The method used by the U.S. Public Health Service for enumeration of asbestos dust on membrane filters

Our plain-English summary of The method used by the U.S. Public Health Service for enumeration of asbestos dust on membrane filters, by G.H. Edwards, J.R. Lynch (The Annals of Occupational Hygiene, 1968).

The study

• Authors: G.H. Edwards, J.R. Lynch
• Journal: The Annals of Occupational Hygiene
• Published: 1968;11(1):1-6
• Cited by: 19 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy.

Abstract

Samples of air-borne mineral dusts are collected by drawing air through cellulose ester membrane filters. Particles smaller than nominal pore size are retained. Varying sampling rate and filter holder controls the distribution of dust across the filter face. Samples are mounted on standard microscope slides using a high viscosity solution of membrane filter material in a mixture of diethyl oxalate and dimethyl phthalate to render the filter transparent. The mount is stable for 28-30 days, but after this period migration of the dust particles causes a loss of areal concentration. The grain and fiber dust particles, which lie on the surface of the filter, are counted with a phase contrast microscope, using 10X eye-pieces and a 4 mm "high-dry" objective. The field area is delineated with a Porton graticule. Count data are recorded in a form allowing data reduction and statistical analysis by electronic computer. The method, although not suitable to determine compliance with the threshold limit value for asbestos that was developed from a different enumeration method, is highly suited for developing basic data for a more precise recommendation of the limit for asbestos.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This 1968 paper from the U.S. Public Health Service described the development and testing of a laboratory method for counting asbestos dust collected on membrane filters. It was not a study of workers' health. Air was drawn through cellulose ester filters, including by battery-powered personal samplers worn by employees. The filters were made transparent with a thick mounting liquid of dissolved filter material in dimethyl phthalate and diethyl oxalate. Grains and fibers (particles at least 3 times longer than wide) were then counted in 20 random fields under a phase contrast microscope. When samples were mounted in the two liquids alone, counts fell to 75 per cent of their original values after one week and to 40 per cent by 30 days. The authors attributed this to dust particles migrating. With the thicker medium, no loss was detected in the first 30 days, but counts fell to 85 per cent by day 46. The authors recommended the method for research studies in dusty trades, but not for checking compliance with the asbestos Threshold Limit Value, which had been developed from a different counting method.

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: Laboratory methods development paper: development and standardization of a membrane filter sampling, mounting and counting method, with stability tests of mounted samples
• Who was studied: No human health population. Air-borne dust samples were collected in connection with the Public Health Service study of the asbestos products industry, including personal samples from employees. The number of samples is not given.
• Exposure: Asbestos dust (fiber types not named) collected on Millipore type AA membrane filters by personal samplers and alongside impinger samples. Filters were mounted in a medium of 0.1 g of membrane filter per ml of a 1:1 dimethyl phthalate and diethyl oxalate solution.
• Main results:
  • Samples mounted in 1:1 dimethyl phthalate and diethyl oxalate only gave 75 per cent of their original counts after one week.
  • With the same two-liquid mount, counts fell to 40 per cent of first-day values by 30 days and stayed there until counting stopped at 76 days.
  • Samples mounted in the high-viscosity medium showed no detectable loss in the first 30 days, but counts declined to 85 per cent of the original value by day 46.
  • Clearing of the filter took about 15 minutes. The authors reported that the medium softened the filter beneath the dust without washing it away or redistributing it.
  • Twenty random fields were counted per sample, and any particle with an aspect ratio of 3 or greater was counted as a fiber.
  • Face caps drilled with 6 additional 4 mm holes reduced the air velocity at the filter. The authors reported that this eliminated the impaction problem while still protecting the filter.
• The authors' conclusion: The authors concluded that the mounting medium clears the filter gently and gives adequate stability for 30 days. They recommended the method for research studies in other dusty trades, but not for application to Threshold Limit Values.
• Limitations the authors state:
  • The method is not suitable for determining compliance with the asbestos threshold limit value, which was developed from a different counting method.
  • Mounts were stable only for about 30 days. The cause of the particle migration was not determined.
  • With the single-opening face cap, larger particles impacted at the filter center can obscure part of the filter, especially in friction product plants.

Topics

Medical terms in this study, explained in our Lexicon: Threshold Limit Value (TLV).

Related studies

• Performance of membrane filters used for TEM analysis of asbestos — J.S. Webber, A.G. Czuhanich, et al. (2007)
• Not just a research method: If used with caution, can job-exposure matrices be a useful tool in the practice of occupational medicine and public health? — M. Fadel, B.A. Evanoff, et al. (2020)
• Comments on the Health Effects Institute-Asbestos Research (HEI-AR) report: "Asbestos in public and commercial buildings," with emphasis on risk assessment methods used — T.D. Sterling, C.W. Collett, et al. (1993)
• A comparison of impinger and membrane filter techniques for evaluating air samples in asbestos plants — H.E. Ayer, J.R. Lynch, et al. (1965)
• The influence of external aerodymanic factors on the measurement of the airborne concentration of asbestos fibres by the membrane filter method — A.M. Johnston, A.D. Jones, et al. (1982)
• Conversion of the asbestos fibre concentration recorded by means of the konimeter and the thermal precipitator to that expected by means of the membrane filter method — R.S. du Toit, T.C. Gilfillan (1979)

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