Relationship between lung biopersistence and biological effects of man-made vitreous fibers after chronic inhalation in rats

Dissolution behavior of stone wool fibers in synthetic lung fluids: Impact of iron oxidation state changes induced by heat treatment for binder removal

Stone wool fiber materials are commonly used for thermal and acoustic insulation, horticulture and filler purposes. Biosolubility of the stone wool fiber (SWF) materials accessed through acellular in vitro dissolution tests can potentially be used in future as an indicator of fiber biopersistence in vivo. To correlate acellular in vitro studies with in vivo and epidemiological investigations, not only a robust dissolution procedure is needed, but fundamental understanding of fiber behavior during sample preparation and dissolution is required. We investigated the influence of heat treatment procedure for binder removal on the SWF iron oxidation state as well as on the SWF dissolution behavior in simulant lung fluids (with and without complexing agents). We used heat treatments at 450 °C for 5 min and 590 °C for 1 h. Both procedures resulted in complete binder removal from the SWF. Changes of iron oxidation state were moderate if binder was removed at 450 °C for 5 min, and there were no substantial changes of SWF's dissolution behavior in all investigated fluids after this heat treatment. In contrast, if binder was removed at 590 °C for 1 h, complete Fe(II) oxidation to Fe(III) was observed and significant increase of dissolution was shown in fluids without complexing agent (citrate). PHREEQC solution speciation modeling showed that in this case, released Fe(III) may form ferrihydrite precipitate in the solution. Precipitation of ferrihydrite solid phase leads to removal of iron cations from the solution, thus shifting reaction towards the dissolution products and increasing total mass loss of fiber samples. This effect is not observed for heat treated fibers if citrate is present in the fluid, because Fe(III) binds with citrate and remains mobile in the solution. Therefore, for developing the most accurate SWF in vitro acellular biosolubility test, SWF heat treatment for binder removal is not recommended in combination with dissolution testing in fluids without citrate as a complexing agent.

Critical review of the evidence for a causal association between exposure to asbestos and esophageal cancer

Esophageal cancers comprise about 1% of all cancers diagnosed in the US but are more prevalent in other regions of the world. Several regulatory agencies have classified asbestos as a known human carcinogen, and it is linked to multiple diseases and malignancies, including lung cancer and mesothelioma. In a 2006 review of the epidemiological literature, the Institute of Medicine (IOM) did not find sufficient evidence to demonstrate a causal relationship between asbestos exposure and esophageal cancer. To reevaluate this conclusion, we performed a critical review of the animal toxicological, epidemiological, and mechanism of action literature on esophageal cancer and asbestos, incorporating studies published since 2006. Although there is some evidence in the epidemiological literature for an increased risk of esophageal cancer in asbestos-exposed occupational cohorts, these studies generally did not control for critical esophageal cancer risk factors (e.g. smoking, alcohol consumption). Furthermore, data from animal toxicological studies do not indicate that asbestos exposure increases esophageal cancer risk. Based on our evaluation of the literature, and reaffirming the IOM's findings, we conclude that there is insufficient evidence to demonstrate a causal link between asbestos exposure and esophageal cancer.

Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers

In 2011, SCOEL classified RCF as a secondary genotoxic carcinogen and supported a practical threshold. Inflammation was considered the predominant manifestation of RCF toxicity. Intrapleural and intraperitoneal implantation induced mesotheliomas and sarcomas in laboratory animals. Chronic nose-only inhalation bioassays indicated that RCF exposure in rats increased the incidence of lung cancer and similar exposures resulted in mesothelioma in hamsters, but these studies may have been compromised by overload. Epidemiological studies in the US and Europe showed an association between exposure and prevalence of respiratory symptoms and pleural plaques, but no interstitial fibrosis, mesotheliomas, or increased numbers of lung tumors were observed. As the latency of asbestos induced mesotheliomas can be up to 50 years, the relationship between RCF exposure and respiratory malignances has not been fully determined. Nonetheless, it is possible to offer useful perspectives. RCF and rock wool have similar airborne fiber dimensions and biopersistence. Therefore, it is likely that these fibers have similar toxicology. Traditional rock wool has been the subject of numerous cohort and case control studies. For rock wool, IARC (2002) concluded that the epidemiological studies did not provide evidence of carcinogenicity. Based on analogies with rock wool (read across), it is reasonable to believe that increases in lung cancer or any mesotheliomas are unlikely to be found in the RCF-exposed cohort. RCF producers have developed a product stewardship program to measure and control fiber concentrations and to further understand the health status of their workers.

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.

The health effects of chrysotile: current perspective based upon recent data

This review substantiates kinetically and pathologically the differences between chrysotile and amphiboles. The serpentine chrysotile is a thin walled sheet silicate while the amphiboles are double-chain silicates. These different chemistries result in chrysotile clearing very rapidly from the lung (T(1/2)=0.3 to 11 days) while amphiboles are among the slowest clearing fibers known (T(1/2)=500 days to infinity). Across the range of mineral fiber solubilities chrysotile lies towards the soluble end of the scale. Chronic inhalation toxicity studies with chrysotile in animals have unfortunately been performed at very high exposure concentrations resulting in lung overload. Consequently their relevance to human exposures is extremely limited. Chrysotile following subchronic inhalation at a mean exposure of 76 fibers L>20 microm/cm(3) (3413 total fibers/cm(3)) resulted in no fibrosis (Wagner score 1.8-2.6), at any time point and no difference with controls in BrdU response or biochemical and cellular parameters. The long chrysotile fibers were observed to break apart into small particles and smaller fibers. Toxicologically, chrysotile which rapidly falls apart in the lung behaves more like non-fibrous mineral dusts while response to amphibole asbestos reflects its insoluble fibrous structure. Recent quantitative reviews of epidemiological studies of mineral fibers have determined the potency of chrysotile and amphibole asbestos for causing lung cancer and mesothelioma in relation to fiber type have also differentiated between these two minerals. The most recent analyses also concluded that it is the longer, thinner fibers that have the greatest potency as has been reported in animal inhalation toxicology studies. However, one of the major difficulties in interpreting these studies is that the original exposure estimates rarely differentiated between chrysotile and amphiboles. Not unlike some other respirable particulates, to which humans are, or have been heavily occupationally exposed, there is evidence that heavy and prolonged exposure to chrysotile can produce lung cancer. The value of the present and other similar studies is that they show that low exposures to pure chrysotile do not present a detectable risk to health. Since total dose over time decides the likelihood of disease occurrence and progression, they also suggest that the risk of an adverse outcome may be low if even any high exposures experienced were of short duration.

Cytotoxic and oxidative effects induced by man-made vitreous fibers (MMVFs) in a human mesothelial cell line

The introduction of man-made vitreous fibers (MMVFs) as a substitute for asbestos in industrial and residential applications raises concerns about their potential health hazards. The aim of our study was to assess cytotoxic and oxidative effects induced on a human mesothelial cell line (MeT-5A) by exposure to glass wool (GW), rock wool (RW) and refractory ceramic fibers (RCF) in comparison with crocidolite asbestos (CR). MeT-5A cells were exposed for 24 h to 2, 5 and 10 microg/cm2 of MMVF and crocidolite fibers and analysed by scanning electron microscope (SEM) for cell surface alterations. Cells were exposed for 2 h to 1, 2, 5 and 10 microg/cm2 of the same fibers and analysed by enzyme Fpg-modified comet test for direct and oxidative DNA damage. SEM revealed loss of microvilli in cells exposed to RCF and numerous blebs in cells exposed to higher doses of RW. Comet test showed significant direct DNA damage in cells exposed to RCF even at the lowest dose. Comet test with Fpg, that permits the detection of oxided DNA bases, showed significant oxidative DNA damage in cells exposed to higher doses of RW. The presence of DNA damage and alterations of cell surface induced by low doses of RCF and the presence of oxidative DNA damage and blebs on cell surface in cells exposed to higher dose of RW suggest possible cytotoxic, oxidative and genotoxic effects for these MMVFs.

Respiratory cancer and exposure to man-made vitreous fibers: a systematic review

BACKGROUND

Man-made vitreous fibers (MMVF's) have some structural features similar to those found in asbestos. This has lead to concern that exposure to MMVF's could increase the risk of respiratory cancer.

METHODS

Bibliographic resources were used to identify 10 case-control and 10 cohort studies, which analyzed the relationship between exposure to MMVF's and cancer of the respiratory system. Standardized mortality ratio's (SMR's) were extracted from the cohort studies for a meta-analysis.

RESULTS

A significant increase in SMR was observed for workers exposed to rock and glass wool, but not in workers exposed to glass filament. Meta-analysis of SMR's after stratification by fiber type resulted in aggregate estimates of risk of 1.23 (95% CI = 1.10-1.38), 1.08 (95% CI = 0.93-1.26), and 1.32 (95% CI = 1.15-1.52) for exposure to glass wool, glass filament, and rock wool, respectively. Some or all of the increased mortality could be attributed to tobacco use.

CONCLUSIONS

The results highlight the difficulty of assessing small increases in risk of respiratory cancer potentially caused by occupational exposure in populations with high prevalence of tobacco use.

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.

Clearance of man made mineral fibres from the lungs of sheep

OBJECTIVES

To compare the clearance rate, the related pathology, and the chemical and morphological changes of three man made mineral fibres (MMMFs) in the sheep model of pneumoconiosis.

METHODS

Fibrous particles were extracted from lung parenchyma and analysed by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS).

RESULTS

The concentration of MMMF11, MMMF21, refractory ceramic fibre (RCF-1), and crocidolite asbestos fibres decreased with time according to a slow and a fast kinetic component. There was a statistical difference in the four regression lines as a function of time and the type of fibres (p < 0.001). The diameter of MMMFs decreased during the course of the time, whereas the crocidolite fibres did not seem to show any change. There was a statistical difference in the four regression lines as a function of time (p = 0.037) and type of fibres (p < 0.001). Ferruginous bodies were counted in the 40 sheep for which the latency period was 2 years. No typical ferruginous bodies were found in the groups exposed to MMMFs. The geometric mean concentration of asbestos bodies in the group exposed to crocidolite was 2421 bodies/g lung tissue (95% CI 385 to 15260).

CONCLUSIONS

The number of initially retained fibres decreased with time according to a slow and a fast kinetic component. MMMF11 and MMMF21 have similar clearance, faster than RCF-1 and crocidolite. The geometric mean diameter and length of MMMF decreased with time, but crocidolite did not. After 2 years in the sheep tracheal lobe, ferruginous bodies were not found in all three MMMF groups but were substantial in the crocidolite group. Clearance is thought to proceed through dissolution and macrophage translocation.

Hazard assessment and risk analysis of two new synthetic vitreous fibers

Isofrax and Insulfrax are two new synthetic vitreous fibers (SVFs) developed for high-temperature insulation (1800-2300 degrees F) applications. In an attempt to significantly reduce or eliminate the potential of adverse health effects, these two fibers were specifically designed to have high solubility and, thus, low in vivo biodurability. In this paper, we review the effects of chemical composition on biodurability, in vitro fiber dissolution rates (K(dis)), and the relevance and relationship of K(dis) to pulmonary fibrosis and lung tumors in chronic rat inhalation studies. We also examine the correlations between K(dis) and weighted in vivo half-life (t(0.5)) of long fibers (>20 microm) and their relation to pulmonary effects in chronic rat inhalation bioassays. Predictions for outcomes of inhalation bioassays and development of nonsignificant risk levels of exposure are provided. Additionally, justification for the use of inhalation versus noninhalation animal data is provided as is a brief review of human health effects of SVFs. We conclude, inter alia, that Isofrax and Insulfrax have low biodurability, would not be expected to produce either pulmonary fibrosis or lung tumors in a well-designed animal inhalation bioassay, have weighted half-lives beneath the threshold established by the European Union for classification as a carcinogen, and based on epidemiological data for SVFs would not be expected to result in incremental cancer in human cohorts. Finally, it is estimated that approximately 90% of workplace exposure concentrations of these materials would be beneath 1 f/cc. At a concentration of 1 f/cc, neither fiber would be expected to result in an incremental working lifetime cancer risk greater than 10(-5).

The occupational physician's point of view: the model of man-made vitreous fibers

This article gives a detailed description of the procedure the occupational physician uses in interpreting the available scientific data to provide useful information for prevention of pulmonary diseases related to man-made mineral fibers, particularly lung cancer and mesothelioma. As it is difficult to reach definite conclusions from human data on the toxicity of specific fibers, an experimental approach is needed. Concerning animal data, we emphasize that adequate inhalation studies are the "gold standard" for extrapolating to humans. However, experiments using intracavitary injection or cells in vitro may represent indicative tests for a possible carcinogenic effect. Such tests should be used to assess the intrinsic carcinogenicity of fibers, but they must be confirmed by adequate inhalation models. Despite the present uncertainties, a proposal is made that could make it possible to classify fibers according to their toxicologic potential, grading them in accordance with physicochemical parameters, in vitro testing, and animal experiments. This procedure may be applicable to nonvitreous fibers and to organic fibers.

An experimental approach to the evaluation of the biopersistence of respirable synthetic fibers and minerals

The biopersistence of fibers and minerals in the respiratory tract is an important parameter in the toxicity of those materials. The biopersistence of respirable synthetic fibers and minerals in man can be most closely evaluated in an animal model. While acellular and in vitro systems are important for initial evaluation of solubility and durability, they cannot simulate the dynamics of inhalation deposition and clearance and the subsequent systemic reaction to fibers and minerals that occurs in the animal. To evaluate the biopersistence of synthetic fibers, male rats were exposed to a well defined rat respirable aerosol of man-made vitreous fibers (MMVF), 6 hr/day for 5 days. Following exposure, subgroups were sacrificed at intervals ranging from 1 hr to 52 weeks. Following sacrifice, the lungs were removed, weighed, and immediately frozen at 20 degrees C for subsequent digestion by low temperature plasma ashing. The number, size distribution, and chemical composition of the fibers in the aerosol and lung were determined. With this animal model the role of biopersistence in altering the geometry and clearance of fibers can be systematically evaluated. The model also can be applied for the evaluation of the biopersistence of nonfibrous minerals.

Durability of ceramic and novel man-made mineral fibers

In vitro solubility testing is an important means of assessing the likely behavior of fibers that are respired and accumulate in the lung. The problem has been that such tests often do not mirror the dissolution and removal mechanisms seen in vivo. Comparison of iron and silica solubility values of various types of mineral fiber showed no obvious correlation. Treating a mineral fiber containing high levels of calcium with normal balanced salt solutions produces a precipitate of calcium phosphate on the fiber surface. This deposit was not seen in fibers isolated from the lung of exposed animals. New solutions have been developed and with variations in the methods of exposing fibers, results similar to those seen in vivo have been obtained. Suitable fluid phases have been examined in static and flow-through systems. The relationship of solubility to biological activity is discussed.

Role of biopersistence in the pathogenicity of man-made fibers and methods for evaluating biopersistence: a summary of two round-table discussions

This paper summarizes two roundtable discussions held at the conclusion of the International Conference on Biopersistence of Respirable Synthetic Fibres and Minerals. The first round table addressed the role of biopersistence in the pathogenicity of fiber-induced disease. The panel included T. W. Hesterberg (Chairman), J.M.G. Davis, K. Donaldson, B. Fubini, N.F. Johnson, G. Oberdoerster, P. Sébastien, and D. Warheit. The second panel addressed the issue of methods for assessing biopersistence. It included R.O. McClellan (Chairman), J. Brain, A. Langer, A. Morgan, C. Morscheidt, H. Muhle, and R. Musselman. The two chairmen acknowledge the excellent contributions of all the members of the panels, whose comments formed the basis of this summary. Nonetheless, the authors assume full responsibility for the written text, recognizing that it was not reviewed by the discussants of the two panels.-Environ Health Perspect 102(Suppl 5):277-283 (1994)