Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

elevant, then an exposure assessment limited
to the durable EMPs collected on a sample
would help to reduce possible analytical interferences
caused by other, nondurable EMPs
and may eliminate the need for mineral identification.
Another such area would be improvement
in EM particle identification techniques,
such as field emission SEM and the capability
to determine the elemental composition
of EMPs with an SEM equipped with EDS. A
third research area, largely conceptual at this
time, would be development of techniques to
more accurately reflect internal dose by quantifying
or estimating the number of particles
likely to be produced by splitting of asbestosfiber
bundles deposited in the lung.
Modifications of current analytical methods
and development of new analytical methods
will require an assessment of their implications
for worker health protection (e.g., how
do the results using improved or new methods
relate to human risk estimates based on counts
of EMPs made by PCM?). To ensure that relevant
toxicological parameters (e.g., dose, dimension,
durability, and physicochemical parameters)
are incorporated in the analysis and
measurement, any changes in analytical methods
should be made in concert with changes in
how asbestos fibers or other EMPs are defined.
3.6.1 Reduce Interoperator and
Interlaboratory Variability of
the Current Analytical Methods
Used for Asbestos Fibers
To ensure the validity of EMP counts on air
samples, it is important to ensure consistency
in EMP counts between and among analysts.
Microscopy counts of EMPs on air sample filters
are based on only a small percentage of
the surface area of the filters, and the counting
90
procedures require the analyst to make decisions
on whether each observed particle meets
specified criteria for counting. Interlaboratory
sample exchange programs have been shown to
be important for ensuring agreement in asbestos
fiber counts between laboratories [Crawford
et al. 1982]. Unfortunately, microscopists from
different laboratories are unlikely to view exactly
the same fields, and this alone accounts for
some of the observed variation in fiber counts
between microscopists. A mechanism to allow
recounts of fibers from the exact same field areas
would remove this variable and allow a better
assessment of the variation attributable to
microscopists in analyzing samples.
A technique is under development for improving
the accuracy of PCM-based fiber-counting
by allowing the same sample fields to be examined
by multiple microscopists or by the
same microscopist on different occasions [Pang
et al. 1984, 1989; Pang 2000]. The method involves
the deposition of an almost transparent
TEM grid onto the sample. Included with the
grid are coordinates that allow relocation of
each grid opening. Photomicrographs of typical
grid openings superimposed on chrysotile and
amosite samples have been published [Pang et
al. 1989]. Slides prepared in this manner have
been used in a Canadian proficiency test program
for many years. The main errors affecting
the counts of various types of fibers (e.g., chrysotile,
amosite, and SVF) have been evaluated
by examining large numbers of slides by large
numbers of participants in this program. A recently
developed scoring system for evaluating
the performance of microscopists is based on
errors compared with a reference value defined
for each slide by the laboratory in which they
were produced [Pang 2002]. A statistical analysis
of the intragroup precision in this study was
able to identify those analysts who were outliers
[Harper and Bartolucci 2003]. In a pilot study,
NIOSH CIB 62 • Asbestos