Differential lung clearance between fibers of chrysotile and the more rod-like amphibole asbestos fibers was found in rats that underwent chronic inhalation exposures (Wagner et al., 1974
). The lung fiber burdens of the amphiboles rose continuously throughout 2 yr of exposure, and declined slowly in the rats removed from exposure after 6 mo. In contrast, the lung burdens in rats exposed to both Quebec and Zimbabwe chrysotile increased much more slowly during exposure, and seemed to decline after 12 mo, even with further exposure.
The biopersistence of chrysotile fibers from other locations was studied by Bernstein and colleagues (2004)
following inhalation exposures to aerosols with large number concentrations of fibers >20 μm in length. For chrysotile from the Cana Brava mine in central Brazil, the clearance half-time of the fibers >20, those 5-20, and those <5 μm in length were 1.3, 2.4, and 23 d, respectively (Bernstein et al., 2004
). For chrysotile from the Coalinga mine in California (Calidria RG144), the clearance half-time of the fibers >20, those 5-20, and those <5 μm in length were 7 h, 7 d, and 64 d, respectively, while for tremolite asbestos there was no clearance from the rat lungs over the 1-yr period of observation (Bernstein et al., 2005a
). The tremolite exposures produced lung inflammation, granulomas, and lung fibrosis, while the chrysotile, despite involving a much higher long fiber concentration (190.5 fibers/cm3
>20 μm for chrysotile vs. 106.2 fibers/cm3
>20 μm for tremolite), did not produce any measurable response. For textile-grade chrysotile from the Eastern Townships of Quebec, the clearance half-time for fibers >20 μm in length was 11.4 d, which was similar to that for glass and stone wools previously studied (Bernstein et al, 2005b
), but a longer half-life than reported earlier in this article for California or Brazilian chrysotile. Data suggest differences in clearance for various types of chrysotile.
Similar differential retention of chrysotile versus amphibole types of asbestos was found in humans. Churg (1994)
reported on analyses of lung tissue for 94 chrysotile asbestos miners and millers from the Thetford region of Quebec, Canada. The retained chrysotile and exposure atmosphere contained a small percentage of tremolite, yet the lungs contained more tremolite than chrysotile, and the tremolite content increased rapidly with the duration of exposure. While most of the inhaled chrysotile was rapidly cleared from the lungs, a small fraction seemed to be retained indefinitely. After exposure ended, there was little or no clearance of either chrysotile or tremolite from the lungs, an unexpected finding. Albin et al. (1994)
studied retention patterns in lung tissues from 69 Swedish asbestos-cement workers and 96 controls. Data showed that chrysotile underwent relatively rapid removal in human lungs, whereas amphiboles (tremolite and crocidolite) displayed a slower removal pattern. Albin et al. (1994)
also noted that (1) chrysotile retention may be dependent on dose rate, (2) chrysotile and crocidolite retention may be increased by smoking, and (3) chrysotile and tremolite retention may be enhanced by the presence of lung fibrosis.
The most direct evidence for the effect of altered dust clearance rates on the retention of inhaled fibers in humans comes from studies of the fiber content of the lungs of asbestos workers in various countries. Timbrell (1982)
developed a model for fiber deposition and clearance in human lungs based on his analysis of the bivariate diameter and length distributions found in air and lung samples collected at an anthophyllite mine at Paakkila in Finland. The length and diameter distributions of the airborne dust at this particular mine were exceptionally broad and historic exposures were high. For workers with the highest exposure and most severe lung fibrosis (Ashcroft et al., 1988
), the fiber distributions in some tissue segments approached those of the airborne fibers. Adjacent tissue, analyzed for extent of fibrosis, showed severe fibrotic lesions. Ashcroft et al. (1988)
concluded that chronic retention was essentially equal to deposition in such segments. In these studies, lung fibrosis was associated with increased fiber retention, and fiber retention is clearly associated with fiber length and diameter. The critical fiber length for mechanical clearance from the lungs is greater than 17 μm. as confirmed in the inhalation model published by Coin et al. (1992)
. More precise descriptions of the effect of fiber loading in the lung on the development of fibrosis need to be based on the use of the most appropriate index of fiber loading.
explained that “From a chemical point of view chrysotile behaves in certain aspects as if it were magnesium hydroxide. This is not unexpected when one considers that the structure generally ascribed to the mineral consists of fundamental layers made up in terms of a unit cell of O6-Si4-O4(OH)2-Mg6-OH6 planes” (p. 894). Pundsack (1955)
found that the behavior of chrysotile fibers can be understood as a magnesium hydroxide layer on a silica substrate and explained that initially at neutral pH, such as that found in lung surfactant, “in contact with relatively pure water the fiber surface dissociates partially until equilibrium of the order of that attained by pure magnesium hydroxide is reached.” In an acid environment (such as might occur in the macrophage), “It is important to note that chrysotile reacts with strong acids to form eventually a hydrated silica residue. Therefore, the particles suspended in initially acid solutions are not chrysotile in the strict sense, but represent instead intermediate reaction products of the acid and the fiber” (p. 895).
Bernstein et al. (1984)
and Hammad (1984)
found evidence of substantial in vivo dissolution of glass fibers. Le Bouffant et al. (1984)
used x-ray analysis on individual fibers recovered from lung tissue to show the exchange of cations between the fibers and tissues. For example, the fibers may lose calcium and gain potassium.
Insight on the solubility of fibers in vivo was also obtained from in vitro solubility tests. Griffis et al. (1981)
found that glass fibers suspended either in buffered saline or serum-like solution at 37°C for 60 d exhibited some solubility and that the sodium content of the residual fiber was reduced. Förster (1984)
used Gamble's saline solution for tests on samples of 18 different SVF at temperatures of 20 and 37°C and for exposure times ranging from 1 h to 180 d using static tests, tests with once-daily shaking, tests with continuous shaking, and tests with single fibers in an open bath. There was some solubility for all fibers. Klingholz and Steinkopf (1982
) studied dissolution of mineral wool, glass wool, rock wool, and basalt wool at 37°C in water and in a Gamble's solution modified by omission of the organic constituents. Most of the tests used a continuous-flow system in which the pH was 7.5-8. There was relatively little dissolution in distilled water in comparison to that produced by the modified Gamble's solution. The surfaces developed a gel layer that, for the smaller diameters, extended throughout the fiber cross section. Thus, fibers may become both smaller in outline and more plastic to deformation.
Scholze and Conradt (1987)
performed a comparative in vitro study of the chemical durability of SVF in a simulated extracellular fluid under flow conditions. Seven vitreous, three refractory, and three natural fibers were involved. Samples of the leachate were analyzed, and the silicon concentrations were used to roughly classify the fibers according to their chemical durability in terms of glass network dissolution. A durability ranking of fiber materials was expressed in terms of a characteristic time required for the complete dissolution of single fibers of given diameter.
SVF exhibited relatively poor durability (with network dissolution velocities ranging from 3.5 to 0.2 nm/d for a glass wool and an E-glass fiber, respectively), whereas natural mineral fibers were persistent against the attack of the biological fluid (e.g., less than 0.01 nm/d for crocidolite).
Davies et al. (1984)
exposed rats to SVF aerosols at 10 mg/m3
for 7 h/d, 5 d/wk for 1 yr as compared to the single exposure of several hours duration used by Morgan and Holmes (1984)
. The percentage of glass fibers with diameters less than 0.3 μm recovered from the lungs was consistently less than that in the original fiber suspension, and the reduction was more marked in the animals that were sacrificed following a period of recovery from the exposures than from those sacrificed at the end of the exposure. The degree of fiber etching increased with residence times in the lungs. Glass wool with and without resin was also etched, but to a lesser extent, and the etching of the rock wool fibers was considerably less.
In a study of dissolution of inhaled fibers by Eastes and Hadley (1995)
, rats were exposed for 5 d to 4 types of airborne, respirable-sized SVF and to crocidolite fibers. The SVF included 2 glass wools, and 1 each of rock and slag wool. After exposure, animals were sacrificed at intervals up to 18 mo, and the numbers, lengths, and diameters of a representative sample of fibers in their lungs were measured. Long fibers (>20 μm) were eliminated from rat lungs at a rate predicted from the dissolution rate measured in vitro. The long SVF were nearly completely eliminated in several months, whereas most long crocidolite asbestos fibers remained at the end of the study. The number, length, and diameter distributions of fibers remaining in the rat lungs agreed well with a computer simulation of fiber clearance that 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. Thus, long SVF were cleared by complete dissolution at the rate measured in vitro, and short fibers did not dissolve and were cleared by macrophage-mediated physical removal.
In an inhalation study using 9 fiber types, Bernstein et al. (1996)
exposed rats to an aerosol (mean diameter of ~1 μm) at a concentration of 30 mg/m3
, 6 h/day for 5 d with post-exposure sacrifices at 1 h, 1 day, 5 d, 4 wk, 13 wk, and 26 wk. At 1 h following the last exposure, the 9 types of fibers were found to have lung burdens ranging from 7.4 to 33 × 106
fibers/lung with geometric mean diameters (GMD) of 0.40-0.54 μm, reflecting the different bivariate distributions in the exposure aerosols. The fibers cleared from the lungs following exposure with weighted half-lives ranging from 11 to 54 d. The clearance was found to closely reflect the clearance of fibers in the 5-20 μm length range. An important difference in removal was seen between the long fiber (L > 20 μm) and shorter fiber (L between 5 and 20 μm and L < 5 μm) fractions, depending upon composition. For all glass wools and the stone wools, the longer fibers were removed faster than the shorter fibers. It was found that the time for complete fiber dissolution based on the acellular in vitro dissolution rate at pH 7.4 was significantly correlated with the clearance half-times of fibers >20 μm in length. No such correlations were noted with any of the length fractions using the acellular in vitro dissolution rate at pH 4.5. Examination of the fiber length distribution and particles in the lung from 1 h through 5 d of exposure indicated that, especially for those fibers that form leached layers, fiber breakage may have occurred during this early period. These results demonstrate that for fibers with high acellular solubility at pH 7.4, the clearance of long fibers is rapid.
Eastes and Hadley (1996)
fitted much of the data just cited into a mathematical model of fiber carcinogenicity and fibrosis. Their model predicts the incidence of tumors and fibrosis in rats exposed to various types of rapidly dissolving fibers in an inhalation study or in an intraperitoneal (ip) injection experiment. This takes into account the fiber diameter and the dissolution rate of fibers longer than 20 μm in the lung, and predicts the measured tumor and fibrosis incidence to within approximately the precision of the measurements. The underlying concept for the model is that a rapidly dissolving long fiber has the same response in an animal bioassay as a smaller dose of a durable fiber. Long, durable fibers have special significance, since there is no effective mechanism by which these fibers may be removed. In particular, the postulation is that the effective dose of a dissolving long fiber scales as the residence time of that fiber in the extracellular fluid. The residence time of a fiber is estimated directly from the average fiber diameter, density, and the fiber dissolution rate as measured in simulated lung fluid at neutral pH.
The incidence of fibrosis in chronic inhalation tests, the observed lung tumor rates, and the incidence of mesothelioma in the ip model were all well predicted by this model. The model allows one to predict, for an inhalation or ip experiment, what residence time and dissolution rate are required for an acceptably small tumorigenic or fibrotic response to a given fiber dose. For an inhalation test in rats at the maximum tolerated dose (MTD), the model suggests that less than 10% incidence of fibrosis would be obtained at the maximum tolerated dose of 1-μm diameter fibers if the dissolution rate were greater than 80 ng/cm2
/h. The dissolution rate that would give no detectable lung tumors in such an inhalation test in rats is much smaller. Thus, a fiber with a dissolution rate of 100 ng/cm2
/h has a nonsignificant chance of producing either fibrosis or tumors by inhalation in rats, even at the MTD. This model provides manufacturers of SVF and other synthesized fibers with design criteria for fibrous products that minimize, if not eliminate, the potential for producing adverse health effects. Support for the use of biopersistence data for the prediction of fibrosis and tumor responses in rats from both ip injection studies and chronic inhalation studies for fibers >5 and >20 μm in length was also provided by Bernstein et al. (2001a
). For the inhalation studies, Bernstein et al. (2001a
) used collagen deposition at the bronchoalveolar junction as a predictor of interstitial fibrosis. Fibrosis was also associated with the development of lung cancers in rats (Davis & Cowie, 1990
). The consensus from these and other studies already mentioned suggests that some SVF, including ceramic fibers, are more durable than others and are less persistent than crocidolite asbestos in the lung.
As discussed earlier, the retention, dose, dimensions, durability, and composition of amphibole asbestos fibers, in contrast to disintegration and/or dissolution of SVF, are critical parameters related to adverse health effects occurrence, with chrysotile asbestos fibers falling between these two extremes. The initial database of well-conducted inhalation toxicology studies from which these concepts are based included primarily studies of SVF, many of which included asbestos-exposed groups as positive controls. More recently, studies on different types of asbestos fibers have extended these concepts, differentiating serpentine (chrysotile) asbestos from amphibole asbestos. The existing database of fiber toxicity studies indicates that human exposure to respirable fibers that are biopersistent in the lung also induces significant and persistent pulmonary inflammation, cell proliferation, or fibrosis, and therefore needs to be viewed with concern.