NIOSH Releases Revised Method for Asbestos Analysis
A revision of NIOSH’s method for analysis of asbestos and other fibers by phase-contrast microscopy (PCM) has been added to the NIOSH Manual of Analytical Methods. Dated June 14, 2019, the revised method presents alternative procedures for preparing and mounting asbestos samples on microscope slides for counting fibers under PCM, a technique that renders visible the changes in light waves as they pass through objects. The revised method also explains the optional but recommended use of special slide coverslips intended to improve laboratory microscopists’ accuracy when counting fibers.
The rules governing which entities seen through the microscope should count as asbestos fibers remain unchanged.
Martin Harper, who coauthored the revised method, emphasized that the changes give laboratories the flexibility to choose among combinations of treatments and mounting media.
“There’s been no major change [in the revised method] that would cause folks to have to rush out and make substantial changes to what they’re doing,” Harper said. “There are now choices and clarifications in the method that are intended to be helpful to people and to allow them the leeway to do some things a little differently.”
The revised method allows laboratories to use a dimethylformamide solution and a resin called Euparal instead of acetone and triacetin resin to mount sample material on slides. Because Euparal and triacetin have almost identical refractive indexes, Harper explained, the use of one or the other will not alter what analysts see through PCM.
The disadvantage of triacetin is that too much of it can cause the fibers on a slide to move, Harper said. Fixing the fibers in place is easier to achieve with Euparal.
Keeping the fibers from moving is necessary for analysts who follow the revised method’s instructions for using slide coverslips with relocatable fields. These coverslips are printed with a numbered and lettered grid that allows analysts to repeatedly return to the same field.
“If there’s any doubt about the counts that were made on a slide, you could go back to that slide and recheck those counts by looking at exactly the same fields that you looked at before,” Harper said. Using slides with relocatable fields “also allows you to do a study of the analysts over time by having them look back at the same fields over and over again.”
Harper is a coauthor of papers in the Journal of Environmental Monitoring that describe the results of round-robin studies among laboratories using relocatable grid slides for asbestos analysis. One finding of the papers is that the relocatable grid slides improved microscopists’ accuracy over time.
“We do see an improvement in their skills after they have been asked to look at grid openings with known fibers and record what they see, and then compare that to what has been seen by others,” Harper said. “With amphibole asbestos, the improvement is small and involves calibrating the accuracy of measuring minimum length, but for chrysotile, where the ability to find hard-to-see fibers is critical, the feedback from examining known samples can be very important for training.”
The method references the AIHA Industrial Hygiene Proficiency Analytical Testing program, or IHPAT, in relation to microscopists' ability to see chrysotile fibers. According to the method, the IHPAT scheme "provide(s) a very robust test of visual acuity and concentration" due to the presence on IHPAT chrysotile filters of a large number of narrow-width chrysotile fibers near the limit of detection.
The revised method also allows microscopists to use the new phase-shift test slide for calibration. Previously, the method stipulated the use of the HSE/NPL test slide, which is no longer available, Harper said.
A PDF of NIOSH method 7400, “Asbestos and Other Fibers by PCM,” is available from the agency’s website. The June 14, 2019, revision of NIOSH 7400 contains editorial corrections not present in an earlier version of the method dated April 29, 2019. NIOSH requests that users of the method ensure that they have downloaded the June 14 version (PDF).
For more information about asbestos fiber counting, see the article “Asbestos in Focus” in the August 2016 issue of The Synergist.