In-vitro biodurabilityThe magnesium hydroxide part of each layer being closest to the fiber surface is reflected in the chemical characteristics of chrysotile, which has poor acid resistance compared to other asbestiform substances. The amphiboles, for example, in which the silicate oxygens are on the “outside” of the layers and the hydroxides are masked within, have better resistance to acids. Hargreaves & Taylor reported that if fibrous chrysotile is treated with dilute acid, the magnesia can be completely removed. The hydrated silica which remains, though fibrous in form, had completely lost the elasticity characteristic of the original chrysotile and had a structure that was “amorphous” or “glassy” in type. Wypych et al. examined what happens to natural chrysotile fibers when acid-leached under controlled conditions. The authors reported that the leached products consisted of layered hydrated disordered silica with a “distorted” structure resembling the silicate layer existing in the original minerals. Extensive characterization techniques confirmed the removal of the brucite-like sheets, leaving silica with an eminently amorphous structure. Suquet reported on the assessment of the structural damage produced by grinding or acid leaching of chrysotile. The author reported that “Acid leaching transformed chrysotile into porous, non-crystalline hydrated silica, which easily fractured into short fragments. If the acid attack was too severe, these fragments converted into shapeless material”.Seshan reported that following exposure to water, strong acids and simulated gastric juices, chrysotile asbestos underwent changes in the physical, chemical and surface properties. The authors reported that the surface becomes silica-like and that upon exposure to water and acid the magnesium is lost from the fibers. The authors also reported that upon acid exposure, the magnesium ions are leached out, leaving a magnesium-free silica network. In addition, the acid treatment also destroyed the X-ray diffraction pattern of chrysotile and changed its refractive index. In contrast, crocidolite asbestos remained unchanged.Larsen evaluated different types of natural and synthetic fibers which had been subjected to systematic solubility tests in vitro in a physiological solution at 37 °C. Included in this evaluation were chrysotile and crocidolite. Solubility was evaluated by the measurement of silicon in a Gamble’s solution similar in composition to lung fluid (without the organic components) using atomic absorption spectrophotometry. The authors reported that the dissolution values ranged from a few nanograms of silicon dissolved per cm2 (chrysotile and crocidolite) to several thousands of ng/cm2 silicon dissolved (glass wools) and that aramide and carbon fibers proved to be practically insoluble. For chrysotile, the authors reported that after a 6-week shaking-table experiment (closed system) that 6 ng/cm2 silicon and 160 ng/cm2 magnesium had dissolved.Oze & Solt investigated the biodurability of chrysotile and tremolite asbestos in simulated lung and gastric fluids. The simulated gastric fluid (SGF) was composed of HCl and NaCl solution at a pH 1.2 and the simulated lung fluid (SLF) was a modified Gamble’s solution at pH 7.4 at 37 °C. The studies were performed under batch conditions using 0.01, 0.1 and 1 g of ground fiber in a 50 ml vial over 720 h in apparently static conditions. There was no discussion of the influence of the large number of fibers present in such quantities on fluid contact and whether the suspensions settled over time. The relative biodurabilities determined under these conditions were (from most to least) tremolite (SLF) > chrysotile (SLF) > tremolite (SGF) > chrysotile (SGF) when accounting for the greater surface area of chrysotile per mass or per fiber compared to tremolite. Silica release from chrysotile was 30–66 times greater under acid conditions as compared to neutral pH. The authors estimated that a chrysotile fiber will dissolve ∼200 × faster in SLF and ∼2.5 × faster in SGF compared to tremolite asbestos. The authors calculated that a 1 × 10 μm chrysotile fiber will completely dissolve in neutral pH in ∼19 months while a tremolite fiber of equal shape will dissolve in ∼4 years. At acid pH, a chrysotile fiber of the same dimensions will dissolve in ∼33 h and a tremolite fiber will dissolve in ∼9 months. The authors pointed out that these values represent approximate fiber lifetimes and do not account for changes in the surface area with respect to time, or for preferential dissolution sites such as crystal defects or edges. In addition, these times do not take into account the inflammatory processes in the lung that have been shown to occur with tremolite and their influence on dissolution rates.In another study using a Gambles solution, Osmon-McLeod et al. assessed the durability of a number of fibers including long fiber amosite and long fiber chrysotile. In this study, the pH of the Gambles solution was adjusted to 4.5 to mimic that inside macrophage phagolysosomes, which the authors described as “potentially the most degradative environment that a particle should encounter following lung deposition and macrophage uptake”. Fiber durability was assessed from the loss of mass of the fiber. The chrysotile was recovered with ∼30% of original weight after the 24-week incubation. The amosite asbestos was recovered with ∼75% of original weight. None of the carbon nano-tube samples included in the study showed a significant loss of mass by week 24 with one exception which was recovered at only ∼70% of its original weight at all time-points from week 3 onward. The authors stated that for chrysotile, the percent recoveries reflect true mass loss, whereas the small mass loss for amosite asbestos over the 24-week period may be due to the loss of small fibers in the sample. The chrysotile showed no difference in average fiber width with incubation, but did show a marked decrease in length. At 0 weeks the chrysotile sample comprised a mixture of fibrils and ropes of fibrils, while at 10 weeks only small fibrils remained. The authors commented that it is probable that the measured loss of length accurately reflects fiber shortening in addition to the breaking up of large fiber bundles. Pathogenicity of these samples was also evaluated in vivo using a mouse model sensitive to inflammogenic effects of fibers. Osmon-McLeod et al. found that the data indicate that long fiber chrysotile showed ∼70% mass loss and a marked decrease in length with long-term incubation in the Gambles solution, with a concomitant mitigation of the pathogenicity seen in mice injected with 0 weeks samples. Long fiber amosite that had been incubated for 10 weeks, however, also showed a loss of mass comparable to one of the long carbon nano-tubes at the same time-point, but no fiber shortening, and did not lose its pathogenicity.These studies illustrate the differences in dissolution rates between chrysotile and amphibole asbestos under both neutral and acidic conditions and provide support for understanding the results of the inhalation studies discussed below.
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