Testing Methodologies

Asbestos Point Count

Dixon Information analyzes bulk samples by stereo microscopy using phase contrast Polarized Light Microscopy (PLM) and dispersion staining whenever possible to determine the layer or layers that are asbestos containing before a point count is performed. Analysis by visual estimation is based on the Interim Method of Determination of Asbestos in Bulk Insulation Samples, EPA-40 CFR Appendix E to Subpart E of Part 763, with guidance from EPA-600/R-93/116: Method for the Determination of Asbestos in Bulk Building Materials as well as the EPA-600/M4-82-20 and OSHA ID-191 methods.

Dixon Information analyzes air samples by the NIOSH method 7400 issue 3 of April 29, 2019 for phase contrast microscopy and polarized light. Sample volume is based on field data. The detection limit for air samples is 5.5 fibers in 100 microscopic fields of view. To express the reporting limit in fibers per CC, divide 2.695 by the volume in liters. Limit of quantitation is based on sample volume.

Point count uses a grid, super imposed onto the field of view in the microscope ocular. If a particle is under the cross-section of two lines in the grid, a point is counted.

  • If it is asbestos, it counts as one asbestos point.
  • If it is a non-asbestos particle, it counts as one non-asbestos point. If both asbestos and non-asbestos can be distinguished under the point, it counts as one point for each. Empty points are not counted.
  • A minimum of 400 points total are counted on a minimum of eight slides.
  • The ratio is calculated to determine the asbestos-percentage.

Asbestos Gravimetry

Gravimetric reduction is a technique used to enhance the ability to accurately quantify asbestos in materials. The sample is first dried and then weighed. It is then subjected to temperatures not to exceed 500 degrees C. to burn off organic material. After cooling, further treatment with hydrochloric acid or other chemicals removes soluble substances in the sample. The remaining material is then washed with plain water, dried and weighed. A point count is performed, and weight factor calculations are used to determine the concentration of asbestos fibers.


  • Some particles cannot be removed with gravimetric procedures and can cause interference in the point count. 
  • Paint can occlude asbestos and produce a severely biased point count.
  • Some samples are not suited for point count analysis and should be rejected and or reported as biased. Other methods of analysis should be recommended.
  • Floor tile analysis by point count is rejected by our laboratory. Our lab uses tetrahydrofuran (THF) solvent to dissolve the plastic matrix and extract fibers from floor tile. Phase contrast microscopy is used to image these fibers as small as 0.2 micron.


  • If asbestos was found using visual estimation using PLM analysis, but not found in the point count, the result is reported as trace. This is a quantity that is above the limit of detection, but below the limit of quantification.
  • If the calculated point count results are less than 1% asbestos (asbestos points divided by the total points and then multiplied by the gravimetric residue) it is reported as less than 1% asbestos.
  • Above 1% asbestos, it is reported to the nearest percentage.
  • Asbestos point count result supersedes the PLM visual estimation result.

Quality Assurance

  • Dixon Information participates in the Bulk Asbestos Proficiency Test by NVLAP (101012), prepared by RTI International for participants in NIST’s National Voluntary Laboratory Accreditation Program for Bulk Analysis by PLM.
  • According to EPA method 600/R-93/116 section 3.4, we do not use “consensus” quantitation results. Instead, results are reported in weight percent with a 95% tolerance interval.