Environ. Sci. Techno/. 1995, 29, 1728-1736
Effect of indirect Sample Preparation Procedures on the Apparent Concentration of Asbestos in Settled Dusts R. J . L E E , * T . V . D A G E N H A R T , G . R , DUNMYRE, I. M . STEWART, AND D . K . VAN O R D E N RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, Pennsylvania 15146
Measurement of the asbestos content of surface dust using transmission electron microscopy has been promoted as a means of assessing past or potential future airborne concentrations of asbestos fibers. The method currently being considered by American Society for Testing and Materials (ASTM) employs an indirect sample preparation technique that requires ultrasonication of the dust. This paper reports the results of a series of experiments that examine the effect of ultrasonication on particles containing bound asbestos. The results demonstrate that ultrasonication of samples containing bound asbestos liberates the asbestos from the particles and breaks down unbound fibers into multiple structures. The degree of the effect is shown to depend on the type of starting material and the length of time the sample is sonicated. As a result, the reported asbestos concentration bears no relationship to the number of free respirable fibers in the dust, thereby negating any potential use as a surrogate for past or future airborne exposures.
Introduction Measurement of asbestos concentrations in settled dust is being advocated as a potential surrogate for airborne concentrations and thus has attracted the attention of regulatory, environmental, and indoor air scientists (1-3), However, there are several problems with this approach: (a) there is disagreement about the appropriate methods for sample collection, preparation, and reporting of results: (b) there is substantial disagreement about the effect of sample preparation on the dust; and (c) there is disagreement about the objectives of dust sampling and, thus, the interpretation of the results. Asbestos in dust can be regarded as one or more of the following: as an indicator of past airborne concentrations, as an indicator of future airborne exposure, or simply as another component in nuisance dust. If the measurement is to reflect past or potential future concentrations, it must reliably measure free respirable fibers in the dust. In addition, if the measurement is to be evaluated in terms of potential future entrainment, it should be performed in a manner that also assessesthe re-entrainment of respirable fibers. If it is of interest as a nuisance dust, a determination of mass fraction is necessary. Achieving any of these objectives is complicated by the fact that the dust is often on a surface in a form that is not suitable for direct microscopic examination. If all asbestos in dust occurred as free unit fibrils, the effects of sample preparation might be inconsequential. However, asbestos in dust is bound in other matrices such as broken floor tile, plaster, fireproofing, or other binding materials. In these cases, the degree of disaggregation induced during sample preparation becomes the dominant factor in determining the number of respirable fibers measured. Attempts to standardize measurement procedures are in progress by ASTM, but many components of the collection and analytical procedures have not been sufficiently characterized to allow meaningful data comparisons among investigators. Most important in this regard is whether direct or indirect preparation procedures are used. Both convert a field sample into a form suitable for transmission electron microscopy (TEM) analysis. Direct procedures nominally maintain the original size and spatial relationships of asbestos particles on the collection medium. Indirect procedures remove the asbestos particles from the collection medium and redeposit them onto a new medium suitable for TEM analysis. Indirect sample preparation procedures can increase the apparent concentration of asbestos structures by breaking up larger complex asbestos structures into more numerous smallerstructures ( 4 ) due to chemical dissolution of soluble matrices andlor physical dismemberment. Chemical dissolution results from the use of aqueous suspension media containing various amounts of surfactants and/or acids. Physical dismemberment can result from grinding, manual shaking of suspensions, or dispersal by sonication. Sonication (ultrasonic dispersal) varies in * E-mail address:
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1728
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 7, 1995
0013-936W95/0929-1728$09.00/0
D 1995 American Chemical Society
effectiveness due to several factors, including frequencyof the ultrasound, maximum energy produced by the ultrasonic source, and geometry of the sonication chamber. Considerable controversy (5) has arisen in recent years as to the origin and significance of discrepancies between asbestos structure concentrations determinedby direct and indirect preparations. The bias caused by indirect preparation is widely recognized but poorly understood because little has been done to characterize the component steps of indirect preparation. In the present paper, the results of studies on the effects of surfactants, suspension liquids, and varying sonication durations are presented. To place the data in the proper perspective, a brief historical overview is provided.
Historical Perspective Data describing large asbestos fiber concentrations in surface dusts started to appear in the mid 1980s (6-8). These data were interpreted by some as implying that fibers existed as free fibers in the settled dust and were readily available for redispersal. This concept was in part engendered by the discussion of modes and rates of fiber dispersal presented in the EPA Guidance Document for AsbestosContaining Materials in SchoolBuildings,published in 1978 (9). In this document, the EPA presents a somewhat simplistic and largely anecdotal model of fall-out and secondary dispersal. Much of the data referenced in this document as indicative of redispersal into the air are based on the phase contrast microscopy (PCM) method, which counts both asbestos and nonasbestos fibers. Thus, PCM counts may bear little or no relation to actual asbestos fiber concentrations. In fact, when transmission electron microscopywas used to examine indoor air samples,the levels detected seldom exceeded background levels and were frequently below the limit of detection of the technique (5, 10, 11). This would be a somewhat incongruous result if the EPAs model of dispersal were correct, consideringthe tens of millions of asbestos structures per square centimeter sometimes reported in surface dust. The incongruity becomes even more apparent when consideringpublished values for theoretical K-factors, the ratio between the concentration of fibers in the air and the reported concentration of fibers in dusts on surfaces (12). Typical K-factors for a variety of particulate materials including asbestos, lie in the range of 10-4-10-7: that is, one fiber is calculated to be airborne for every 104-107 fibers on surfaces. Airborne concentrations from buildings with reported levels of surface concentrations of about lo7 asbestos fibers cmzrange from
