Asbestos Analytical Methods - PLM, Point Counting, TEM, and PCM
When to use PLM, 400-point count, TEM, gravimetric reduction, and PCM for asbestos analysis. Detection limits and practical guidance.
Overview
Selecting the right analytical method for asbestos depends on what you are testing (bulk building material, air, soil, dust) and what regulatory program governs the work. This guide covers the methods most commonly used in environmental consulting and abatement work.
Bulk Sample Analysis (Building Materials)
PLM Visual Estimation
Method: EPA 600/R-93/116 - Method for the Determination of Asbestos in Bulk Building Materials
What it does: Uses polarized light microscopy to identify the type of asbestos (chrysotile, amosite, crocidolite, tremolite, actinolite, anthophyllite) and estimate the percentage of asbestos in the sample by visual area estimation.
Detection limit: Approximately 1% by visual estimation. Analysts examine a minimum of three slide preparations from each sample.
When to use: This is the standard first-line method for all bulk building material samples. It is the most economical and widely used technique. Results are reported as percent asbestos by type for each layer of the sample.
Limitations: Visual estimation is subjective, particularly at low concentrations. Studies have shown a pervasive tendency for laboratories to both overestimate and underestimate asbestos content at concentrations below 10%. At or near the 1% ACM threshold, the margin of error can be significant. For this reason, NESHAP requires point counting for samples with low asbestos concentrations.
400-Point Count
Method: EPA 600/R-93/116 (point counting procedure within the same method)
What it does: The analyst systematically examines a grid of 400 points across a minimum of eight slide preparations. Each point is classified as asbestos, non-asbestos, or empty. The ratio of asbestos points to total points is calculated to determine the asbestos percentage.
Detection limit: 0.25% (for 400 points)
When to use: Request a 400-point count when the PLM visual estimation result is less than 10% asbestos, particularly when the result is near the 1% threshold. Many consulting firms routinely request point counts for any sample estimated below 5% to get a more defensible result.
Under NESHAP, if the visual estimate is less than 10%, the owner or operator may either assume the material contains more than 1% asbestos and treat it as ACM, or require verification by point counting. If the point count result differs from the visual estimate, the point count result supersedes.
Key point: A sample in which no asbestos is detected by PLM does not need to be point counted. Point counting will not improve detection of asbestos where none was seen by visual examination - the detection limit for point counting is actually higher than for careful PLM examination, because point counting only identifies asbestos fibers that happen to fall directly under the crosshair.
Cost: More expensive than visual estimation due to the additional labor, but can save significant money in abatement costs if it demonstrates a material is below 1%.
1,000-Point Count
Method: Same as 400-point count but with 1,000 points
Detection limit: 0.1%
When to use: Primarily used in California, where some regulations require the lower detection limit. Occasionally used elsewhere for litigation or when a very precise determination is needed. Not commonly requested for routine building surveys in Ohio.
Gravimetric Reduction
Method: Used in conjunction with PLM or point counting for non-friable organically bound materials (NOB)
What it does: The sample is dried, weighed, then heated to burn off organic binders (up to 500 degrees C). The residue is treated with acid to dissolve calcium carbonate and other soluble minerals. The remaining material is dried, weighed, and analyzed by PLM or point count. The asbestos percentage is calculated using weight factors to account for the removed material.
When to use: For non-friable materials with high organic content where standard PLM is insufficient - primarily floor tile, vinyl sheet flooring, asphalt shingles, caulk, mastic, and similar materials. The organic matrix can obscure asbestos fibers under standard PLM analysis, making gravimetric reduction necessary for accurate identification.
Reference method: Chatfield method or New York State Department of Health Method 198.6 (for NOB samples analyzed by PLM)
Air Sample Analysis
Phase Contrast Microscopy (PCM)
Methods: NIOSH 7400 / OSHA ID-160
What it does: Air is drawn through a mixed-cellulose ester (MCE) filter cassette using a calibrated sampling pump. A wedge-shaped portion of the filter is cleared, mounted on a slide, and examined under phase contrast illumination at 400x magnification. The analyst counts all fibers meeting the counting criteria: length greater than or equal to 5 micrometers, with an aspect ratio of at least 3:1.
Units: Results reported as fibers per cubic centimeter (f/cc)
When to use: PCM is the OSHA-required method for compliance monitoring against the Permissible Exposure Limit (PEL) of 0.1 f/cc and the excursion limit of 1.0 f/cc. All personal exposure monitoring and most area monitoring during asbestos abatement and demolition uses PCM.
Critical limitation: PCM counts all fibers meeting size criteria - it cannot distinguish asbestos fibers from non-asbestos fibers (fiberglass, cellulose, mineral wool, etc.). In environments with non-asbestos fibrous materials, PCM results may overestimate actual asbestos exposure. Despite this limitation, PCM is the regulatory standard because it provides continuity with the historical epidemiological studies on which the PEL is based.
Detection limit: Approximately 0.01 f/cc depending on sample volume. Minimum countable is 5.5 fibers per 100 fields.
Transmission Electron Microscopy (TEM) for Air Samples
Methods: NIOSH 7402 / AHERA TEM (40 CFR 763 Appendix A)
What it does: Uses electron microscopy with energy dispersive X-ray analysis and selected area electron diffraction to positively identify asbestos fibers by morphology, crystalline structure, and elemental composition.
When to use:
- AHERA clearance - After asbestos abatement in schools, AHERA requires TEM analysis for clearance air samples (aggressive air sampling after the containment is cleaned). The clearance criterion is 0.01 fibers/cc by TEM.
- Fiber identification - When PCM results are elevated and there is a question of whether the fibers are actually asbestos or a non-asbestos interferent
- Litigation - When positive fiber identification is critical for legal purposes
Important: OSHA does not accept TEM as a substitute for PCM for compliance monitoring. TEM may be used to identify fibers, but PCM fiber counts determine compliance with the PEL.
Cost: Significantly more expensive than PCM, with longer turnaround times.
Asbestos in Soil
ASTM D7521
What it does: A multi-step method for quantifying asbestos in soil. The sample is dried, crushed, homogenized, and sieved. A 1,000 or 400-point count is then performed on the prepared sample.
Detection limits: 0.1% for 1,000-point count, 0.25% for 400-point count
When to use: When soil contamination with asbestos is suspected - from demolition debris, illegal dumping, naturally occurring asbestos (NOA), or historical industrial activity. Standard PLM methods (EPA 600/R-93/116) are designed for homogeneous building materials and do not perform well on heterogeneous soil samples.
Method Selection Summary
| Sample Type | Primary Method | When to Upgrade |
|---|---|---|
| Bulk building material (friable) | PLM visual estimation | Point count if visual estimate is <10% |
| Bulk building material (non-friable, organically bound) | Gravimetric reduction + PLM/point count | Standard for floor tile, mastic, roofing |
| Air (OSHA compliance) | PCM (NIOSH 7400 / OSHA ID-160) | TEM only for fiber identification, not compliance |
| Air (AHERA school clearance) | TEM (40 CFR 763 Appendix A) | Required - PCM not acceptable for AHERA clearance |
| Soil | ASTM D7521 | Use 1,000-point count for 0.1% detection limit |
| Dust/wipe | PLM or TEM depending on purpose | TEM for quantitative fiber counts |
Practical Guidance
When to Request Point Counting
Many firms establish an internal policy to request 400-point counts on any PLM result below a certain threshold - commonly 5% or less. This is a risk management decision. The cost difference between PLM visual estimation and a 400-point count is typically $20-50 per sample, which is negligible compared to the cost of incorrectly classifying a material as ACM and triggering unnecessary abatement.
The decision framework:
- PLM result: No asbestos detected - No point count needed. Three slides were examined in their entirety.
- PLM result: >10% asbestos - No point count needed. Material is clearly ACM.
- PLM result: 1-10% asbestos - Consider point counting, especially if the result is close to 1% and the classification will significantly impact project cost.
- PLM result: Trace or <1% - Point counting can confirm whether the material is truly below 1%. Remember that even below 1%, OSHA still regulates any amount of asbestos.
Laboratory Selection
Ensure the laboratory performing your analysis is accredited through the National Voluntary Laboratory Accreditation Program (NVLAP) for the applicable method. NVLAP accreditation is required for AHERA analyses and is the standard of care for all asbestos analytical work.
Resources
- EPA 600/R-93/116 - Method for the Determination of Asbestos in Bulk Building Materials
- NIOSH 7400 - Asbestos and Other Fibers by PCM
- NIOSH 7402 - Asbestos by TEM
- OSHA ID-160 - Asbestos in Air
- OSHA Asbestos - Evaluating and Controlling Exposure
Frequently Asked Questions
What is the difference between PLM and point counting?
Both use polarized light microscopy, but they differ in how asbestos content is quantified. Standard PLM (EPA 600/R-93/116) uses visual estimation by a trained analyst to estimate the percentage of asbestos in a bulk sample. Point counting is a more rigorous procedure within the same method where the analyst systematically examines a grid of points on the sample and categorizes each point as asbestos or non-asbestos. A 400-point count has a detection limit of 0.25%, compared to roughly 1% for visual estimation. The point count result supersedes the visual estimation result.
When should I request a 400-point count?
Request a point count when the PLM visual estimation result is less than 10% asbestos. At low concentrations, visual estimation is less reliable - analysts tend to both over- and under-estimate asbestos content near the 1% threshold. Many firms routinely request 400-point counts on any sample where the visual estimate is below a certain percentage (often 5% or less) to get a more defensible determination of whether the material is above or below the 1% ACM threshold.
What analytical method does OSHA require for air monitoring?
OSHA requires Phase Contrast Microscopy (PCM) for compliance monitoring of airborne asbestos fibers. Specifically, OSHA references its own method (OSHA ID-160) and NIOSH Method 7400. PCM counts all fibers meeting size criteria (length greater than 5 micrometers, aspect ratio of at least 3:1) - it cannot distinguish asbestos fibers from non-asbestos fibers. TEM can identify fibers but cannot be used to quantify airborne concentrations for OSHA compliance purposes.
What method is used for asbestos in soil?
Asbestos in soil is typically analyzed using ASTM D7521, which uses a combination of sample preparation (drying, sieving) and either PLM or TEM analysis. A 1,000-point count is commonly requested for soil samples to achieve a detection limit of 0.1%. Standard PLM bulk analysis methods (EPA 600/R-93/116) are designed for homogeneous building materials and are not well suited to heterogeneous soil samples.