--- title: "What Is PLM Asbestos Testing? How the Lab Reads Your Sample" description: "PLM is the microscope method most labs use on bulk asbestos samples. See how it works, what the 1 percent line means, and where PLM can miss floor tile." url: https://www.aes-stl.com/blog/what-is-plm-asbestos-testing publisher: Advanced Environmental Services phone: +1-314-865-2662 quote: https://www.aes-stl.com/contact --- Practical: 6 min read, published June 10, 2026 # What Is PLM Asbestos Testing? How the Lab Reads Your Sample PLM is polarized light microscopy, the main lab method for finding asbestos in bulk building samples such as tile, plaster and pipe wrap. An analyst identifies fibers by their optical traits and estimates the percent. Missouri DNR counts material above one percent as asbestos-containing. PLM can miss very fine fibers, so ask about backup methods. [Request a Quote](https://www.aes-stl.com/contact)[(314) 865-2662](tel:+1-314-865-2662) ![Plastic containment sealed with orange tape inside a home during AES abatement work](https://www.aes-stl.com/_astro/AESI-067.BzNrt3Pt_Z28j9hJ.webp) ![Insulated boiler tanks and piping in a mechanical room before AES asbestos abatement](https://www.aes-stl.com/_astro/Asbestos-pg-3-photo-1.DrFsYFHB_1Dz3mY.webp) ![AES crew member in a protective suit and orange hard hat removing wall material from scaffolding](https://www.aes-stl.com/_astro/098.hgZ7AvUK_QfCE8.webp) ## 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. What the report says What it usually means What to ask Asbestos detected, above 1% Material is asbestos-containing Is it friable? What category? What does my planned work disturb? Asbestos detected, below 1% A trace, below the line DNR uses Was this by visual estimate or point count? Does my planned work still need care? None detected The lab saw no asbestos in this sample Was this a tile, glue or other material where PLM can miss fibers? ## 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. Visual estimate on your report What to ask Asbestos detected, above 1% Which material and how much of it will my work disturb? Asbestos detected, below 1% Was it point counted? Does my planned work still need care? None detected on tile or glue Was a second method considered? Mixed results across samples of the same material Which sample was positive, and where was it taken? ## 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](https://www.aes-stl.com/glossary/mastic) and [asbestos-containing material](https://www.aes-stl.com/glossary/asbestos-containing-material-acm) 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](https://www.aes-stl.com/glossary/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](https://www.aes-stl.com/blog/asbestos-test-kit-vs-professional-survey) compares the two. If you are unsure which materials to sample, start with [where asbestos hides in old St. Louis houses](https://www.aes-stl.com/blog/where-is-asbestos-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](https://www.aes-stl.com/asbestos-survey) page explains what you get, and the [asbestos testing](https://www.aes-stl.com/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](https://www.aes-stl.com/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](https://www.aes-stl.com/contact). Ask about [asbestos testing](https://www.aes-stl.com/asbestos-testing) and we will explain the results in plain terms. ## Frequently asked questions ### What does PLM stand for in asbestos testing? PLM stands for polarized light microscopy. A lab analyst views a prepared sample through a microscope with two polarizing filters and reads the optical traits of the fibers. The EPA method for bulk building materials calls PLM the primary technique for asbestos determination. ### What percent of asbestos counts as a positive result? Missouri DNR defines asbestos-containing material as material containing more than one percent asbestos. Your lab report should state the percent and fiber type found. EPA's method says asbestos may be detected at less than one percent by area if enough material is analyzed. ### Can PLM miss asbestos? Yes. The EPA method says PLM can show significant bias leading to false negatives and false positives for certain materials. It also says floor tiles may contain fibers too small for PLM to resolve, so those fibers may not be detected by this method. ### Is PLM used for air samples? No. PLM is a method for bulk samples, meaning a piece of material such as tile, plaster or insulation. Air monitoring after work is a separate test. If your project needs one, such as a clearance check, ask the inspector which method applies. ### What should I ask a lab about my PLM result? Ask the percent and fiber type found, the reporting limit, whether it used visual estimate or point counting, and whether a negative result on tile or glue will be checked with a second method. Keep the chain of custody form with the report. ## Terms on this page - [Asbestos-containing material (ACM)](https://www.aes-stl.com/glossary/asbestos-containing-material-acm) - [Clearance air test](https://www.aes-stl.com/glossary/clearance-air-test) - [Asbestos survey](https://www.aes-stl.com/glossary/asbestos-survey) - [Mastic](https://www.aes-stl.com/glossary/mastic) ## Sources 1. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), PLM as primary technique](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 2. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), how PLM works](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 3. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), stereomicroscope](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 4. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), detection limit](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 5. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), floor tile](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 6. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), mastics](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 7. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), visual estimate](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 8. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), point counts](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 9. [US EPA: Protect Your Family from Exposures to Asbestos (air monitoring)](https://www.epa.gov/asbestos/protect-your-family-exposures-asbestos) 10. [US EPA: Method for the Determination of Asbestos in Bulk Building Materials (EPA/600/R-93/116), other techniques](https://www.nist.gov/system/files/documents/nvlap/EPA-600-R-93-116.pdf) 11. [US EPA via eCFR: 40 CFR 763.87 Analysis (AHERA school rule), one sample](https://www.ecfr.gov/current/title-40/chapter-I/subchapter-R/part-763/subpart-E/section-763.87) 12. [Missouri DNR: Asbestos Requirements for Demolition and Renovation Projects (PUB2157), one percent](https://dnr.mo.gov/document-search/asbestos-requirements-demolition-renovation-projects-pub2157/pub2157) 13. [Missouri DNR: Asbestos Requirements for Demolition and Renovation Projects (PUB2157), lab analysis](https://dnr.mo.gov/document-search/asbestos-requirements-demolition-renovation-projects-pub2157/pub2157) 14. [Missouri DNR: Asbestos Requirements for Demolition and Renovation Projects (PUB2157), lab results with notice](https://dnr.mo.gov/document-search/asbestos-requirements-demolition-renovation-projects-pub2157/pub2157) Written by Advanced Environmental Services and checked against the sources above on June 10, 2026. Rules change, so check the linked page before you act. ## Keep reading - [Asbestos Test Kit vs Professional Survey: What You Actually GetA test kit tells you about one sample you cut yourself. A professional survey tells you about every suspect material the work will touch, and Missouri requires a certified inspector before work on regulated structures. The EPA says taking samples yourself is not recommended, and that sampling done wrong can be more hazardous than leaving the material alone.](https://www.aes-stl.com/blog/asbestos-test-kit-vs-professional-survey) - [Where Is Asbestos in Old St. Louis Houses? A Job-by-Job CheckAsbestos in an old house usually sits in floor tile and glue, pipe and furnace wrap, textured ceilings and patching compound, siding and roofing shingles, and pour-in vermiculite insulation. About 87% of St. Louis city housing units were built before 1980. You cannot spot it by sight, so match each planned job to the material it would disturb, then test.](https://www.aes-stl.com/blog/where-is-asbestos-in-old-st-louis-houses) - [Asbestos in older St. Louis homes: where it hides and what to doAsbestos can hide in floor tile, floor adhesive, siding, roof shingles, textured walls, pipe wrap, furnace parts and attic insulation. About 87% of St. Louis city housing units and 68% of St. Louis County units were built before 1980. The only way to know is a lab test from a certified inspector.](https://www.aes-stl.com/guides/asbestos-in-older-st-louis-homes) - [Testing an older St. Louis home: which test applies when you buy, sell or renovateWhich test applies depends on what you are doing. Selling or buying a pre-1978 home triggers federal lead disclosure. Renovating or demolishing a regulated building triggers an asbestos inspection and notice. The City of St. Louis requires an occupancy inspection on sales. No federal standard sets a mold limit, so a mold test is your choice.](https://www.aes-stl.com/guides/testing-an-older-st-louis-home) - [How to read a PFAS lab report: ppt, PFOA, PFOS and limitsA PFAS lab report lists each compound it tested for, such as PFOA and PFOS, with a result in parts per trillion (ppt), also written ng/L. Compare PFOA and PFOS to EPA's 4.0 ppt drinking water limits. EPA decides compliance by a running annual average, so one result alone does not prove a violation.](https://www.aes-stl.com/blog/how-to-read-a-pfas-lab-report) - [What drives asbestos removal costAsbestos removal cost depends mainly on how much material there is, where it sits, how hard it is to reach, and what testing, notice fees and disposal the job needs. Angi's 2026 national average is $2,260. AES publishes no asbestos prices and quotes each job after a survey.](https://www.aes-stl.com/guides/asbestos-removal-cost) ## Request your custom environmental remediation estimate Contact us for a free environmental remediation or restoration estimate and a personalized solution to your problems. [Or call (314) 865-2662](tel:+1-314-865-2662)