What Is PLM Asbestos Testing? How the Lab Reads Your Sample

What is PLM asbestos testing?

PLM is polarized light microscopy. It is the main lab method for finding asbestos in bulk building samples, such as a piece of floor tile, plaster or pipe wrap. An analyst prepares the sample and reads the optical traits of its fibers. The EPA’s method for bulk building materials calls PLM “the primary technique used for asbestos determination.”

If you hired a certified inspector in Missouri, the samples went to a lab, and your report probably says “PLM” next to each result. This post explains what that means and what to ask.

How does the lab actually test a sample?

The lab looks at your sample in two stages and reports a percent. The steps below follow the EPA method.

1. Stereomicroscope first. The method says “A preliminary visual examination using a simple stereomicroscope is mandatory for all samples.” The analyst checks the sample for layers, color, texture and visible fibers.
2. Sample preparation. The analyst picks suspect fibers and mounts them, and in some cases removes materials that interfere.
3. PLM identification. “A light microscope equipped with two polarizing filters is used to observe specific optical characteristics of a sample.” Each asbestos type has a recognizable set of traits under polarized light.
4. Quantity estimate. The analyst estimates the percent asbestos by visual area estimate, which the method calls “a semi-quantitative method that must relate back to calibration materials.” Point counting is a second semi-quantitative way. The method notes a “tendency for visual estimates to be high when compared to point-count data.”
5. Report. You get the material, fiber type and percent, and whether it exceeds the regulatory line.

Missouri DNR expects the lab results, plus the chain of custody for each sample, to be part of the inspection report. On a regulated abatement project, “A copy of the asbestos inspection report and laboratory analytical results must accompany the notification.”

What does “one percent” mean on my report?

One percent is the line between “asbestos-containing” and “not.” DNR defines its nonfriable categories as materials “containing more than one percent asbestos.” If the lab reports 2 percent chrysotile in your floor tile, that tile is asbestos-containing material. If it reports “none detected,” the lab saw no asbestos in the sample it analyzed.

Two notes on the line. First, the EPA method says asbestos “may be detected in concentrations of less than one percent by area if sufficient material is analyzed.” A trace result under one percent is possible. Second, how the lab handles a result near one percent matters. Ask whether it used point counting for low results.

Does one positive sample make the whole area positive?

Under the federal school rule, yes. EPA’s AHERA analysis rule says “A homogeneous area shall be determined to contain ACM based on a finding that the results of at least one sample collected from that area shows that asbestos is present in an amount greater than 1 percent.” That rule covers schools, not homes. It shows how the one percent line works in practice: one sample above it is enough to treat the whole matching material as asbestos-containing.

Where can PLM miss asbestos?

PLM is strong, but not perfect. The EPA method says it “can show significant bias leading to false negatives and false positives for certain types of materials.” Two weak spots matter for St. Louis homes.

Floor tile. The method states: “Since floor tiles may contain fibers too small to be resolved by PLM (< 0.25 µm in diameter), detection of those fibers by this method may not be possible.” Vinyl floor tile is on the EPA’s list of places asbestos may be found in homes, so this is no small footnote.

Glue and mastic. The method says “Many organic mastics are soluble in refractive index liquids and, unless removed prior to PLM examination, may affect the refractive index measurement of constituent materials.” The black adhesive under old tile can be hard to read by PLM alone.

For these cases the EPA lists backup tools: “x-ray diffraction (XRD), analytical electron microscopy (AEM), and gravimetry.” The method says these are needed when PLM cannot give a positive identification or a reasonable accuracy for the amount. If your tile or glue came back “none detected” and you still doubt it, ask the inspector whether a second method is worth it before you proceed with demolition. Mastic and asbestos-containing material are defined in our glossary.

Is PLM the same as the test for air?

No. PLM is for bulk samples, meaning a piece of material. Air monitoring is a separate test. The EPA says that after work is complete, an inspector or independent air testing contractor may monitor the air “to make sure there is no increase of asbestos fibers in the air.” If your project needs a clearance air test, your inspector will say which method applies.

How do I get a PLM test in St. Louis?

A mail-in kit also ends at a PLM lab, but you take the sample. Our post on test kits versus a professional survey compares the two. If you are unsure which materials to sample, start with where asbestos hides in old St. Louis houses.

Hire a certified inspector to take the samples. The EPA says “Taking samples yourself is not recommended,” and DNR says regulated structures “must be thoroughly inspected by a Missouri certified asbestos inspector.” The inspector sends the samples to a lab and gives you the report. Our asbestos survey page explains what you get, and the asbestos testing page lists where inspectors look.

What should I ask before I accept a PLM report?

Use this list.

• Which lab ran the samples? Which method? Ask whether it follows EPA/600/R-93/116.
• Which layers of each sample were tested? Tile and glue are different layers.
• Did the lab use visual estimate or point counting? What is the reporting limit?
• Will a negative result on tile or mastic be checked with a second method?
• Does the report list the amount, location, category and condition of each suspect material? DNR says it should.
• Is the chain of custody form attached?

Bring the report to your contractor. A good bid starts from lab results, not a guess. If you plan work above Missouri’s thresholds, a registered contractor does the removal, as covered under our asbestos abatement service.

Next step

If you have a PLM report you do not understand, or an older home to test, call Advanced Environmental Services at (314) 865-2662 or use our contact page. Ask about asbestos testing and we will explain the results in plain terms.