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

mineralogical identification of airborne EMPs
and (2) technical difficulties in generating test
aerosols of size-specific EMPs representative of
worker exposures so that sampling and analytical
methods can be tested and validated.
Improvements in exposure assessment methods
are needed to increase the accuracy of the
methods used to identify, differentiate, and count
EMPs captured in air-sampling filter media. Until
new analytical methods are developed and
validated, it will be necessary to investigate the
various proposals that have been made to modify
current analytical methods, such as those discussed
in Section 3.6.2, and additional modifications
to the current analytical methods.
Manual microscopy methods are labor- intensive
and error-prone. Automated analyses would
permit examination of larger sample fractions
and improve the accuracy of particle classification.
Developing a practical method that accurately
counts and sizes all EMPs could improve
risk assessments and exposure assessments done
in support of risk management. Automated
methods could reduce operator bias and interlaboratory
variability, providing more consistent
results for risk assessments.
Some barriers to improving current analytical
methods have been identified. Increasing the
optical resolution of PCM analysis may help
to increase counts of thinner asbestos fibers.
However, any increases in optical microscopy
resolution will not be sufficient to detect all asbestos
fibers. In addition, any improvements in
counting EMPs (e.g., an increase in the number
of EMPs observed and counted) will need to
be evaluated by comparing them with counts
made by the current PCM method. The use of
electron microscopy (EM) would improve the
capability to detect thin fibers and also provide
a means to identify many types of minerals.
NIOSH CIB 62 • Asbestos
However, the routine use of EM would
• require the development of standardized
analytical criteria for the identification of
various EMPs;
• require specialized experience in microscopy
and mineral identification;
• increase analytical costs; and
• potentially increase the lag time between
collecting the sample and obtaining results.
In some workplace situations, such as in construction,
increases in the time needed to analyze
samples and identify EMPs could potentially
delay the implementation of appropriate
control measures to reduce exposures.
Several potential sampling and analytical improvements
are currently under study. Some
of the studies are aimed at improving the accuracy
of current techniques used for monitoring
exposures to asbestos. One such study
is evaluating the use of thoracic samplers for
the collection of airborne EPs, and another
concerns the use of gridded coverslips for
PCM analyses. The proposed use of gridded
coverslips for sample evaluation can aid in the
counting of asbestos and other EMPs and can
provide a means for “recounting” fibers at specific
locations on a filter sample. Another study
is evaluating the proposed ASTM method to
determine whether interoperator variability of
differential counting (to distinguish fibers of
asbestos minerals from other EMPs) is within
an acceptable range.
Research to develop new methods is warranted.
One such research area could be development
of methods that would permit assessment of
the potential biopersistence (e.g., durability) of
EMPs collected on air sampling filters prior to
their evaluation by PCM or other microscopic
methods. If durability is deemed biologically
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