WHAT DO POSITIVE NOA LAB RESULTS MEAN (PART I): CONSIDERATIONS WHEN ORDERING AND INTERPRETING POLARIZED LIGHT (PLM) AND PHASE CONTRAST MICROSCOPY (PCM) ANALYSES OF ROCK/SOIL AND AIR SAMPLES

Accurate identification and quantitation of naturally occurring asbestos (NOA) in rock/soil and air samples is fraught with difficulties, any of which can lead to serious error. Bench-top optical methods of analyzing NOA include: a) polarized light microscopy (PLM) and 2) phase contrast microscopy (PCM).

The PLM technique is applied to rock/soil materials following either the Cal ARB 435 or modified EPA 600/R-93/116 methods. Samples are ground, placed onto a microscope slide, and analyzed using standard optical mineralogy procedures. Analytical results can be affected by numerous sample preparation and analysis variables, as well as the expertise of the analyst. Unfortunately, PLM's limited magnification (~400X max) capability is unable to resolve small asbestos fibers, or practically determine asbestos concentrations below 0.25 %, which may necessitate using transmission electron microscopy (TEM).

The PCM technique is applied to air sample cassettes and is primarily used for meeting OSHA requirements concerning monitoring the asbestos exposure of workers. It is a pure fiber counting method and does not differentiate between asbestos and non-asbestos fibers. Like PLM, it also has limited ability to resolve small asbestos fibers.

Geologists must exercise considerable caution in relying upon PLM analytical results for NOA samples produced by commercial testing laboratories. Most testing labs offering CARB 435 sample analysis, do not follow method sample preparation procedures, resulting in widely varying results. PLM methods are being pushed to their limits with the recent lowering of accepted regulatory amounts of NOA to 0.25 % for general rock/soil. Unfortunately, the method does not address statistical uncertainties present at this low concentration. It also has not been established that commercial asbestos testing laboratories as a group, are up to the task of providing accurate, reproducible NOA PLM analytical results. Most labs are geared toward analyzing high quality, ore grade asbestos used in building materials, not weathered dirt. No certification, proficiency testing, or well defined test methods currently exist for NOA analysis. As a result, it is important to rigorously assess the credentials and capabilities of labs used for NOA analysis.