09/01/88
Asbestos Content in Bulk Insulation Samples: Visual Estimates
and Weight Composition
September, 1988
ASBESTOS CONTENT IN BULK INSULATION SAMPLES:
Visual Estimates and Weight Composition
By
Ian M. Stewart
RJ Lee Group
Monroeville, PA 15146
Prepared for:
Midwest Research Institute
Kansas City, MO 64110
EPA Contract No. 68-02-4252
Work Assignment 43
MRI Project 8861-A43
Field Studies Branch
Exposure Evaluation Division
Office of Toxic Substances
U.S. Environmental Protection Agency
Washington, DC 20460
DISCLAIMER
This report was prepared under contract to an agency of the United States
Government. Neither the United States Government nor any of their
employees makes any warranty, expressed or implied, or assumes any legal
liability for any third party's use of or the results of such use of any
information, apparatus, product, or process disclosed in this report, or
represents that its use by such third party would not infringe on
privately owned rights. Mention of trade names or commercial products
does not constitute endorsement for use.
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INTRODUCTION
In April 1973, the U.S. Environmental Protection Agency ( EPA ) issued the
National Emissions Standards for Hazardous Air Pollutants ( NESHAP ) for
asbestos ( 38 FR 8820 ). The NESHAP regulation governs the removal,
demolition, and disposal of asbestos-containing bulk wastes. An asbestos-
containing product, as stated by the regulation, was defined for the first
time to be a product with greater than 1% asbestos, by weight. The intent
of the 1% limit was:
...to ban the use of materials which contain significant
quantities of asbestos, but to allow the use of materials
which would: (1) contain trace amounts of asbestos which
occur in numerous natural substances, and (2) include very
small quantities of asbestos ( less than 1 percent ) added
to enhance the material's effectiveness. ( 38 FR 8821 )
It must be clearly understood that the EPA NESHAP definition of 1% by
weight was not established to be a health-based standard.
In May 1982, EPA issued a regulation which required schools to inspect and
sample suspect friable surfacing materials for their asbestos content.
EPA maintained consistency in its definition of an asbestos-containing
material ( ACM ) by defining it as 1% by weight. At that time, the Agency
investigated the available methodologies for measurement of asbestos
fibers.
the regulation included an interim methodology entitled "Interim Method
for the Determination of Asbestos in Bulk Insulation Samples" ( 47 FR
23376 ). The polarized light microscope ( PLM ) protocol issued by the
Agency was prepared by expert mineralogists and has been generally
accepted by the analytical community as the appropriate analytical tool
for measurement of asbestos content in bulk samples.
The interim method includes a description of its quantitation procedure.
This procedure employs a technique called "point counting" to provide a
determination of the area percent of asbestos in the sample. Based on a
measurement made by point counting, the 1982 rule states "...reliable
conversion of area percent to dry weight is not currently feasible unless
the specific gravities and relative volumes of the material are known."
EPA amended this statement in a correction to the regulation in September
1982 ( 47 FR 38535 ). EPA altered paragraph 1.7.2.4 of Appendix A of the
rule by stating, "Paragraph 1.7.2.4 of Appendix A of the rule was intended
to provide for a point counting procedure or an equivalent estimation
method for determining the amount of asbestos in bulk samples." This
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correction, acknowledged the practical and economic limitations of the
point counting method and permitted the use of the visual estimation
methodology. Visual estimation methodology is employed by most PLM
laboratories and gives results which are very similar to a volume
percentage.
In the following discussion, the validity of the assumptions that are made
in extrapolating an area / volume percentage estimation to a weight
percentage estimation of the asbestos content of insulation and other
building materials will be examined. The reader should note that this
discussion considers only the expected variation from the true weight
percentage as is found when applying the visual estimate technique to
determine the asbestos content in a bulk sample. The questions of
laboratory / analyst variability of such visual estimations are not
considered in this discussion.
RELATIONSHIPS BETWEEN AREA, VOLUME, AND WEIGHT PERCENTAGE
The principles of stereology are well documented ( see, for example,
"Quantitative Stereology," Underwood ) 1/ and will not be reiterated
here other than to state that in classical stereology, with the assumption
of a homogeneous distribution of phases within a solid, there is a direct
relationship between the volume fraction of a phase present in the solid
and the area fraction of that phase observed in a section taken through
the solid.
1/ Underwood, E.E., Quantitative Stereology, Addison-Wesley Publishing
Company, (1970)
That is to say, ( See original text )
where V(p) refers to the volume of the phase p present in the total volume
V, and A(p) represents the area projection of that phase in a planar
section of that solid of total area A. It should be noted that, for the
classical rules of stereology to apply in a transmission sample, the
section through the sample should be no thicker than the thickness or
diameter of the smallest component.
The point counting method has been criticized as a technique for observing
ACM because it does not take into consideration the fact that the asbestos
fibers present may be comparatively thin in the Z direction relative to
the other components present. Thus, if the volume percentage of asbestos
present is extrapolated from the projected area obtained by the point
counting technique, the volume percent of asbestos present will generally
be
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overestimated. As a result, the majority of laboratories analyzing ACM
have adopted a visual estimate which allows a certain amount of latitude
on the part of the microscopist to compensate for this thickness factor
when examining samples on the microscope slide. In most instances, the
visual estimation of asbestos content is made on a stereomicroscope with
which the microscopist may more readily estimate the third dimension.
Therefore, these estimates may be more readily extrapolated to a volume
percentage than those from the point count method. This technique is
essentially that which is proposed in the Interim American Society for
Testing and Materials ( ASTM ) Method. Currently, this method is being
considered for adoption by the National Institute of Standards and
Technology ( formerly the National Bureau of Standards ) as part of its
National Voluntary Laboratory Accreditation Program for the determination
of bulk asbestos in samples. This procedure will provide a measurement of
the asbestos in the sample which may be easily extrapolated to a volume
measurement.
CURRENTLY ACCEPTED EXPERIMENTAL METHOD
The currently accepted and most generally used methodology for the
identification of asbestos in building materials is compatible with both
the EPA interim method and the proposed ASTM method. Identification of
the asbestos type present using polarized light microscopy follows
accepted mineralogical practices. The quantification of the asbestos
content by visual estimation which is used is acceptable under the
amendment to the 1982 Regulation published in the Federal Register and is
substantially the same as that recommended in the ASTM method. It can be
seen that there is continuity of approach and direct correlation between
existing data and that which may be produced under the ASTM procedure.
While the visual estimation procedure is generally called the polarized
light microscopy method, the microscopist, in fact, uses a combination of
a low magnification stereo-microscope for preliminary examination and
estimation of the percentage of each fiber type, followed by a detailed
examination, using the polarized light microscope, of individual fibers
removed from the bulk material. The procedure has been outlined in a
draft to ASTM Committee D22.05 da/ted January 14, 1988---"Standard Method
of Testing for Asbestos-Containing Materials by Polarized Light
Microscopy."
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The method calls for bulk samples of building materials to be first
examined with a low power binocular microscope. By use of such a
microscope, the following observations can be made.
(1) The fibers can be detected.
(2) The homogeneity of the material can be determined.
(3) A preliminary identification of the fibers present can be
made.
(4) An estimate of fiber content by volume can be made.
(5) Fibers may be separated from the matrix for more detailed
analysis of subsamples with the polarized light microscope.
The method has been used, essentially in its present form, by the majority
of the participants in the EPA Bulk Sample Analysis Round Robin program.
These results indicate generally good reproducibility and good accuracy in
assessing the volume percentage of an asbestos mineral present in an
insulating material. The accuracy of such an analysis does not differ
very greatly from the expected inhomogeneity ( or homogeneity ) of the
material being analyzed ( manufacturers' specifications generally show a
range of composition for any one product which frequently was additionally
modified at the point of application ). In the ASTM technique,
quantification of asbestos content is discussed in the following terms:
"A quantitative estimate of the amount of asbestos present is most readily
obtained by visual comparison of the bulk sample in slide preparations to
other slide preparations and bulk samples with known amounts of asbestos
present in them." The document goes on to state that estimates of the
quantity of asbestos obtained by the method are neither volume nor weight
percent estimates, but are based on estimating the projected area, from
observation, of the distribution of particles over the two dimensional
surface of the glass slide, and on an observation of bulk material, and
that a basis for correcting to a weight or volume percent has not been
established. It is this latter aspect which will be discussed more fully
in this document. The ASTM method, however, provides for the percentage
to be first assessed from the bulk material as observed on the
stereomicroscope; it would seem, therefore, that this percentage is a
closer approximation to a volume percentage rather than a projected area
one. In addition the ASTM document states, "However, the error introduced
by assuming that the estimates are equivalent to weight percent is
probably within the precision of the visual estimate technique."
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CORRELATION OF WEIGHT PERCENTAGE WITH VOLUME PERCENTAGE
To correlate the weight fraction of the phase to its area or volume
fraction, it is necessary, as is pointed out in The EPA Test Method, that
the specific gravities and relative volume fractions of all the phases
present in the material are known. 1/
1/ Interim Method for The Determination of Asbestos in Bulk Insulation
Samples EPA 600/MA-82-020, December, 1982.
In any multicomponent consisting of n components, the weight percent of
component i is given by the following formula:
( See original text )
where P(i) is the specific gravity of the its component and V(i) is the
volume of the ith component. From this formula, it is clear that if the
volume percent and the density of each individual element in a bulk
insulation sample is known, it would be possible to obtain a weight
percentage for any particular component and specifically for those
components which are classed as asbestos. To determine this information
experimentally would, however, be extremely time consuming, requiring the
separate identification of each component in the matrix, determining its
specific gravity from reference tables, and applying these factors in the
formula.
An alternative conversion is therefore suggested in which an average
density is assumed for the nonasbestos matrix. In this model, the weight
percentage, W(a), of a particular asbestos type present at a volume
percentage of V(a) and having a density of P(a) present in a matrix of
density P(m) is given by the formula ( See original text ).
The density value ascribed to the nonasbestos matrix should be selected
taking into consideration the major constituents of the matrix but, for a
large range of commonly encountered inorganic matrices, a value of
2.5g/cm(3) may be assumed.
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PRACTICAL APPLICATION
These formulas will be applied to a range of samples. In applying formula
1 to determine actual weight percentages, published values for the several
components were used. To determine the weight percentages using the model
described by formula 2, a matrix density of 2.5 g/cm(3) was assumed.
Sample 1 Acoustical Material
Sample 1 is a sample of an acoustical material taken from an actual
ceiling
treatment.
Component Vol% Wt% ( Actual
)Wt% ( Model )
Chrysotile 15.0 15.1215.
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Glass Fiber 60.0 60.47
Carbonate 10.0 10.85
Cement 3.0 3.26
Clay 10.0 8.53
Gypsum 2.0 1.78
( Appendix 1 shows in detail how these weight percentages are calculated.
)
Sample 2 Round Robin Sample from Independent QC Ring
Sample 2 is from an independent round robin sample series in which four
laboratories participated. Reported values for amosite content were 30%,
30-40%, 45%, and 15-20%. The results from the second laboratory were
taken using the midpoint of the reported compositional range ( the
midpoint of the reported range for sample two was selected as most
probably representing the actual composition, lying between the reported
values of one and three, with four regarded as an outlier ).
Component Vol% Wt% ( Actual ) Wt% ( Model )
Amosite 35.0 38.82 41.55
Carbonate 35.0 32.94
Cement 30.0 28.24
Sample 3 Sample A EPA Bulk Sample Analysis Round Robin No.16
Sample 3 is sample A from the EPA Bulk Sample Analysis Round Robin series,
Round number 16.
Component Vol% 1/ Wt% ( Actual ) Wt% ( Model )
Amosite 3.0 4.04 3.92
Glass 87.0 92.29
Cellulose 10.0 3.67
1/ Volume percentage data for samples 3, 4, 5 and 6 are averages taken
from EPA Round Robin reports and would not normally be reported to
this level of significance.
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Sample 4 Sample D EPA Bulk Sample Analysis Round Robin No. 16
Component Vol% Wt% ( Actual ) Wt% ( Model )
Chrysotile 3.0 3.53 3.12
Clay 97.0 96.47
Sample 5 Sample D EPA Bulk Sample Analysis Round Robin No. 17
Sample 5 is Sample D from the EPA Bulk Sample Analysis Round Robin series,
Round Number 17.
Component Vol% Wt% ( Actual ) Wt% ( Model )
Chrysotile 2.9 2.56 3.01
Amosite 30.7 34.40 36.90
Cement 66.3 63.04
Sample 6 Sample A EPA Bulk Sample Analysis Round Robin No. 17
Sample 6 is Sample A from the EPA Bulk Sample Analysis Round Robin series,
Round Number 17.
Component Vol% Wt% ( Actual ) Wt% ( Model )
Crocidolite 97.0 97.52 97.78
Cement 3.0 2.48
It is clear from these data that, for most samples, the weight percentage
of the asbestos content is not substantially different from the volume
percentage which is normally reported and is within the expected variation
both of the analytical procedure and the sample homogeneity. A close
estimate of the weight percentage can be derived from a simple model which
assumes an average matrix density of 2.5 g/cm(3).
Plots of the difference between observed volume percentage and calculated
weight percentage for chrysotile, density 2.6 g/cm(3), ( Figure 1 ) and
crocidolite, density 3.4 g/cm(3), ( Figure 2 ) are shown calculated using
this model. The maximum deviation between the numerical values of weight
and volume percentage occurs near the 50% mark and, in the worst case
( crocidolite ), is less than 10%.
Exceptions will be found in samples whose matrices have significantly
higher or lower densities than the asbestos observed. Figure 3 presents
the extreme case of crocidolite ( density 3.4 g/cm(3) ) in a matrix of
cellulose with an estimated average density of 0.9 g/cm(3).
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The magnitude of the discrepancy in the critical region near 1% is shown
in figure 4. If only the volume percentage estimate is used, mass
percentages as high as 3% would be reported as below the definition of
ACM. In this case, a conversion to weight percentage is necessary if the
weight percentage is not to be grossly underestimated.
SAMPLE TREATMENT
Some samples, for example floor tiles, roofing felts, and some
cementitious products, may require special treatment ( ashing, solvent or
acid extraction ) to separate the asbestos from other materials in order
to facilitate analysis. In such cases, the resulting weight loss of the
sample due to treatment must be recorded and any volume to weight
percentage correction applied to the remaining material must be further
corrected to take this weight loss into consideration. For example, if
30% asbestos is detected in a sample after processing which resulted in a
25% weight loss, then the corrected asbestos content is 0.75 x 30 = 22.5%
CONCLUSIONS AND RECOMMENDATIONS
An assessment has been made of the validity of extrapolating to a weight
percentage the area or volume percentage of asbestos present in a sample
as determined by polarized light microscopy. A model has been presented
which can be applied to area or volume percentage data to give a more
accurate estimation of the weight percentage. With the exception of
asbestos-containing materials having a substantial density differential
between matrix and asbestos, generally low density cellulosic or perlitic
matrices, the magnitude of this correction is smaller than the expected
variability imposed by both the analytical variation and the inhomogeneity
of the sample. As a result, the weight percentage of asbestos present can
generally be equated with the observed area or volume percentage.
The following recommendations are made:
(1) For samples whose approximate average matrix density is close to
that of the asbestos species observed ( within 0.5 g/cm(3) ),
assume equivalence of weight and area or volume percentage.
(2) For samples whose approximate average matrix density differs from
that of the asbestos species present by more than 0.5 g/cm(3),
convert the observed area or volume percentage to weight
percentage using formula 2, using a matrix density consistent
with the principal matrix components.
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OMITTED TEXT: Table I; Table II; Table III; Table IV; Figure 1; Figure 2;
Figure 3; Figure 4; Appendix I
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