Environ. Sci. Technol. 1996, 30, 3010-3015
Interlaboratory Evaluation of the Breakup of Asbestos-Containing Dust Particles by Ultrasonic Agitation R. J. LEE, D. R. VAN ORDEN,* AND G. R. DUNMYRE RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, Pennsylvania 15146
Measurement of asbestos in settled dust has been suggested as a surrogate for past and potential future airborne exposures. While conceptually appealing, identification and quantitation of those respirable particles that either were airborne or could become airborne through re-entrainment are complex and difficult tasks. Round robin testing of a draft ASTM method [The method has recently been balloted and passed by ASTM as Method D-5755.] for determining asbestos in dust was performed to assess the variability of the method and the impact of large, non-respirable asbestos-containing particles on the reported numerical concentration of respirable asbestos structures. Tests conducted using single, nonrespirable particles of asbestos or asbestoscontaining materials (ACM) indicate that these large particles can give apparent concentrations of asbestos in the dust on the order of millions of structures per square centimeter. Coefficients of variation (CV) ranged up to 2 for these tests, primarily the result of variable application of the indirect preparation procedure. A second series of tests was conducted using simulated building dusts. ACM dust was blown throughout a controlled chamber, and the resulting settled dust was sampled and distributed to participating laboratories. These results indicate that the interlaboratory CV is similar to that of the first tests, but can be reduced to 0.8 if restrictions are placed on the indirect sample preparation procedure. There was no statistically significant difference in the concentration of asbestos collected from three different surfaces, but this is the result of limited data and highly variable results. Overall, these tests indicate that the analysis of surface dusts using indirect sample preparation should be limited to qualitative evaluation as to the presence of asbestos in the surface dust.
* Author to whom correspondence should be addressed; fax: (412)733-1799; e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
Introduction Asbestos fibers are ubiquitous in our environment, occurring naturally in rocks and soils and occurring in manmade products such as building materials and friction products (1, 2). Therefore, it is not unexpected that asbestos can be present in surface dusts found in buildings. The concentration of asbestos in settled dust has been proposed as a potential surrogate for airborne concentration (and, thus, an estimate of health risk), and this has attracted attention from regulators, environmentalists, and indoor air scientists (3-7). Data describing astronomically large asbestos fiber concentrations in settled dusts started to appear in the mid 1980’s (8, 9). It has been argued that these data implied that the asbestos existed as free fibers that settled in the dust and were thus readily available for re-dispersal. In fact, when an transmission electron microscopy was used to examine indoor air samples from buildings nationwide, the levels detected in the air seldom exceeded background levels and were frequently below the limit of detection of the technique (1, 10-12). If it is postulated that the presence of asbestos in settled dust is due to the settling of airborne fibers, this is a somewhat anomalous result considering the billions of asbestos structures per square foot typically reported in settled dust. These results indicate that either airborne levels are, at best, very tenuously related to surface dust concentrations or that the measurements of asbestos fiber concentration are flawed. A method has been proposed that specifies indirect preparation of dust samples for transmission electron microscopy (TEM) analysis (13, 14). This method requires the sample to be collected using a microvacuum technique (14). Following sample collection, the dust is rinsed out of the cassette and into a beaker. The beaker and suspension are then ultrasonically agitated for 3 min, and an aliquot of the suspension is re-deposited onto a filter. This filter is then prepared using standard direct preparation methods (15). Thus, direct preparation procedures maintain the original size and spatial relationship of particles on the preparation filter while indirect preparation removes the particles from the filter and re-deposits them onto a new medium suitable for TEM analysis. There has been considerable controversy (16) in recent years about the validity of asbestos structure concentrations determined by indirect sample preparation methods as specified in various measurement protocols. The large, positive, asbestos concentration bias resulting from indirect preparation is widely recognized but poorly understood because not enough has been done systematically to characterize the component steps of indirect preparation. Recently, however, the effects of surfactants and of sonication duration on asbestos dusts have been explored. It has been shown (17-19) that increasing sonication times result in higher apparent asbestos concentrations, presumably as a result of physical and chemical release of asbestos from matrices, from dissolution of bundles into individual fibrils, and from comminution of the fibers into more numerous, shorter and thinner fibrils. In the interim, one study has been published indicating that reasonable reproducibility can be achieved by the method when used on a laboratory-prepared mixture of
S0013-936X(96)00011-9 CCC: $12.00
1996 American Chemical Society
calcite and chrysotile (20). This study used known mass proportions of asbestos and non-asbestos, which is appealing. However, concern arises as to the suitability of this simple mixture to model complex real-world mixtures. For example, chrysotile in some brands of spray-on fireproofing is intimately enveloped in gypsum that has crystallized around the asbestos fibers. Thus, fireproofing asbestos is not easily separated from its non-asbestos matrix. In contrast, chrysotile and calcite are easily separated. In this paper, the earlier study (17) on the effect of indirect sample preparation of settled dusts is extended in two parts to include interlaboratory testing on individual pieces of asbestos-containing material (ACM) debris and, in the second part, interlaboratory testing of simulated building dusts containing ACM debris.
Tests on Large Asbestos-Containing Particles One aspect of critical importance for any asbestos dust sampling and analysis method is a proper definition of dust. The aforementioned ASTM method (14) includes small debris particles up to 1 mm in size in the definition of dust. These tests on debris specifically evaluate the impact of large, non-respirable asbestos-containing particles on the apparent asbestos concentration in settled “dust”. Methods. In the present study, nine laboratories evaluated three types of dust samples to test a draft version of the ASTM method (14). The first sample type (spray-on fireproofing containing a nominal 10% chrysotile, with vermiculite and gypsum) consisted of a single 0.5-mm piece of asbestos-containing material (ACM). The second sample type included a single 1-mm ACM piece taken from the same material. The third sample type consisted of a single 1-mm bundle of chrysotile picked from commercial grade 7 M chrysotile, the grade most commonly used in building materials. Using a binocular microscope and a calibrated scale, each particle was hand-picked from a bulk material and placed into a new 25-mm air cassette. Photographs of the particles were taken to document particle size. All nine laboratories chosen for this program claimed previous experience in the analysis of dust samples. All but one of the laboratories were accredited by the National Voluntary Laboratory Accreditation Program (NVLAP) for TEM asbestos analysis. Eight of the laboratories received one sample of each of the 0.5- and 1-mm ACM particles. The ninth laboratory analyzed seven samples of the 0.5- and 1-mm ACM particles. All laboratories received two samples of the 1-mm 7 M bundles, with the ninth laboratory receiving additional samples. The samples were randomly chosen for distribution to the laboratories. Each laboratory was asked to perform a dust analysis on each sample following the proposed protocol specified in the draft test method (14). To ensure uniformity, each laboratory was provided a copy of the most recent draft of the protocol. (The final, approved version of the ASTM method includes a step to calibrate the ultrasonic bath and a change in the fiber counting rules.) In addition, each laboratory was instructed to use EPA Level II counting rules (15) and to return their completed reports to RJ Lee Group. There was no reported documentation of the sample condition upon receipt at each laboratory. Results were calculated in terms of structures per centimeter squared, assuming a hypothetical sample collection area of 100 cm2. The data were evaluated to determine relative differences of measured concentration
TABLE 1
Apparent Asbestos Structure Concentration (Million of Structures/cm2) 7 M chrysotile
0.5 mm ACM
1 mm ACM
all structure all structure all structure structures g 5 µm structures g 5 µm structures g 5 µm lab A lab A lab B lab B lab C lab C lab D lab D lab E lab F lab F lab G lab G lab H lab H lab H lab H lab H lab H lab H lab H lab H lab I lab I
0.02 0.70 1.11 1.22 6.31 3.37 22.50 4.26 0.20 0.02 1.12 0.22 3.71 2.56 2.36 4.74 3.48 5.72 3.95 3.05 5.59 4.91 0.06 0.04
0.02 0.43 0.42 0.67 1.68 0.54 7.66 1.38 0.05
