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

the pooled relative standard deviations, without
the outliers, met the requirements for an unbiased
air-sampling method. Further study is
needed to validate these findings and to identify
other techniques that can reduce interlaboratory
and interoperator variability in counting
asbestos fibers and other EMPs by PCM.
Reference slides made from proficiency test
filters from the American Industrial Hygiene
Association (AIHA) have been created and circulated
to laboratories and individual microscopists
recruited from AIHA laboratory quality
programs [Pang and Harper 2008; Harper
et al. 2009]. The results illustrate an improved
discrimination of fiber counts when the proficiency
test materials have a more controlled
composition. These reference slides have also
been evaluated in Japan, the United Kingdom,
and elsewhere in Europe. Further research will
be useful in determining the value of these
slides for training purposes.
3.6.2 Develop Analytical Methods
with Improved Sensitivity
to Visualize Thinner EMPs
to Ensure a More Complete
Evaluation of Airborne
Exposures
Most PCMs can visualize EMPs with widths
>0.25 µm, which is the approximate lower
resolution limit when the microscope is operated
at a magnification of 400× and calibrated
to NIOSH 7400 specifications [NIOSH 1994a].
However, higher-end optical microscopes can
resolve thinner widths, and, for crocidolite,
they may resolve widths as thin as 0.1 µm.
Improvement in the optical resolution may be
possible with use of an oil-immersion 100× objective
with a numerical aperture of 1.49. Also,
NIOSH CIB 62 • Asbestos
the use of 15× eyepiece oculars would help improve
the visibility of small particles and thin
EMPs in samples. However, using oil immersion
has several drawbacks. When exposed to
air for more than a few hours, the oil on the
slide dries and its optical properties change.
Also, the oil cannot be wiped off because the
cover slip is likely to be moved and thus ruin
the sample. For these reasons, using oil immersion
does not permit recounts or further analysis
for quality control purposes and is not an
attractive alternative.
Other methods may also allow for increased
resolution with optical microscopes. Anecdotal
information on the use of PCM with dark-
medium (DM) objectives, presented at a meeting
in November 2007, suggests that analysts
using DM objectives could resolve more blocks
of the Health and Safety Executive/National
Physical Laboratory (HSL/ULO) test slide ‡‡
than are allowable for the method and produced
higher counts of chrysotile fibers than
expected [Harper et al. 2009]. The implication
is that using DM objectives can resolve thinner
chrysotile fibers than the accepted method. This
methodology should be explored further to determine
its resolution and potential application
in asbestos exposure assessment.
As stated previously, because risk estimates for
workers exposed to asbestos fibers have been
‡‡ The HSE/NPL Mark II or HSL/ULO Mark III Phase
Shift Test Slide checks or standardizes the visual detection
limits of the PCM. The HSL/ULO Test Slide
consists of a conventional glass microscope slide with
seven sets of parallel line pairs of decreasing widths.
The microscope must be able to resolve the blocks of
lines in accordance with the certificate accompanying
the slide. Only slides where at least one block of lines is
intended to be invisible should be used. Microscopes
that resolve fewer or greater numbers of blocks than
stated on the certificate cannot be used in NIOSH
Method 7400.
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