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Photographic gallery.  Thousands of particles under the microscope.

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.

HSE/NPL Phase Contrast Test Slide

Phase Contrast Test Slide

10X Objective, Darkfield Illumination 10X 0.25 NA Darkfield Image of Phase Contrast Test Slide
40X Phase Contrast Lines 4 through 6 40X 0.65 NA Phase Contrast Image of Phase Contrast Test Slide
40X Phase Contrast Lines 5 through 7 40X 0.65 NA Phase Contrast Image of HSE Groups 5, 6, and 7

Importance of Phase Plate Alignment

The Phase Anulus and the Phase Filter

Field 1 with Improper Phase Alignment Field 1 with Proper Phase Alignment

Field 2 with Improper Phase Alignment Field 2 with Proper Phase Alignment

Asbestos Fibers and Visibility Through the Light Microscope

Much has been made of the need for transmission electron microscopy (TEM) "because the light microscope can't see asbestos fibers thinner than 0.25 micrometers". This quote is in error on many counts, but is a common misconception. First, it is not the microscope that "sees", but rather the human eye establishes the detection limit. If the light source is not "on" the resolution limit for the microscope has not changed but the eye won't see anything. Second, the ability to "see" (detect) an object is not dependent on resolution. This point has been made for the general case above but will be made again here in the specific case of asbestos fibers. Third, after over thirty years of diligent searching there has been no evidence that asbestos fibers shorter than five micrometers cause any health effect more detrimental than nuisance dust. Fresh crystalline silica is more detrimental in this respect. That challenges the "need" for an instrument that can "see" particles that are not apparently detrimental as a replacement for light microscopy. We will continue monitoring with the TEM because it is required in many governement and industrial regulations, but it is not a "superior" method in the case of hazardous asbestos detection or identification at sites of asbestos removal projects.

Cleared Filter Containing Amosite, Phase Contrast 40X Objective 10X 0.25 NA Darkfield Image of Cleared Filter Containing Amosite Cleared Filter Containing Amosite, Phase Contrast 40X Objective 10X 0.25 NA Darkfield Image of Cleared Filter Containing Amosite Cleared Filter Containing Chrysotile, Phase Contrast 40X Objective 10X 0.25 NA Darkfield Image of Cleared Filter Containing Chrysotile 40X 0.65 NA Phase Contrast Image of HSE Groups 5, 6, and 7