Size-selective preparation of inorganic fibers for biological experiments

Size Separation of Amosite by Filtration and Shaking Methods

The objectives of this study are (1) to separate fibrous grunerite (amosite) by its length using filtration and shaking techniques utilized in a previous study and (2) to create two distinct length groups (short and long) of the amosite with higher output in a cost-effective way. The shaking system included an electrodynamic exciter, a linear power amplifier, and an audio-frequency signal generator and was attached to a cowl sampler as a funnel loaded with a polycarbonate filter. A suspension of amosite was passed through the 10-μm pore size polycarbonate filter in the shaking system and was transferred to a filtration system through five different pore sizes of polycarbonate membrane filters in series from the top: 10-, 5-, 2-, 1-, and 0.2-μm pore sizes. Each polycarbonate filter was tightly clamped with two conductive 25-mm spacers with a 25-mm stainless steel support screen to prevent leakage. The amosite length and diameter were manually measured with images from a field emission scanning electron microscope (FESEM). A sequence of fields was selected at random locations, and an image of each field was acquired. The length and width of approximately 500 fibers for each sample were measured with ImageJ software. Two significantly different length groups (short and long) of amosite were collected (p <0.05). Approximately 95% of separated amosite (n = 499) using the filtration system were shorter than 5 μm (short fiber group), and approximately 80% of separated amosite (n = 503) using the shaking system were longer than 5 μm (long fiber group).

A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos

For more than 40 years, intensive research has been devoted to shedding light on the mechanisms of asbestos toxicity. Given the key role of fibre length in the mechanisms of asbestos toxicity, much work has been devoted to finding suitable comminution routes to produce fibres in desired size intervals. A promising method is cryogenic milling that, unlike other mechanical size reduction techniques, preserves the crystal–chemical properties of materials. In this study, the effect of cryogenic milling on the physical–chemical properties of commercial Russian chrysotile was studied in order to produce precise size fractions with invariant properties compared to the pristine fibres. In particular, two batches with fibres > 5 µm and < 5 µm were prepared, as this limit sets their potential toxicity. The results are fundamental for future in vitro toxicity testing of this commercial product, widely used in chrysotile-friendly countries but not yet adequately studied. Results show that fibre length can be controlled by milling time under cryogenic conditions without inducing structural defects or amorphization; short fibres (95% L < 5 µm) can be obtained by cryogenic milling for 40 min, while 10 min is enough to yield long chrysotile fibres (90% L > 5 µm).

Aerodynamic size separation of glass fiber aerosols

The objective of this study was to investigate the efficacy of an aerodynamic separation scheme for obtaining aerosols with nearly monodisperse fiber lengths as test samples for mechanistic toxicological evaluations. The approach involved the separation of aerosolized glass fibers using an Aerodynamic Aerosol Classifier (AAC) or a multi-cyclone sampling array, followed by the collection of separated samples on filter substrates, and the measurement of each sample fiber length distribution. A glass fiber aerosol with a narrow range of aerodynamic sizes was selected and sampled with the AAC or multi-cyclone sampling array in two separate setups. The fiber length and diameter were measured using a field emission scanning electron microscope. The glass fiber aerosol was separated in distinct groups of eight with the AAC and of four with the multi-cyclone sampling array. The geometric standard deviations of the fiber length distributions of the separated aerosols ranged from 1.49 to 1.69 for the AAC and from 1.6 to 1.8 for multi-cyclone sampling array. While the separation of glass fiber aerosols with an AAC is likely to produce two different length fiber groups and the length resolution may be acceptable, the overall mass throughput of these separation schemes is limited.

Dielectrophoretic classification of fibres: principles and application to glass fibres suspended in air

Particles exposed to an electric field experience forces that influence their movement. This effect can be used for filtration of air, or for size classification of aerosols. The motion of charged particles in a non-uniform electric field is called electrophoresis. Two processes are involved in this phenomenon: 1) charging of particles and 2) electrical mobility separation. If fibres are exposed to electrophoresis, they are separated on the basis of two parameters: diameter and length. Regrettably, as naturally occurring fibres are polydisperse both in diameter and length, the electrophoresis is not very efficient in length classification. In contrast, dielectrophoresis is the motion of electrically neutral particles in a non-uniform electric field due to the induced charge separation within the particles. As deposition velocity of fibres induced by dielectrophoretic force strongly depends on length and only weakly on diameter, it can be used for efficient length classification.

Principles of length classification of conducting and non-conducting fibres are presented together with design of a fibre classifier. Lastly, images of motion of fibres recorded by high-speed camera are depicted.

Assessment of the potential hazard represented by natural raw materials containing mineral fibres-The case of the feldspar from Orani, Sardinia (Italy)

This work describes the nature of the potentially hazardous fibrous amphibole found in the Orani's feldspar mine (Sardinia, Italy). To identify its nature, a protocol of analysis including morphometric, chemical and crystallographic characterizations was applied. Thanks to this approach, it was possible to classify the observed fibres as tremolite after comparing chemical data, SEM/TEM observations, FTIR/ Raman spectra and X-ray diffraction data with those reported for a standard sample. The unit cell parameters of the investigated tremolite phase are a = 9.82(1) Å, b = 18.08(3) Å, c = 5.27(1) Å, and the angle β corresponds to 104.4(1)°. The mean concentration of asbestos tremolite in the Orani's feldspar is 0.28 wt%. Most of the fibres (0.26 wt%) are respirable 'regulated' fibres, representing a potential hazard. Because the total amount of tremolite in the sample is 0.6 wt%, a large fraction of it has a crystal habit other than fibrous-asbestiform or acicular. The obtained results allowed us to suggest possible solutions for a safe exploitation and mineral processing of the Orani's mine. The procedure proposed herein may be a general tool suitable to identify the mineralogical nature of fibrous minerals in raw materials and assess if they may represent a potential health/environmental hazard.

Methodologies for determining the sources, characteristics, distribution, and abundance of asbestiform and nonasbestiform amphibole and serpentine in ambient air and water

Anthropogenic and nonanthropogenic (erosion) processes contribute to the continuing presence of asbestos and nonasbestos elongated mineral particles (EMP) of amphibole and serpentine in air and water of urban, rural, and remote environments. The anthropogenic processes include disturbance and deterioration of asbestos-containing materials, mining of amphibole- and serpentine-bearing rock, and disturbance of soils containing amphibole and serpentine. Atmospheric dispersal processes can transport EMP on a global scale. There are many methods of establishing the abundance of EMP in air and water. EMP include cleavage fragments, fibers, asbestos, and other asbestiform minerals, and the methods employed do not critically distinguish among them. The results of most of the protocols are expressed in the common unit of fibers per square centimeter; however, seven different definitions for the term "fiber" are employed and the results are not comparable. The phase-contrast optical method used for occupational monitoring cannot identify particles being measured, and none of the methods distinguish amphibole asbestos from other EMP of amphibole. Measured ambient concentrations of airborne EMP are low, and variance may be high, even for similar environments, yielding data of questionable value for risk assessment. Calculations based on the abundance of amphibole-bearing rock and estimates of asbestos in the conterminous United States suggest that amphibole may be found in 6-10% of the land area; nonanthropogenic erosional processes might produce on the order of 400,000 tons or more of amphibole per year, and approximately 50 g asbestos/km(2)/yr; and the order of magnitude of the likelihood of encountering rock bearing any type of asbestos is approximately 0.0001.

Efficacy of screens in removing long fibers from an aerosol stream--sample preparation technique for toxicology studies

Fiber dimension (especially length) and biopersistence are thought to be important variables in determining the pathogenicity of asbestos and other elongate mineral particles. In order to prepare samples of fibers for toxicology studies, it is necessary to develop and evaluate methods for separating fibers by length in the micrometer size range. In this study, we have filtered an aerosol of fibers through nylon screens to investigate whether such screens can efficiently remove the long fibers (L >20 µm, a typical macrophage size) from the aerosol stream. Such a sample, deficient in long fibers, could then be used as the control in a toxicology study to investigate the role of length. A well-dispersed aerosol of glass fibers (a surrogate for asbestos) was generated by vortex shaking a Japan Fibrous Material Research Association (JFMRA) glass fiber powder. Fibers were collected on a mixed cellulose ester (MCE) filter, imaged with phase contrast microscopy (PCM) and lengths were measured. Length distributions of the fibers that penetrated through various screens (10, 20 and 60 µm mesh sizes) were analyzed; additional study was made of fibers that penetrated through double screen and centrally blocked screen configurations. Single screens were not particularly efficient in removing the long fibers; however, the alternative configurations, especially the centrally blocked screen configuration, yielded samples substantially free of the long fibers.

Surface reactivity and cell responses to chrysotile asbestos nanofibers

High aspect-ratio nanomaterials (HARNs) have recently attracted great attention from nanotoxicologists because of their similarity to asbestos. However, the actual risk associated with the exposure to nanosized asbestos, which escapes most regulations worldwide, is still unknown. Nanometric fibers of chrysotile asbestos have been prepared from two natural sources to investigate whether nanosize may modulate asbestos toxicity and gain insight on the hazard posed by naturally occurring asbestos, which may be defined as HARNs because of their dimensions. Power ultrasound was used to obtain nanofibers from two different chrysotile specimens, one from the dismissed asbestos mine in Balangero (Italian Western Alps) and the other from a serpentine outcrop in the Italian Central Alps. Electron microscopy, X-ray diffraction, and fluorescence spectroscopy revealed that the procedure does not affect mineralogical and chemical composition. Surface reactions related to oxidative stress, free radical generation, bioavailability of iron, and antioxidant depletion, revealed a consistent reduction in reactivity upon reduction in size. When tested on A549 human epithelial cells, the pristine but not the nanosized fibers proved cytotoxic (LDH release), induced NO production, and caused lipid peroxidation. However, nanofibers still induced some toxicity relevant oxidative stress activity (ROS production) in a dose-dependent fashion. The reduction in length and a lack of poorly coordinated bioavailable iron in nanochrysotile may explain this behavior. The present study provides a one-step procedure for the preparation of a homogeneous batch of natural asbestos nanofibers and shows how a well-known toxic material might not necessarily become more toxic than its micrometric counterpart when reduced to the nanoscale.

New detoxification processes for asbestos fibers in the environment

Airborne asbestos fibers are associated with many serious detrimental effects on human health, while the hazard posed by waterborne fibers remains an object of debate. In adopting a precautionary principle, asbestos content in water needs to be kept as low as possible and polluting waters with asbestos should be avoided. Turci et al. (2008) recently reported a method for the decontamination of asbestos-polluted waters or landfill leachates from chrysotile that combines power ultrasound (US) with oxalic acid (Ox), an acidic chelating molecule. In the previous study, the occurrence of antigorite, a polymorph of serpentine, the mineral group encompassing chrysotile asbestos, acted as a confounding factor for complete removal of chrysotile from water. The effects of US + Ox on pure chrysotile asbestos from Val Malenco, Italian Central Alps, were examined in this investigation. In the absence of mineral contaminants, a more rapid removal of pure chrysotile from water was undertaken with respect to the previous specimen. After only 12 h of combined US + Ox acid treatment, imaging (SEM) of mineral debris indicated complete loss of fibrous habit. In addition, crystallography and vibrational features of chrysotile were not detectable (x-ray powder diffraction [XRPD] and micro-Raman spectroscopy) and elemental analysis showed a low Mg/Si ratio, i.e., the loss of the brucitic layer in chrysotile (x-ray fluorescence, XRF). Some nanometric rod-shaped debris, observed in the previous study and tentatively recognized as serpentine antigorite, was now found to be made of amorphous silica, which is relatively safe and noncarcinogenic to humans, providing further assurance regarding the safety of treated product. Thus, data indicated the proposed method was effective in detoxifying waterborne chrysotile asbestos fibers.

A new approach to the decontamination of asbestos-polluted waters by treatment with oxalic acid under power ultrasound

A suspension of chrysotile asbestos fibres in aqueous 0.5M oxalic acid was subjected to power ultrasound with the aim to disrupt and detoxify the mineral by the leaching action of oxalic acid on its structural cations acting simultaneously with a vigorous acoustic cavitation. Sonication was performed in a "cavitating tube", a vertical hollow vibrating cylinder made of titanium, operating at 19.2 kHz and 150 W. Treatment lasted from 2.5 to 21 h. Scanning electron microscopy (SEM) showed that the joint action of the chelating agent and ultrasound (though not of either when applied independently) mostly converted asbestos fibres into micrometric aggregates and nano-sized debris, whose morphology totally differed from asbestos fibres. When treated suspensions were filtered through CA membranes (pore size 0.20 microm), more than half of the asbestos went through the filter because it had either been brought in solution or dispersed in the form of extremely small particles. Most of the structural metal ions were brought into solution (ICP-AES). After the treatment the BET surface area of the recovered solid was tenfold greater than the original. The crystalline fraction of residual solids, though resembling the original sample in XRD, was shown by micro-Raman spectra to be made of antigorite, a polymorph form of serpentine. Furthermore, as the length of these antigorite fibrils lay outside the fibre range rated as a health hazard under worldwide regulations, our procedure can be employed for the decontamination of chrysotile-polluted waters and sediments.

Cytotoxic effects of quartz and chrysotile asbestos: in vitro interspecies comparison with alveolar macrophages

Cytotoxic effects of DQ12 quartz and chrysotile asbestos on alveolar macrophages of different animal species were compared in vitro. The type of cell reaction toward the cytotoxic dusts was always the same: a loss of cell viability (trypan blue dye exclusion test) was accompanied by the release of cytoplasmic and lysosomal enzymes. The extent of cellular destruction depended upon the amount of dust applied. In the range of 50-100 micrograms/ml quartz or chrysotile asbestos, species-specific variations were observed in the sensitivity of the cells. At this concentration alveolar macrophages of dogs, monkeys, and human patients were damaged to a greater extent than the cells from rats and cattle. Simultaneous incubation of the cells with quartz and L-alpha-dipalmitoyl lecithin resulted in a reduction of the cytotoxic quartz effect. The extent of the protective effect varied according to the species. In the case of chrysotile asbestos no reduction of the fibers cytotoxicity was observed in the presence of L-alpha-dipalmitoyl lecithin.

Activation of the alternative complement pathway and generation of stimulating factors for granulocytes by glass fibers

Continuous-filament glass fibers coated with organic agents, candidate asbestos substitutes, were assessed for their ability to elicit from normal human serum complement-derived cleavage products which are able to stimulate the chemotaxis and the respiratory burst of polymorphonuclear leukocytes. Glass fibers generated chemoattracting and respiratory stimulating factors for polymorphonuclears from human serum. The effect was dose related for chemotaxis from the serum fiber concentration of 75 micrograms/ml to 1,250 micrograms/ml. The serum chemoattracting activity, as well the respiratory stimulation, were dramatically impaired when serum had been preliminarily absorbed with antiC5 antiserum. Since the impairment of chemotactic activity occurred also in the presence of EDTA, but not in the presence of EGTA, we assumed an activation of the alternative complement pathway. Glass fibers were studied in comparison to a UICC sample of Canadian chrysotile asbestos, which is able to activate in vitro the alternative complement pathway. Glass fibers exhibited less ability than asbestos fibers to generate complement cleavage products with chemotactic activity for polymorphonuclears; however, they produced an activity about equal to 80% of a chemotactic standard stimulus such as zymosan-activated plasma.

Carcinogenicity studies on fibres, metal compounds, and some other dusts in rats

About 50 dusts were examined on their carcinogenicity in rats mainly after intraperitoneal injection and some after intratracheal instillation. In the i.p. test, very low doses between 0.05 and 0.5 mg asbestos led to tumour incidences of about 20 to 80%. Polyvinyl-pyridine-N-oxide prolonged the tumour latency after injection of actinolite. 60 mg attapulgite from three sources with short fibre lengths were not shown to be carcinogenic but an attapulgite sample with longer fibres had a moderate effect. Relatively thick rock and ceramic fibres (median greater than 1 micron) induced tumours, but slag and wollastonite fibres did not, probably because of their better solubility. Intratracheal instillations of glass microfibres (20 X 0.5 mg) led to lung tumours in 5 of 34 rats (0 in control). The carcinogenic potency of an inorganic fibre depends on its size and persistency, and possibly also on other properties, especially on the surface. Nickel powder, nickel oxide, nickel subsulfide and cadmium sulfide were all found to be carcinogenic in the two tests. Cadmium chloride and cadmium oxide could only be administered in very low doses because of their high acute toxicity. A high amount of magnetite (15 X 15 mg i.tr.) led to an unexpected lung tumour incidence of 69%. The i.p. test in rats proved to be very sensitive for detecting the carcinogenic potency of non-acute toxic natural and man-made mineral dusts as well as metal compounds. This means that, if a high dose of one of these dusts does not induce tumours in this test, no suspicion of carcinogenic potency can be substantiated.

Influence of well-defined mineral fibers on proliferating cells

The effects of well-defined asbestos and man-made mineral fibers, as well as glass and synthetic fluoroamphibole, on phagocytizing permanent rat tumor cells were tested. The following parameters were compared: cell proliferation as determined by cell count and 3H-thymidine incorporation, RNA synthesis by 3H-uridine uptake, protein synthesis by incorporation of 3H-labeled amino acids, protein content and plasma membrane permeability by release of lactic dehydrogenase. The dosage of most of the dusts was estimated gravimetrically, but for some dusts also numerically. Because of the wide range of different fibers lengths, diameters and specific weights, it was sometimes difficult to compare chemically and physically differing fiber fractions with the same fiber counts. In some cases, resulting weights are so different that a direct comparison of the conclusions is impossible. The results with fibers of diverse sources showed the same trends: the toxicity of fibers increases with increasing length and dose. In this test system we found an inhibition of DNA and RNA synthesis. Protein synthesis as measured by amino acid uptake per total cell culture decreased, but the protein content of the single cell increased as determined by the Lowry method. The increase of plasma membrane permeability as determined by lactic dehydrogenase was also dependent on fiber length and concentration. Generally the thinner the fiber, the greater the toxicity when gravimetrical dosage and the same length distributions are employed. Beyond that we can state that the toxicity of fibers from different sources with similar fiber dimensions is similar. One of the glass fiber fractions has a comparable geometry (length, diameter) to the UICC fraction of chrysotile and exhibits the same high toxicity.

Macrophage functions after exposure to mineral fibers

UICC, other well-defined asbestos samples and different man-made mineral fibers (MMM) such as glass fiber and synthetic amphibole asbestos were studied in vitro by using rat and guinea pig lung macrophages. These samples had relatively narrow length and diameter spectra. Most of the fiber samples were added to the cultures on a gravimetric basis, although some were added on a numerical basis. Electrocorundum and DQ12 (Dorentruper Quartz) were used as controls at comparable gravimetrical concentrations. The assays used were the release of lactate dehydrogenase (to demonstrate plasma membrane permeability) and the release of beta-glucuronidase (to indicate lysosomal permeability). Carbohydrate metabolism was monitored by the measurement of lactic acid production and, as one of the tests for macrophage function, the production of lysozyme was determined. The phagocytic ability of the cells was measured, after the addition of opsonized zymosan, by bioluminescence following luminol enhancement. Only some results could be evaluated, however, due to technical difficulties. A length- and dose-dependent cytotoxicity of the fibers was found in this system which was similar to that previously described with permanent cell lines. No great differences were found between fibers having different physicochemical compositions if their geometric dimensions were similar. Long, very thin fibers of glass, chrysotile, crocidolite and synthetic fluoroamphiboles were all toxic in the test system.

Cytotoxicity of a short-fiber chrysotile asbestos for human alveolar macrophages: preliminary observations

Studies were performed to compare the cytotoxicity for human alveolar macrophages of a naturally occurring short-fiber chrysotile asbestos (RG 144) to that of a standard reference mixed-fiber (long and short) chrysotile asbestos (UICC chrysotile A. Rhodesian). Parallel studies were also performed with quartz (Min-U-Sil 15), a known macrophage toxin. On a mass basis, and after 24 hr incubation, RG 144 was more cytotoxic than the UICC standard reference fiber and less toxic than quartz (silica). The cytotoxic potential of RG 144 chrysotile was further enhanced after size reduction by milling. These findings may have important biologic implications with respect to the use of short-fiber asbestos in industry.