Laboratory Analysis Methods and Interpretation of Results
Candidates must know the principal methods, what each can and cannot see, and how to read a report critically.
Polarized light microscopy (PLM) is the workhorse for bulk samples. It uses refractive index, birefringence, dispersion staining, and morphology to identify mineral fibers. Standard PLM has a practical detection limit around 1% by weight; below that, fibers may be present but not reliably quantified. Point counting improves the estimate of low-percentage asbestos. PLM cannot reliably resolve very thin fibers or distinguish some amphiboles without careful optical work, and it does not measure airborne concentration.
Transmission electron microscopy (TEM) offers higher magnification, electron diffraction, and energy-dispersive X-ray analysis, so it can identify fibers at much lower concentrations and is used for air samples, clearance, and difficult bulk questions. Phase contrast microscopy (PCM), used for air monitoring, counts fibers but cannot tell asbestos from other fibers. X-ray diffraction (XRD) can quantify bulk mineral content but is less common for routine bulk work.
Reading a report: check the method, the detection limit, whether point counting was used, the analyst's description of layers, and whether the result is reported as a percentage by weight or as a qualitative 'detected/not detected.' 'None detected' is not the same as 'zero'; it means below the method's reporting limit. Traps: treating a <1% PLM result as proof of no asbestos; ignoring that multiple layers were analyzed separately; and assuming a lab result transfers to a different homogeneous area.
Rule of thumb: match the method to the question. Bulk identification = PLM (point count when low percentages matter); low-level or confirmation questions = TEM; airborne fiber counts = PCM, with TEM for asbestos-specific confirmation.