Once a sample of a suspected asbestos-containing material is taken, it must be sent to an accredited laboratory for testing. Workers at accredited labs will be trained to recognize asbestos and asbestiform fibers in the material, should they be present. The National Institute for Standards and Technology keeps and regularly updates a list of these labs, some of which use Polarized Light Microscopy tests and some of which use Transmission Electron Microscopy.
Polarized Light Microscopy
As the name suggests, Polarized Light Microscopy (PLM) makes use of polarized light, which is light whose waves vibrate in only one direction. It is used frequently in optical mineralogy, which classifies minerals by their optical properties, or how they react to light. Asbestiform minerals have a property called anisotropy, meaning that their atoms are arrayed in a regular pattern in two or three main directions. PLM can identify this property, though it requires sophisticated knowledge and analysis on the part of the microscopist. This method is most commonly used to analyze bulk building materials, as it can screen a large number of samples relatively quickly.
Transmission Electron Microscopy
However, visible light has its limits when it comes to creating images of very, very small things. The most sophisticated available method is Transmission Electron Microscopy (TEM), in which a thin sample of the material is bombarded with electrons, some of which pass through the material and some of which are reflected back. The result is an extremely detailed picture of the sample. Three properties are needed to make a positive identification of asbestos: morphology, chemistry, and structure. The last of these, structure, can only be determined with certainty using TEM.
Phase Contrast Microscopy
The above two methods are appropriate for testing “bulk,” or solid, samples of asbestos-containing materials, but a different process is used for initial testing of air samples. This is known as Phase Contrast Microscopy (PCM) and measures the concentration of fibers in a cubic centimeter of air. PCM is quicker and cheaper than other methods, but will not say for sure whether asbestos is present, only fibrous material in general. If fibers smaller than 0.25 microns are identified, the sample will generally be sent for TEM analysis. Because of its precision, TEM is the current standard for identifying asbestos in air samples.