Synthetic Vitreous Fibers Research Articles

End-User Exposures to Synthetic Vitreous Fibers: II. Fabrication and Installation Fabrication of Commercial Products

This article summarizes the results of exposure monitoring conducted during the installation and fabrication of commercial synthetic vitreous fiber (SVF) products. Included in this investigation were fiberglass duct insulation and construction applications (duct board, duct liner, and duct wrap), pipe and vessel insulation, batt insulation for prefabricated homes, and general fiberglass products. Commercial mineral wool products sampled as a part of this investigation included ceiling tiles, building safing, and loose insulation for prefabricated homes. A total of 520 valid air samples were collected as a part of this investigation and were analyzed using gravimetric, phase contrast microscopy (PCM), or scanning electron microscopy (SEM). Airborne fiber-size distributions were also determined for a subset of samples collected for SEM analysis. As a result of the task-based sampling strategy used in this study, sample times reflect exposures over the time the person was actually engaged in SVF-related work activities, and exposure results are therefore presented as task-length averages (TLAs). Thirty-five total dust samples were collected as a part of this investigation, resulting in 14 TLAs ranging from 0.3 to 7.6 mg/m3. A total of 125 PCM-based TLAs were collected, with the mean TLA time for all product and occupation categories ranging from 277 to 443 minutes. The mean PCM-based TLAs for all product/occupations were below 1.0 f/cm3, ranging from 0.04 to 0.68 f/cm3. A total of 116 SEM TLAs were determined. Average SEM-based TLA concentrations were slightly lower than the PCM-based estimates and ranged from < 0.01 to 0.16 f/cm3

End-user exposures to synthetic vitreous fibers: II. Fabrication and installation fabrication of commercial products.

This article summarizes the results of exposure monitoring conducted during the installation and fabrication of commercial synthetic vitreous fiber (SVF) products. Included in this investigation were fiberglass duct insulation and construction applications (duct board, duct liner, and duct wrap), pipe and vessel insulation, batt insulation for prefabricated homes, and general fiberglass products. Commercial mineral wool products sampled as a part of this investigation included ceiling tiles, building safing, and loose insulation for prefabricated homes. A total of 520 valid air samples were collected as a part of this investigation and were analyzed using gravimetric, phase contrast microscopy (PCM), or scanning electron microscopy (SEM). Airborne fiber-size distributions were also determined for a subset of samples collected for SEM analysis. As a result of the task-based sampling strategy used in this study, sample times reflect exposures over the time the person was actually engaged in SVF-related work activities, and exposure results are therefore presented as task-length averages (TLAs). Thirty-five total dust samples were collected as a part of this investigation, resulting in 14 TLAs ranging from 0.3 to 7.6 mg/m3. A total of 125 PCM-based TLAs were collected, with the mean TLA time for all product and occupation categories ranging from 277 to 443 minutes. The mean PCM-based TLAs for all product/occupations were below 1.0 f/cm3, ranging from 0.04 to 0.68 f/cm3. A total of 116 SEM TLAs were determined. Average SEM-based TLA concentrations were slightly lower than the PCM-based estimates and ranged from <0.01 to 0.16 f/cm3. The geometric mean fiber diameters for commercial products and occupations sampled as a part of the investigation ranged from 0.8 microm to 1.9 microm. Geometric mean fiber length varied by a factor of approximately three, ranging from 9.5 microm to 29.5 microm.

Synthetic Vitreous Fibers: A Review of Toxicology Research and Its Impact on Hazard Classification

Because the inhalation of asbestos, a naturally occurring, inorganic fibrous material, is associated with lung fibrosis and thoracic cancers, concerns have been raised about the possible health effects of synthetic vitreous fibers (SVFs). SVFs include a very broad variety of inorganic fibrous materials with an amorphous molecular structure. Traditionally, SVFs have been divided into three subcategories based on composition: fiberglass, mineral wool (rock, stone, and slag wools), and refractory ceramic fiber. For more than 50 years, the toxicologic potential of SVFs has been researched extensively using human epidemiology and a variety of laboratory studies. Here we review the research and its impact on hazard classification and regulation of SVFs.Large, ongoing epidemiology studies of SVF manufacturing workers have provided very little evidence of harmful effects in humans. Several decades of research using rodents exposed by inhalation have confirmed that SVF pulmonary effects are determined by the “Three D's”, fiber dose (lung), dimension, and durability. Lung dose over time is determined by fiber deposition and biopersistence in the lung. Deposition is inversely related to fiber diameter. Biopersistence is directly related to fiber length and inversely related to fiber dissolution and fragmentation rates. Inhaled short fibers are cleared from the lung relatively quickly by mobile phagocytic cells, but long fibers persist until they dissolve or fragment. In contrast to asbestos, most of the SVFs tested in rodent inhalation studies cleared rapidly from the lung (were nonbiopersistent) and were innocuous. However, several relativley biopersistent SVFs induced chronic inflammation, lung scarring (fibrosis), and thoracic neoplasms. Thus, biopersistence of fibers is now generally recognized as a key determinant of the toxicologic potential of SVFs. In vitro dissolution of fibers in simulated extracellular fluid correlates fairly well with fiber biopersistence in the lung and pulmonary toxicity, but several exceptions suggest that biopersistence involves more than dissolution rate.Research demonstrating the relationship between biopersistence and SVF toxicity has provided a scientific basis for hazard classification and regulation of SVFs. For a nonhazardous classification, legislation recently passed by the European Union requires a respirable insulation wool to have a low lung-biopersistence or be noncarcinogenic in laboratory rats. U.S. fiberglass and mineral wool industries and the Occupational Health and Safety Administration (OSHA) have formed a voluntary Health and Safety Partnership Program (HSPP) that include: a voluntary permissible exposure level (PEL) in the workplace of 1 fiber/cc, a respiratory protection program for specified tasks, continued workplace air monitoring, and, where possible, the development of fiber formulations that do not persist in the lung. RCF manufacturers have implemented a Product Stewardship Program that includes: a recommended exposure guideline of 0.5 fibers/cc; a 5-year workplace air monitoring program; and research into the development of high-temperature-resistant, biosoluble fibers.

Potential Pulmonary Effects of Man-Made Organic Fiber (MMOF) Dusts

In the first half of the twentieth century epidemiologic evidence linked elevated incidences of pulmonary fibrosis and cancer with inhalation of chrysotile and crocidolite asbestos, a family of naturally occurring inorganic fibrous materials. As the serpentine and amphibole forms of asbestos were phased out, synthetic vitreous fibers (SVFs; fiber glass, mineral wool, and refractory fiber) became increasingly utilized, and concerns were raised that they too might cause adverse health effects. Extensive toxicological research on SVFs has demonstrated that their pulmonary effects are directly related to fiber dose in the lung over time. This is the result of deposition (thin fibers deposit in the lower lung more efficiently than thick fibers) and lung-persistence (“biopersistence” is directly related to fiber length and inversely related to dissolution and fragmentation rates). In rat inhalation studies, asbestos was determined to be 7- to 10-fold more biopersistent in the lung than SVFs. Other than its effect on biopersistence, fiber composition did not appear to play a direct role in the biological activity of SVFs. Recently, the utilization of man-made organic fibers (MMOFs) (also referred to by some as synthetic organic fibers) has increased rapidly for a variety of applications. In contrast to SVFs, research on the potential pulmonary effects of MMOFs is relatively limited, because traditionally MMOFs were manufactured in diameters too thick to be respirable (inhalable into the lower lung). However, new developments in the MMOF industry have resulted in the production of increasingly fine-diameter fibers for special applications, and certain post-manufacturing processes (e.g., chopping) generate respirable-sized MMOF dust. Until the mid-1990s, there was no consistent evidence of human health affects attributed to occupational exposure to MMOFs. Very recently, however, a unique form of interstitial lung disease has been reported in nylon flock workers in three different plants, and respirable-sized nylon shreds (including fibers) were identified in workplace air samples. Whether nylon dust or other occupational exposures are responsible for the development of lung disease in these workers remains to be determined. It is also unknown whether the biological mechanisms that determine the respirability and toxicity of SVFs apply to MMOFs. Thus, it is appropriate and timely to review the current data regarding MMOF workplace exposure and pulmonary health effects, including the database on epidemiological, exposure assessment, and toxicology studies.

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.

Depletion of glutathione and ascorbate in lung lining fluid by respirable fibres

The use of synthetic vitreous fibres has increased along with a decline in the utilisation of asbestos. There remains concern that these synthetic fibres pose a health risk to workers because of the generation of respirable fibres which can enter the lung and cause adverse health effects. An improved understanding of the mechanism of fibre pathogenicity should allow more rational short-term testing regimes for new fibres as they are developed. We hypothesised that carcinogenic fibres have greater free radical activity compared with non-carcinogenic fibres and that they contribute to disease by causing oxidative stress in the lung. We examined a panel of respirable fibres, designated as being carcinogenic or non-carcinogenic based on previous animal studies for ability to deplete antioxidants from lung lining fluid. On the basis of inhalation studies, a panel of fibres was divided into three carcinogenic fibres-amosite asbestos, silicon carbide, and refractory ceramic fibre 1 (RCF1) and three non-carcinogenic fibres-man-made vitreous fibre 10 (a glass fibre MMVF10), Code 100/475 glass fibre, and refractory ceramic fibre 4 (RCF4). We measured the levels of glutathione (GSH) and ascorbate, two antioxidants present in lung lining fluid (LLF) after fibre treatment. All of the experiments were carried out at equal fibre number. Fibres had the ability to deplete both GSH and ascorbate from both LLF and pure solutions, an effect which was fibre number dependent. The greatest depletion of antioxidants was observed with the two non-carcinogenic glass fibres, and this effect was observed when A549 lung epithelial cells were treated with fibres. Our results show that antioxidant depletion in cell free solution and lung lining fluid solely is not a simple indicator of the ability of fibres to cause lung pathology and that other biological events in the lung are involved.

Depletion of glutathione and ascorbate in lung lining fluid by respirable fibres

Objective—The use of synthetic vitreous fibres has increased along with a decline in the utilisation of asbestos. There remains concern that these synthetic fibres pose a health risk to workers because of the generation of respirable fibres which can enter the lung and cause adverse health effects. An improved understanding of the mechanism of fibre pathogenicity should allow more rational short-term testing regimes for new fibres as they are developed. We hypothesised that carcinogenic fibres have greater free radical activity compared with non-carcinogenic fibres and that they contribute to disease by causing oxidative stress in the lung. We examined a panel of respirable fibres, designated as being carcinogenic or non-carcinogenic based on previous animal studies for ability to deplete antioxidants from lung lining fluid.Methods—On the basis of inhalation studies, a panel of fibres was divided into three carcinogenic fibres—amosite asbestos, silicon carbide, and refractory ceramic fibre 1 (RCF1) and three non-carcinogenic fibres—man-made vitreous fibre 10 (a glass fibre MMVF10), Code 100/475 glass fibre, and refractory ceramic fibre 4 (RCF4). We measured the levels of glutathione (GSH) and ascorbate, two antioxidants present in lung lining fluid (LLF) after fibre treatment. All of the experiments were carried out at equal fibre number.Results—Fibres had the ability to deplete both GSH and ascorbate from both LLF and pure solutions, an effect which was fibre number dependent. The greatest depletion of antioxidants was observed with the two non-carcinogenic glass fibres, and this effect was observed when A549 lung epithelial cells were treated with fibres.Conclusions—Our results show that antioxidant depletion in cell free solution and lung lining fluid solely is not a simple indicator of the ability of fibres to cause lung pathology and that other biological events in the lung are involved.

Lung Biopersistence and in Vitro Dissolution Rate Predict the Pathogenic Potential of Synthetic Vitreous Fibers

Here we review the past decade of research on inorganic fiber toxicology, which demonstrates that fiber biopersistence and in vitro dissolution rate correlate well with fiber pathogenicity. Test fibers for these studies included eight synthetic vitreous fibers (SVFs)—refractory ceramic fiber (RCF1), four fiber glasses (FCs), rock wool, slag wool, HT stone wool—and two asbestos types (crocidolite and amosite). Fiber toxicology and biopersistence were investigated using rodents exposed by inhalation. To evaluate chronic inhalation toxicity, rodents were exposed nose-only to ∼ 100 fibers >20 µm in length (F > 20 µm)/cm3, 6 h/day, 5 days/wk, for 2 yr (rats) or 1½ yr (hamsters). To evaluate lung biopersistence, rats were exposed nose-only for 5 days to fiber aerosol; lung burdens were then analyzed during 1 yr postexposure. In vitro dissolution rate was evaluated in a flow-through system using physiological solutions that mimic the inorganic components of extra- and intracellular lung fluids. The 10 test fibers encompassed a range of respiratory toxicities, from transient inflammation only to carcinogenesis. Lung clearance weighted half-times (WT½) for F > 20 µm were 6–15 days for stonewool, building insulation FCs, and slag wool; 50–80 days for rock wool, 2 special-application FCs, and RCFI; and >400 days for asbestos. WT½ correlated with pathogenicity: The rapidly clearing fibers were innocuous (insulation FCs, slag wool, and stonewool), but the more biopersistent fibers were fibrogenic (rock wool) or fibrogenic and carcinogenic (special-application FCs, RCFI, amosite and crocidolite asbestos). In vitro dissolution rates (k dis= ng/cm2/h) of the 10 fibers at pH 7.4 or 4.5 ranged from < 1 to >600. Fibers that dissolved rapidly in vitro also cleared quickly from the lung and induced only transient inflammation in the chronic studies. In contrast, fibers that dissolved slowly in vitro were biopersistent in the lung and tended to induce permanent pathogenicity. Other in vitro studies of fiber degradation suggest that, in addition to fiber dissolution, fiber leaching and subsequent transverse breakage may also be important mechanisms in lung biopersistence and hence pathogenicity. The validity of using lung biopersistence for predicting the potential pathogenicity of SVFs is confirmed by this research. The research also supports the use of in vitro fiber degradation at pH 7.4 and/or pH 4.5 as an indicator of SVF potential pathogenicity.

Particle Contamination in Experimental Fiber Preparations

Two samples of refractory ceramic fibers (RCF1 and RCF1a) were tested in rats exposed by inhalation for 3 wk and followed thereafter for 12 mo. RCF1 and RCFIa have similar fiber chemistry but differ in the fiber size distributions and the contents of nonfibrous particles (2% for RCFIa, 25% for RCFli. For both test samples the target aerosol concentration was 130 fibers/ml > 20 µm but the RCF1 aerosol contained more short fibers (less than 20 µm long) and more nonfibrous particles. Radioactive tracer measurements carried out during a period of 90 days after exposure demonstrated an almost complete abolition of alveolar clearance in RCFI-exposed animals. With RCFIa the half-time of46Sc2O3 particles was 80 days, compared to 60 days for controls (not statistically different). Biochemical and cytological analyses were carried out in bronchoalveolar lavage at days 3, 17, 31, 94, and 365 postexposure. They revealed a more important and more persistent inflammation in RCFI-exposed animals compared to RCFIa. These observations show that the biological activity of synthetic vitreous fibers (SVFs) assessed by inhalation experiments depends not only on the chemical composition of the fiber type but also on the physical characteristics of the test sample. They are in line with results of previous studies and support the hypothesis of a synergistic effect between fibrous and nonfibrous particles. They raise questions about the interpretation of the so called RCC experiments in which the several SVFs samples tested were not prepared using the same method and differed in their physical characteristics. In particular, these differences existed especially between the RCF and MMV F samples.

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.

Estimation of Dissolution Rate From In Vivo Studies of Synthetic Vitreous Fibers

Although the dissolution rate of a fiber was originally defined by a measurement of dissolution in simulated lung fluid in vitro, it is feasible to determine it from animal studies as well. The dissolution rate constant for a fiber may be extracted from the decrease in long fiber diameter observed in certain intratracheal instillation experiments or from the observed long fiber retention in short-term biopersistence studies. These in vivo dissolution rates agree well with those measured in vitro for the same fibers. For those special types of fibers, the high-alumina rock wool fibers that could not be measured in vitro, the method provides a way of obtaining a chemical dissolution rate constant from an animal study. The inverse of the in vivo dissolution rate, the fiber dissolution time, correlates well with the weighted half life of long fibers in a biopersistence study, and the in vivo dissolution rate may be estimated accurately from this weighted half-life.

Gender differences in the healthy worker effect among synthetic vitreous fiber workers.

The aim of this study was to determine whether the healthy worker effect and its component parts operate similarly for women and men. A cohort of workers from 14 synthetic vitreous fiber factories in seven countries, employed for at least 1 year between 1933 and 1977 and followed up to the early 1990s, included 375 deaths and 53,608 person-years among females and 2,568 deaths and 210,073 person-years among males. Standardized mortality ratios for all-cause and circulatory diseases were adjusted for country, age, calendar time, and gender. In addition, internal comparisons were adjusted for time since hire and employment status. The analyses addressed the following: 1) the healthy hire effect, 2) the time since hire effect, and 3) the healthy worker survivor effect. In this cohort, an overall healthy worker effect was not present in either gender. The healthy hire effect, based on standardized mortality ratios for years 1-4 since hire, was observed in males (standardized mortality ratio (SMR) = 0.8; 95% confidence interval (CI): 0.7, 1.0) but was less in females (SMR = 0.9; 95% CI: 0.5, 1.6). The relative risks increased slightly with time since hire in males but not in females. Higher mortality ratios were seen among those leaving employment than among those who remained actively employed; however, this effect was substantially greater for women (relative risk (RR) = 3.4; 95% CI: 1.8, 6.3) than men (RR = 1.8; 95% CI: 1.5, 2.1). The gender difference for active versus inactive status was stronger up to age 60 (men: RR = 1.7; 95% CI: 1.4, 2.0; women: RR = 3.6; 95% CI: 1.8, 7.1) than above that age. In conclusion, it appears that there is a stronger selection of healthy men than women into the workforce, while health-related selection out of the workforce is stronger for women than men.

A risk assessment for exposure to glass wool.

Synthetic vitreous fibers (SVFs) have been widely used as insulation material in places where asbestos was used many years ago and therefore the hazards have been compared. Since the three principal types of asbestos fibers types have caused lung cancer at high exposures, there is a widely held belief that all fibers are carcinogenic if inhaled in large enough doses. Hence, on a morphological basis, SVFs have been studied for their carcinogenic potential. However, there is considerable evidence that differences exist among fibers in their potency to produce a carcinogenic response. In this attempt to carry out a numerical risk assessment for the installers of blown glass wool (fiber) insulation, we start with a characterization of the material; then we review the exposures both in manufacturing and installation. Neither the epidemiological studies of human exposure nor the animal studies have shown a marked hazardous effect from glass wool and we can therefore be sure that any effect that might exist is small. But in this case, as in many other situations where there is a potential hazard, society desires further reassurance and therefore we have made a mechanistic calculation. There are good estimates of the risk associated with exposure to chrysotile asbestos at high exposures and doses. We have therefore taken these numbers and discussed how much less risky an exposure to glass wool fibers might be. We conclude that for a given fiber count, glass wool is five to ten times less risky (and of course the risk might be zero). The risk for a nonsmoking installer of glass wool fiber insulation who wears a respirator is about 6 in a million (and might be zero) per year. This means that out of a million installers there might be six lung cancers from this cause every year or out of 10,000 installers there might be one in 16 years. The low risk of 6 in a million per year of a worker blowing glass wool is consistent with the fact that no one has found any of cancer attributable to the manufacture or installation of glass wool fibers in spite of diligent searches. This is compared with several other occupational risks. Nonetheless common prudence suggests that any installer of blown glass wool fiber insulation wear a respirator.

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).

Potential significance of airborne fiber dimensions measured in the U.S. refractory ceramic fiber manufacturing industry.

To determine dimensions of airborne fibers in the U.S. refractory ceramic fiber (RCF) manufacturing industry, fibers collected through personal air sampling for employees at RCF manufacturing and processing operations have been measured. Data were derived from transmission electron microscopy analyses of 118 air samples collected over a 20-year period. Characteristics of sized fibers include: diameter measurements of <60; 0.19 to 1.0 micron, m of which 75% are less than 0.6 micron and length ranging from < 0.6 to > 20 micron, with 68% of fibers between 2.4 and 20 micron. Exposures in RCF manufacturing include airborne fibers with dimensions (diameter < 0.1-0.4 micron, length < 10 micron) historically associated with biological effects in pleural tissues. Air sampling data and a review of studies relating fiber size to pleural effects in animals and humans support the belief that information on fiber dimensions is essential for studies with synthetic vitreous fibers.

Comparison of NIOSH Method 7400 A and B Counting Rules for Assessing Synthetic Vitreous Fiber Exposures

A number of counting rules are available to quantify airborne concentrations of synthetic vitreous fibers (SVFs), including the National Institute for Occupational Safety and Health (NIOSH) Method 7400 A and B rules and the World Health Organization European Reference Method for Man-Made Mineral Fibers. Differences in counting rules have led to a lack of comparability of sampling results across studies. The purpose of this article is to compare NIOSH Method 7400 A and B fiber counts using samples collected as a part of a large assessment of end-user exposures to SVFs. This comparison provides a link between previous investigations that utilized A rules and more recent investigations that used B rules. A total of 792 airborne fiber samples were collected according to NIOSH Method 7400 and separately evaluated using both NIOSH A and B counting rules. The results of this investigation indicate that A-rule fiber counts give statistically higher fiber density estimates than corresponding B-rule fiber counts, w...

Comparison of NIOSH Method 7400 A and B Counting Rules for Assessing Synthetic VitreousFiber Exposures

Comparison of NIOSH Method 7400 A and B Counting Rules for Assessing Synthetic VitreousFiber Exposures

In situ microscopic analysis of asbestos and synthetic vitreous fibers retained in hamster lungs following inhalation.

Hamsters breathed, nose-only, for 13 weeks, 5 days/week, 6 hr/day, either man-made vitreous fiber (MMVF)10a, MMVF33, or long amosite asbestos at approximately 300 World Health Organization (WHO) fibers/cc or long amosite at 25 WHO fibers/cc. [World Health Organization fibers are longer than 5 microm and thicker than 3 microm, with aspect ratio >3.] After sacrifice, fiber burden was estimated (left lungs) by ashing and scanning electron microscopy (ashing/SEM) or (right middle lobes) by confocal laser scanning microscopy (CLSM) in situ. In situ CLSM also provided three-dimensional views of fibers retained, undisturbed, in lung tissue. Fibers of each type were lodged in alveoli and small airways, especially at airway bifurcations, and were seen fully or partly engulfed by alveolar macrophages. Amosite fibers penetrated into and through alveolar septa. Length densities of fibers in parenchyma (total length of fiber per unit volume of lung) were estimated stereologically from fiber transsections counted on two-dimensional optical sections and were 30.5, 25.3, 20.0, and 81.6 mm/mm3 for MMVF10a, MMVF33, and low- and high-dose amosite, respectively. Lengths of individual fibers were measured in three dimensions by tracking individual fibers through series of optical sections. Length distributions of amosite fibers aerosolized, but before inhalation versus after retention in the lung were similar, whether determined by ashing/SEM or in situ CLSM. In contrast, the fraction of short MMVF10a and MMVF33 fibers increased and the geometric mean fiber lengths of both MMVFs decreased by approximately 60% during retention. Most likely due to fiber deposition pattern and differences in sampling, fiber burdens [MMVF10a, MMVF33, and amosite (high dose; 269 WHO fibers/cc)] determined by ashing/SEM were 1.4, 1. 5, and 3.5 times greater, respectively, than those calculated from in situ CLSM data. In situ CLSM is able to provide detailed information about the anatomic sites of fiber retention and also fiber lengths and burdens in good agreement with ashing/SEM results.

In Situ Microscopic Analysis of Asbestos and Synthetic Vitreous Fibers Retained in Hamster Lungs Following Inhalation

In Situ Microscopic Analysis of Asbestos and Synthetic Vitreous Fibers Retained in Hamster Lungs Following Inhalation

The Importance of Fiber Biopersistence and Lung Dose in Determining the Chronic Inhalation Effects of X607, RCF1, and Chrysotile Asbestos in Rats

The chronic inhalation effects in rats of X607 (a rapidly dissolving synthetic vitreous fiber) were compared with those previously reported for RCF1 (a refractory ceramic synthetic vitreous fiber) and chrysotile asbestos. Of primary concern was the importance of biopersistence as a mechanism of fiber toxicity. Fischer rats were exposed to fiber aerosol by nose-only inhalation for 6 h/day, 5 days/week for 2 years. X607 and RCF1 aerosols were similar in concentration (∼200 fibers/cc) and average dimensions (approximately 20 × 1 μm). Chrysotile aerosol was higher in concentration (10,600 fibers/cc) and an order of magnitude smaller in average dimensions. Lung fiber deposition after 6 h inhalation was greater for X607 than for RCF1. However, at later time points, fibers/lung (especially long fibers) were much lower for X607 than for RCF1, suggesting less biopersistence for X607. X607 was neither fibrogenic nor tumorigenic and induced only minimal lung cellularity that reversed after exposure was terminated. In contrast, RCF1 and chrysotile asbestos induced pulmonary fibrosis and thoracic neoplasms (chrysotile induced 32% more pulmonary neoplasms than RCF1). Lung deposition and fiber lengths did not explain the toxicologic differences between the three fibers. Fiber biodurability, including chemical and physical parameters, appears to be a major toxicologic determinant here. Chemical analysis of lung fibers revealed rapid leaching of X607 compared to RCF1. Inin vitrodissolution tests, X607 underwent rapid dissolution (kdis= 990 ng/cm2/h) and transverse fragmentation, RCF1 dissolved slowly (kdis= 6 ng/cm2/h) and did not fragment, and chrysotile dissolution was negligible (<0.1 ng/cm2/h).