PCM cassettes manufactured by Zefon Inc. Image is a courtesy of Zefon Inc.
Phase Contrast Microscopy and specifically the method NIOSH 7400 is a common technique that is used for quantification of airborne fibers. This method is recommended by the Ontario Asbestos Regulation 278/05, Chapter 17, Paragraph 6, as a primary technique for the assessment and concentration of asbestos fibers during asbestos removal projects.
The method is relatively simple, and gives the construction industry a fast and affordable technique to measure the fibers suspended in the air. The method is very sensitive, and reports all fibers that are at least 5 micron long in the laboratory report.
The method is designed to measure ALL fibers regardless of their nature and chemistry. Typical fibers that are common for construction sites and can be detected by Phase Contrast Microscopy using NIOSH 7400 are:
- Asbestos fibres
- Refractory fibers
- Fibrous wood and paper dust (debris of cellulose dust)
- Mica dust (ground mica plates)
- Fibrous gypsum crystals
Chrysotile fibers under PCM
Some common fibers cause little or no interference to the NIOSH 7400 test because they are either too long or too heavy to be detected with high magnification microscopy:
- Hair, wool
- Glass insulation
- Cotton, jute, copra, plant fibers
- Man-made fibers (Nylon, Acrylic, carpet fibers etc.)
In conclusion: when you use the NIOSH 7400 method to quantify fibers in the air, the laboratory provides you with the Total Fibers. Often the results are discussed as the “worst case scenario”.
For example, if the sample was collected as a part of asbestos clearance procedure and the total fiber count is below 0.01 ff/cc, regardless if all fibers in the sample are asbestos or not, it still passed the test. If the concentration of fibers exceeds the regulated limit, you may need to use a more definitive testing procedure, such as NIOSH 7402. NIOSH 7402 employs Transition Electron Microscopy (TEM), in conjunction with Electron Diffraction. If your project requires the specifications of the NIOSH 7402 procedure, LCS Laboratory will be able to assist you with the test.
