A comparison of historic asbestos measurements using a thermal precipitator with the membrane filter-phase contrast microscopy method

Our plain-English summary of A comparison of historic asbestos measurements using a thermal precipitator with the membrane filter-phase contrast microscopy method, by G. Burdett (The Annals of Occupational Hygiene, 1998).

The study

• Author: G. Burdett
• Journal: The Annals of Occupational Hygiene
• Published: 1998;42(1):21-31
• Cited by: 4 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The published results and analytical methods used in a 1938 survey of the spinning area in a UK crocidolite asbestos factory have been described and re-interpreted, by comparing the method used with the current membrane filter-phase contrast microscopy (MF-PCM) method for asbestos. By good fortune, most of the original microscope, and thermal precipitator sampling heads similar to those used for sampling were available, as well as guidance from the factory inspector who collected and analysed the original samples. A textile grade crocidolite asbestos was used to generate a fibrous dust cloud whose size distribution was characterised by scanning and transmission electron microscopy and found to give a close approximation to the fibre size distributions monitored in 1938. Samples taken over the same sampling time, but at lower concentrations than originally sampled, showed that the thermal precipitator-oil immersion microscopy (TP-OI) method used at x 2000 magnification, gave higher > 5 microns long fibre counts by a factor of between 3 and 4, than the current MF-PCM method. The differences in performance could be explained by the superior resolving power of the TP-OI method for fine crocidolite fibres. Exposures of airborne asbestos fibres in the spinning area were found to be equivalent to about 20 f ml-1 for personal samples and 10 f ml-1 for area samples, but due to the high levels of ventilation on the day the samples were collected, the average levels throughout the year may have been somewhat higher.

In plain English

This laboratory study re-examined dust measurements taken in 1938 in the spinning area of the J.W. Roberts Ltd crocidolite asbestos factory in Leeds. In 1938, personal and static samples were collected with thermal precipitators and counted under an oil-immersion microscope at about x2000 (TP-OI). To relate those counts to the current membrane filter and phase contrast microscopy method (MF-PCM), the author used the original microscope and similar sampling heads. The author generated a textile grade crocidolite dust cloud and sampled it with both methods side by side in five runs. In the laboratory dust, fibres longer than 5 micrometres had sizes very similar to those reported in 1938. The two thermal precipitator heads averaged 11.1 and 10.8 fibres per ml by TP-OI. Membrane filters counted by PCM averaged 3.36 fibres per ml, about 30% of the TP-OI count. The author concluded that the 1938 levels were equivalent to about 20 fibres per ml for personal samples and 10 for static samples under the current method. Windows were open that day, so levels in colder months may have been higher.

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 side-by-side comparison of a historic sampling and counting method with the current method, used to re-interpret published 1938 factory measurements
• Who was studied: Original 1938 survey: one personal and one static thermal precipitator sample at each of three asbestos flyer spinning frames in the spinning area of the J.W. Roberts Ltd factory, Armley, Leeds, UK, on 20 July 1938. Laboratory part: five repeat sampling runs in a dust chamber.
• Exposure: Crocidolite asbestos. In 1938 the frames handled a single asbestos thread, an asbestos and cotton thread, or two asbestos threads. The laboratory tests used textile grade crocidolite at 4 to 16 fibres per ml (measured by TP-OI). Fibres counted were those longer than 5 micrometres.
• Main results:
  • In the laboratory runs, the two thermal precipitator heads gave average TP-OI counts of 11.1 and 10.8 fibres per ml.
  • When the same thermal precipitator deposits were counted with x500 phase contrast optics instead of x2000 oil immersion, only 45% of the fibres were counted (averages 4.8 and 5.0 fibres per ml).
  • Membrane filter samples counted by PCM averaged 3.36 fibres per ml, about 30% of the TP-OI count. The TP-OI method gave counts of fibres longer than 5 micrometres that were higher by a factor of 3 to 4.
  • Fibres longer than 5 micrometres in the laboratory dust had size distributions very similar to those Goodall reported in 1938.
  • The author found that the differences between the counting methods could be explained almost entirely by the theoretical performance of the optics, which resolve fine crocidolite fibres differently.
• The authors' conclusion: The author concluded that the laboratory relationship gives a reasonable basis for converting Goodall's 1938 counts to present-day terms. Under the current method (MDHS 39/3), the 1938 levels would have been reported about three times lower. The 1930 'dust datum' for spinning areas was equivalent to a personal exposure of about 20 fibres per ml and a static exposure of about 10 fibres per ml.
• Limitations the authors state:
  • The 1938 samples were taken with all the windows open. Ventilation would be lower in colder months, so the figure represents only a seasonal estimate.
  • Laboratory concentrations were lower than those originally measured, because membrane filters could become overloaded at higher levels.
  • The estimates of how the two microscopes perform were described as only approximate.
  • Fibre counting depends on the person counting and varies between counters and laboratories.
  • The 1938 thermal precipitator inlet might have been clogged by large fibre clumps, which would lower the counts. The author judged this unlikely.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Crocidolite.

Medical terms in this study, explained in our Lexicon: Thermal Precipitator, phase contrast microscopy (PCM), Crocidolite, Transmission Electron Microscopy (TEM), Personal Breathing Zone Sample.

Related studies

• 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)
• A routine for the control of the performance of microscopists evaluating airborne asbestos fibre samples on membrane filters by phase contrast microscopy — J.R. Hoyes, R. Clayton (1980)
• 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)
• Estimates of historical exposures by phase contrast and transmission electron microscopy in North Carolina USA asbestos textile plants — J.M. Dement, D. Myers, et al. (2009)
• Comparison of a fibrous aerosol monitor (FAM) with the membrane filter method for measuring airborne asbestos concentrations — P.J. Iles, T. Shenton-Taylor (1986)
• A comparison of fibre counting stopping rules in the application of the membrane filter method — N.P. Crawford, W.M. Maclaren, et al. (1987)

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