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NIOSH 7400 Asbestos Fiber Counting
The ability to detect asbestos fibers using phase contrast microscopy is not
controlled by the resolution limit of the microscope.
fibers at least an order of magnitude below the resolution limit of the
microscope can be detected provided they are longer than
five micrometers. This is similar to the phenomena that allows us to see the
stars in the night sky, all of which are far below
the resolution limit of our eyes. When the HSE Test Slide is used to "calibrate
the microscope" it is the combination of the
microscope and the specific microscopist that are being calibrated. Using the
40X, NA 0.65 phase contrast objective some
individuals may be able to detect the lines of group 6 or even 7 on the HSE
slide when others may not detect fully the lines of
group 5. Microscopes of sufficiently poor quality may restrict the ability of
the microscopist to see the higher numbered groups
on the HSE slide but building inferior light microscopes is not a desirable
direction to drive the industry. That has already
happened in reducing the NA of 40X phase contrast objectives. NA's of 0.75 were
standard in the 1070's and an NA 0.85 was common.
With NIOSH 7400 the Microscope Industry was forced to reduce the NA of the 40X
objective to 0.65 in order to take advantage of
the large market in the asbestos remediation industy. It is not the light
microscope that is so limited as it is the wisdom of
those who try to reduce the quality of the analysis to some vague lowest common
limit and then try and compensate by requiring
that each field of view be examined for at least 30 seconds.
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