Recommended Procedures for Sampling and Counting Asbestos Fibers

Our plain-English summary of Recommended Procedures for Sampling and Counting Asbestos Fibers (1975).

The study

• Published: American Industrial Hygiene Association Journal, February 1975
• Type of article: Journal article
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This was not a study of workers or patients. It set out recommended procedures from the Joint AIHA-ACGIH Aerosol Hazards Evaluation Committee for sampling airborne asbestos fibers at work and counting them by microscope. The method was tied to the 1974 ACGIH Threshold Limit Value of 5 fibers longer than 5 micrometers per milliliter of air. Air was to be collected on membrane filters in protected holders. The two or three samples taken at each location were to be averaged. Breathing-zone samples should be taken close to the worker's nose over as much of the workday as possible, and calibrated battery-powered personal pumps were called satisfactory. Results were to be expressed as 8-hour time-weighted averages. Fibers were to be counted by phase-contrast microscopy at 400 to 450 times magnification. Kohler illumination and the Porton reticle were recommended, though other reticles meeting its specifications were allowed. Slides were to be counted within two days, or within two hours if an alternate medium was used. Pumps were to be recalibrated after every 24 to 36 hours of use. The committee noted that the precision of the method is limited.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Recommended methods document (sampling and counting procedure) from a professional committee; not a research study
• Exposure: Occupational exposure to airborne asbestos fibers. The 1974 ACGIH Threshold Limit Value was 5 fibers longer than 5 micrometers per milliliter of air. The article does not name specific fiber types.
• Main results:
  • Two or three samples should be collected at each sampling location, and the results averaged.
  • Breathing-zone samples should reflect the worker's entire normal workday and be collected as close to the worker's nose as possible.
  • Calibrated battery-powered personal pumps were described as satisfactory for breathing-zone sampling.
  • Exposure should be expressed as an 8-hour time-weighted average.
  • More than 150 particles per field, including non-fibrous background, may interfere with counting.
  • Counting should be done by microscope at a total magnification of 400 to 450 times.
  • The Kohler method of illumination was recommended.
  • The Porton reticle was recommended to define the counting field, but other reticles with suitable markings could be used.
  • Slides made with the preferred medium must be counted within two days, because fibers tend to migrate and crystals tend to grow.
  • Slides made with the alternate medium must be counted within two hours after preparation.
  • Personal sampling pumps should be recalibrated after each 24 to 36 hours of use.
  • Millipore field monitor holders preloaded with 37-mm Type AA filters were recommended. Membrane filters of other types and makers could not be recommended at the time.
  • Fiber concentration is calculated from four things: the number of fibers counted, the number of fields counted, the ratio of filter area to counting-field area, and the sample volume (sampling rate times time).
• Limitations the authors state:
  • The committee stated that the precision of the method is limited, so averaged results should be reported to the nearest significant figure.
  • Membrane filters of other types and manufacturers could not be recommended. The stated reasons were that they do not become optically clear in the mounting medium, or that they have too much particle background.

Topics

Medical terms in this study, explained in our Lexicon: Threshold Limit Value (TLV), Time-Weighted Average (TWA).

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