400, and the calculated dissolution rates agreed with the in vivo values typically within a factor of 4. The method presented here is similar to one developed previously for borosilicate glass fibers that was accurate to a factor of 1.25. The present coefficients work over a much broader range of composition than the borosilicate ones but with less accuracy. The dissolution rate constant of a fiber may be used to estimate whether disease would occur in animal inhalation or intraperitoneal injection studies of that fiber.     

Biopersistence of Man-Made Vitreous Fibers and Crocidolite Asbestos in the Rat Lung Following Inhalation

This study investigated possible relationships between fiberbiopersistence in the lung and previously observed differencesin pulmonary toxicity between asbestos and man-made vitreousfibers (MMVF) following inhalation exposure. Fischer 344/N ratswere exposed nose only, 6 hr/day for 5 days to 30 mg/m³ MMVF(two fiberglass compositions, rock wool, or slag wool) or to10 mg/m³ crocidolite asbestos. At eight time points up to 1year postexposure, lung fiber burdens were analyzed for number/lungand bivariate dimensions using scanning electron microscopy(SEM) and for chemical composition using SEM energy dispersivespectroscopy. After 365 days, >95% of long (>20 µm)MMVFs had disappeared from the lung compared to only 17% oflong crocidolite fibers. Longer MMVFs disappeared more rapidlythan short MMVFs, suggesting that long fibers were dissolvingor breaking. Mean diameters and lengths of the MMVFs decreasedwith time, while the mean diameter of crocidolite remained unchangedand its mean length showed an apparent increase, probably relatedto macrophage-mediated clearance of short fibers. Leaching ofoxides occurred in the fibrous glasses and slag wool and correlatedwith morphological changes in the fibers over time. No chemicalor morphological changes were observed in crocidolite fibers.These changes in MMVF number, chemistry, and morphology overtime in lung tissue compared to crocidolite asbestos demonstratethe relatively low biological persistence of some MMVFs in thelung and may explain why these MMVFs are not tumorigenic inrats, even after chronic exposure at high concentrations.     

STUDIES ON THE INHALATION TOXICOLOGY OF TWO FIBERGLASSES AND AMOSITE ASBESTOS IN THE SYRIAN GOLDEN HAMSTER. PART II. RESULTS OF CHRONIC EXPOSURE

Fiberglass (FG) is the largest category of man-made mineral fibers (MMVFs). Many types of FG are manufactured for specific uses-building insulation, air handling, filtration, and sound absorption. In the United States, &gt; 95% of FG produced is for building insulation. Several inhalation studies in rodents of FG building insulation have shown no indication of pulmonary fibrosis or carcinogenic activity. However, because of increasing use and potential for widespread human exposure, a chronic toxicity/carcinogenicity inhalation study of a typical building insulation FG (MMVF 10a) was conducted in hamsters, which were shown to be highly sensitive to the induction of mesotheliomas with another MMVF. A special-application FG (MMVF 33) and amosite asbestos were used for comparative purposes. Groups of 140 weanling male Syrian golden hamsters were exposed via nose-only inhalation for 6 h/day, 5 days/wk for 78 wk to either filtered air (chamber controls) or MMVF 10a, MMVF 33, or amosite asbestos at 250-300 WHO fibers/cm3 with two additional amosite asbestos groups at 25 and 125 WHO fibers/cm3. They were then held unexposed for 6 wk until 10-20% survival. After 13, 26, 52, and 78 wk, various pulmonary parameters and lung fiber burdens were evaluated. Groups hamsters were removed from exposure at 13 and 52 wk and were held until 78 wk (recovery groups). Initial lung deposition of long fibers (&gt; 20 mum in length) after a single 6-h exposure was similar for all 3 fibers exposed to 250-300 fibers/cm3. MMVF 10a lungs showed inflammation (which regressed in recovery hamsters) but no pulmonary or pleural fibrosis or neoplasms. MMVF 33 induced more severe inflammation and mild interstitial and pleural fibrosis by 26 wk that progressed in severity until 52 wk, after which it plateaued. While the inflammatory lesions regressed in the recovery animals, pulmonary or pleural fibrosis did not. A single multicentric mesothelioma was observed at 32 wk. No neoplasms were found in the remainder of the study. Amosite asbestos produced dose-related inflammation and pulmonary and pleural fibrosis as early as 13 wk in all 3 exposure levels. The lesions progressed during the course of the study, and at 78 wk severe pulmonary fibrosis with large areas of consolidation was observed in the highest 2 exposure groups. Progressive pleural fibrosis with mesothelial hypertrophy and hyperplasia was present in the thoracic wall and diaphragm in most animals and increased with time in the recovery hamsters. While no pulmonary neoplasms were observed in the amosite exposed hamsters, a large number of mesotheliomas were found; 25 fibers/cm3, 3.6%; 125 fibers/cm3, 25.9%; and 250 fibers/cm3, 19.5%. For the 3 fiber types, the severity of the lung and pleural lesions generally paralleled the cumulative fiber burden, especially those &gt;20 mum length, in the lung, thoracic wall, and diaphragm. They also inversely paralleled the in vitro dissolution rates; that is, the faster the dissolution, the lower were the cumulative lung burdens and the less severe the effects.     

COMPARISON OF THE CHEMICAL EVOLUTION OF MMVF FOLLOWING INHALATION EXPOSURE IN RATS AND ACELLULAR IN VITRO DISSOLUTION

In the process of developing new fibers or evaluating existing fibers, the dissolution coefficient is often determined using an acellular in vitro assay. This coefficient has been found useful in selecting fibers based on solubility and has been shown to be related to the clearance half-time of long fibers (&gt;20 m) following inhalation in rats. The chemical evolution of fibers following in vitro dissolution and inhalation was examined to evaluate the comparability of these two methods. The fibers studied ranged from compositions similar to commercial insulation materials to new glasses and stone wools that have been developed for enhanced solubility. The inhalation studies were performed using Fischer 344 male rats exposed to a well-defined rat respirable aerosol (mean diameter of 1 m) at a concentration of 30 mg/m3, 6 h/day for 5 days, with postexposure sacrifices at 1 h, 1 day, 5 days, 4 wk, 13 wk, and 26 wk. The in vivo results reported have been from fibers recovered from the lung that were primarily less than 20 m in length. The in vitro dissolution measurements were performed using a continuous-flow model with simulated lung fluid based upon a modified Gamble's media using either 1- or 10- m-diameter fibers without discrimination of length. The inhalation studies show that the composition of many of the fibers recovered from the lung changes with time in the lungs, with a depletion in Na O2, CaO, and MgO and a relative enrichment in SiO2 and Al2O3, with this evolution strongly depending upon glass composition. B2O3 has been found (on 10- m-diameter fibers in vitro as analyzed by secondary ion mass spectrometry) to also be depleted. The chemical evolution of these shorter fibers (L &lt; 20 m) is consistent with that obtained from the in vitro experiments at pH 7.4, although the rate of variation observed with the two methods varied. These results clearly demonstrate that those fibers whose composition changes quickly during in vivo studies are the same as those whose composition changes quickly during in vitro assays. These studies demonstrate that the chemical evolution of the shorter fibers recovered following in vivo inhalation studies mirrors closely the changes found from the in vitro assays. These results complement those already reported for fibers longer than 20 m, which have been shown to be removed from the lung primarily by dissolution. &lt;/abs&gt;     

A science-based paradigm for the classification of synthetic vitreous fibers

Synthetic vitreous fibers (SVFs) are a broad class of inorganic vitreous silicates used in a large number of applications including thermal and acoustical insulation and filtration. Historically, they have been grouped into somewhat artificial broad categories, e.g., glass, rock (stone), slag, or ceramic fibers based on the origin of the raw materials or the manufacturing process used to produce them. In turn, these broad categories have been used to classify SVFs according to their potential health effects, e.g., the International Agency for Research on Cancer and International Programme for Chemical Safety in 1988, based on the available health information at that time. During the past 10-15 years extensive new information has been developed on the health aspects of these fibers in humans, in experimental animals, and with in vitro test systems. Various chronic inhalation studies and intraperitoneal injection studies in rodents have clearly shown that within a given category of SVFs there can be a vast diversity of biological responses due to the different fiber compositions within that category. This information has been further buttressed by an in-depth knowledge of differences in the biopersistence of the various types of fibers in the lung after short-term exposure and their in vitro dissolution rates in fluids that mimic those found in the lung. This evolving body of information, which compliments and explains the results of chronic animal studies clearly show that these "broad" categories are somewhat archaic, oversimplistic, and do not represent current science. This new understanding of the relation between fiber composition, solubility, and biological activity requires a new classification system to more accurately reflect the potential health consequences of exposure to these materials. It is proposed that a new classification system be developed based on the results of short-term in vivo in combination with in vitro solubility studies. Indeed, the European Union has incorporated some of this knowledge, e.g., persistence in the lung into its recent Directive on fiber classification.     

ESTIMATING IN VITRO GLASS FIBER DISSOLUTION RATE FROM COMPOSITION

A method is presented for calculating the dissolution rate constant of a borosilicate glass fiber in the lung, as measured in vitro, from the oxide composition in weight percent. It is based upon expressing the logarithm of the dissolution rate as a linear function of the composition. It was found that the calculated dissolution rate constant agreed with the measured value within the variation of the measured data in a set of compositions in which the dissolution rate constant ranged over a factor of 100. The method was shown to provide a reasonable estimate of dissolution over a considerably wider range of composition than what was used to determine the parameters, such as a set of data in which the dissolution rate constant varied over a factor of 100,000. The dissolution rate constant may be used to estimate whether disease would ensue following animal inhalation or intraperitoneal studies.     

Estimating in vitro glass fiber dissolution rate from composition

A method is presented for calculating the dissolution rate constant of a borosilicate glass fiber in the lung, as measured in vitro, from the oxide composition in weight percent. It is based upon expressing the logarithm of the dissolution rate as a linear function of the composition. It was found that the calculated dissolution rate constant agreed with the measured value within the variation of the measured data in a set of compositions in which the dissolution rate constant ranged over a factor of 100. The method was shown to provide a reasonable estimate of dissolution over a considerably wider range of composition than what was used to determine the parameters, such as a set of data in which the dissolution rate constant varied over a factor of 100,000. The dissolution rate constant may be used to estimate whether disease would ensue following animal inhalation or intraperitoneal studies.     

Response to Dr. Morfeld's Letter

Abstract

The biopersistence of airborne fibers is felt to play an important role in their potential toxicity. Since the dissolution rate of fibers can be measured in cell-free systems, the current study was undertaken to determine if the dissolution rate of fibers in the lung was related to the dissolution rate of fibers in vitro, and whether dissolution serves to remove fibers from the lung. To determine dissolution rates in vivo, suspensions of fibers were administered to rats by intratracheal instillation, and the numbers, lengths, and diameters of fibers recovered from the lungs at intervals up to 1 yr after administration were measured by phase-contrast optical microscopy. Five different glass fibers were used that had dissolution rates ranging from 2 to 600 ng/cm²/h measured in vitro in simulated lung fluid at pH 7.4. Examination of the diameter distributions of fibers longer than 20 μm showed that the peak diameter decreased steadily with time after instillation, at the same rate measured for each fiber in vitro, until it approached zero. Measurements of the total number of fibers remaining in the rats' lungs at times up to 1 yr after instillation suggest that not many of the administered fibers were being cleared by macrophage-mediated transport via the conducting airways. A computer simulation of the fibers in the lungs was performed in which each of the administered long fibers (20 μm or longer) was decreased in diameter according to the rate measured in vitro, while the short fibers (less than 20 μm long) were unaffected. The ratio of long to short fibers predicted by this simulation agreed well with this quantity measured from the fibers recovered from the rats' lungs at each time interval after instillation. It was concluded that long glass fibers, at least those longer than 20 μm, are removed from the lung by dissolution at much the same rate measured in vitro.