Point Counting

Point counting is a way of measuring how much asbestos is in a bulk sample under a polarized light microscope. The analyst looks at a set number of points (often 400) spread across the sample and records how many land on asbestos fibers; the share of points gives the percentage of asbestos.

Origin

Point counting is an established technique in petrography for estimating mineral proportions. EPA built it into its bulk-sample methods for asbestos, and under the asbestos NESHAP a sample whose asbestos content is visually estimated at less than 10 percent may be point counted to confirm whether it is above or below the 1 percent regulatory threshold.

Significance in Asbestos Context

Most labs report asbestos content by visual estimation, which is fast but subjective, especially near the 1 percent line that decides whether a material is regulated. Point counting is more objective but still has limits: very thin fibers below the resolution of the light microscope are missed, and results depend on sample preparation. Low or "trace" results in old surveys are sometimes used to argue that a product was effectively asbestos-free; a point count, or TEM analysis, can show otherwise.

Example Sentence

The visual estimate was "less than 1 percent," but a 400-point count found 2.5 percent chrysotile, making the floor tile a regulated asbestos-containing material.

Related Terms

Polarized Light Microscopy (PLM), Bulk Sample Analysis, EPA Method 600/R-93/116, PLM Detection Limit, Transmission Electron Microscopy (TEM), NESHAP

Notes

• If a lab report says "trace" or "less than 1 percent," ask whether a point count or TEM analysis was done.
• Keep the original lab reports, chain-of-custody forms and any remaining sample material.