Levels of workplace exposure

Our plain-English summary of Levels of workplace exposure, by A.L. Rickards (The Annals of Occupational Hygiene, 1994).

The study

• Author: A.L. Rickards
• Journal: The Annals of Occupational Hygiene
• Published: 1994;38(4):469-75, 409
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A brief description was presented of the evolution of workplace exposure monitoring. Measurements began in the 1930s and developed through particle counting to the introduction of the membrane filter, light microscope method in the mid-1960s. It is important to be aware that most data from the past were collected in relation to workplace control. The membrane filter phase contrast microscope method provides an index of exposure. Fibres with diameters less than 0.2 microns are unlikely to be counted. The method is not capable of analysing individual fibre types and consequently all fibres are counted. There have been several changes to the detail of the membrane filter method. The possible effect of these changes were presented. The Asbestos International Association (AIA) has implemented a fibre counting performance trial and also an annual return of workplace monitoring data. A WHO project is in progress with the objective of harmonizing the membrane filter method for fibre counting of all fibre types using phase contrast microscopy.

In plain English

This paper was a brief review of how workplace asbestos exposure has been measured, read at a 1993 workshop on chrysotile asbestos. The author wrote that most past data were gathered to monitor dust control, not for epidemiology. The membrane filter method counts fibres under a light microscope. It cannot tell fibre types apart, and fibres thinner than about 0.15 micrometres are likely to be missed. The author listed changes to the method over time and stated that each change increased the number of fibres counted. A 1936 report had measured around 100 fibres per cubic centimetre in chrysotile textile work. Using what the author called a crude and approximate correction, this would be roughly 3200 fibres per millilitre by modern methods. In a 1992-1993 Asbestos International Association counting trial, 91 counters took part. The author described the Association's worker exposure surveys as crude but said good control was achieved in most member operations. The author concluded that control, not prohibition, should be the aim, especially in emerging nations.

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: Narrative review of asbestos workplace exposure measurement methods and data, presented as a workshop paper
• Who was studied: No human study population. The paper reviewed historical and present-day workplace asbestos measurements. It included a 1936 dataset (Middleton), an Asbestos International Association (AIA) fibre counting trial in 1992-1993 (27 countries, 54 laboratories, 91 counters), and AIA annual workplace dust surveys of its members since 1984.
• Exposure: Airborne asbestos fibres in workplaces, measured by methods including the thermal precipitator and the membrane filter phase-contrast microscope method. The historical data were for chrysotile ("white" asbestos) textile processing.
• Main results:
  • The membrane filter method has limits. Fibres thinner than 0.5 micrometres become progressively harder to detect, down to about 0.15 micrometres, below which they are likely to be undetected. The method gives an index of exposure but cannot analyse which fibre types are present.
  • The author listed changes in the membrane filter method (filter clearing, eyepiece graticule, counting rules, conversion from thermal precipitator) with approximate factors. The author stated that each of these changes increased the number of fibres counted.
  • Middleton's 1936 data indicated levels of around 100 fibres per cubic centimetre for chrysotile textile processing. After the author's crude and approximate correction for method changes, this corresponded to about 3200 fibres per millilitre by modern methods.
  • The author stated that counts in the hundreds of fibres match factory accounts of past dustiness better than values in the 10-100 range.
  • In the 1992-1993 AIA fibre counting trial, the 91 counters were classified as Group 1: 39 (42.9%), Group 2: 40 (44.0%) and Group 3: 12 (13.1%). In 1991-1992 the 94 counters were split 44 (46.8%), 29 (30.9%) and 21 (22.3%).
  • According to the AIA survey data, good control was achieved in most operations within AIA influence, though problems still existed.
• The authors' conclusion: The author concluded that the aim should be control rather than prohibition. Control should take priority over epidemiology and should be encouraged in 'emergent nations'. Users of exposure data must keep in mind how measurement methods changed and the shortcomings of the data.
• Limitations the authors state:
  • The factors given for method changes can only be approximate.
  • The correction applied to the 1936 data was crude and approximate.
  • The AIA survey data are crude, and their suitability for epidemiology is questionable.

Topics

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

Medical terms in this study, explained in our Lexicon: Membrane Filter Method, phase contrast microscopy (PCM).

Related studies

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• Workplace protection factors of HSE approved negative pressure full-facepiece dust respirators during asbestos stripping: preliminary findings — S.N. Tannahill, R.J. Willey, et al. (1990)
• Monitoring of airborne asbestos fibers in an urban ambient air of Shahryar City, Iran: levels, spatial distribution, seasonal variations, and health risk assessment — F. Taghizadeh, A.J. Jafari, et al. (2019)
• Fiber exposure reassessed with the new indices — T. Schneider, J. Skotte (1990)
• Exposure of UK industrial plumbers to asbestos, Part I: Monitoring of exposure using personal passive samplers — G. Burdett, D. Bard (2007)
• Spatio-seasonal variation of airborne asbestos concentration in urban areas of Shiraz, Iran — M.K. Fathi Fathabadi, A. Abdolahnejad, et al. (2017)

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