Within the realm of asbestos sampling, there are several analytical techniques. The most general way to test for asbestos-containing material is through Phase Contrast Microscopy (PCM), Transmission Electron Microscopy (TEM), and Polarized Light Microscopy (PLM).
The first two methods, PCM and TEM are used to analyze air samples.
The third method: PLM, is used to detect asbestos in a “bulk” sample such as a piece of floor tile, wall texture, hard plaster, pipe insulation, etc. This discussion will be limited to bulk analysis using the PLM method.
Why Use Point Counting When Testing for Asbestos?
When a bulk sample of material is found to contain 10% asbestos or less, the sample may be presumed to contain greater than 1% asbestos and treated as such or the sample can be further analyzed via Point Counting.
This additional step can be employed to more accurately determine the percent of asbestos that is in the material being sampled. Additionally, samples analyzed via PLM frequently are determined to contain “trace” amounts of asbestos usually written as <1% asbestos. Any sample determined to contain <1% asbestos must be point counted to prove it contains less than 1%. Otherwise, it assumed to be positive regardless of the PLM results. Non-friable materials such as floor tile, roofing materials, and their associated mastic binders are not good candidates for point count analysis, they are better analyzed by gravimetric methods.
Typical materials that are candidates for point count analysis include drywall-joint compound, ceiling or wall texture, hard plaster, etc. The main purpose to point count material is to find out if it contains less than 1% asbestos since anything 1% or less is legally considered “non-asbestos containing”. If point counting is not used, many materials could be mistakenly considered to contain more than 1% asbestos. This mistaken assumption could lead to an expensive asbestos remediation project that could otherwise be avoided.
Using the Point Counting Procedure
For each layer of material to be point counted, eight mounts are made by dispersing 8 pinches of a sample in a suitable fluid. A reticule is placed on the eyepiece of the microscope that superimposes a grid of points over the field of view. Fifty non-empty points are examined for each mount, yielding 400 points, some of which would be identified as asbestos and the rest as non-asbestos material. A simple calculation gives the percentage of asbestos; 4 points in 400 would be 1.0%.