The nature, hazards and assessment of occupational exposure to airborne asbestos dust: a review

Our plain-English summary of The nature, hazards and assessment of occupational exposure to airborne asbestos dust: a review, by W.H. Walton (The Annals of Occupational Hygiene, 1982).

The study

• Author: W.H. Walton
• Journal: The Annals of Occupational Hygiene
• Published: 1982;25(2):117-247
• Type of article: Review
• Cited by: 52 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 003952).

Abstract

Hygiene measurement parameters for airborne asbestos are discussed from mineralogical and biological standpoints. It is concluded that at present there is no practicable alternative to the membrane filter/phase contrast optical microscope (MF/OM) method for routine use, despite its many limitations, but that some features, including the fibre size limits, should be revised.

On present evidence fibre counts appear to provide a better index of hazard than respirable mass concentration. Considerations of respirability and biological activity together suggest that the counts should be of fibres in the size range: length greater than 5-10 µm but less than 100 µm; width less than 1.5-2 µm (no minimum specified); aspect ratio greater than 5:1 to 10:1. Compared with present definitions (l>5 µm, d<3 µm, l/d>3:1) this reduces the maximum diameter and increases the minimum aspect ratio.

The main drawback of the MF/OM method is that fibres thinner than ca. 0.4-0.2 µm (depending on the microscope, method of mounting the sample and the observer's visual acuity) cannot be seen, although they contribute substantially to the number to an extent that varies with asbestos variety. This makes standardization of the method extremely difficult and is the main reason why different hygiene standards are needed for different asbestos varieties, especially between the amphiboles crocidolite and amosite. Further studies of the lung deposition of fibres thinner than 0.1-0.2 µm are needed.

In animal experiments chrysotile has generally been more damaging than the amphiboles, in contrast to human epidemiological experience. This is probably due to the lesser durability of chrysotile when retained in the body over long periods, as evidenced by the surprisingly small amounts of chrysotile compared with amphibole found in human lungs at autopsy despite the much greater industrial usage of chrysotile.

Many non-asbestiform amphiboles and other minerals give rise to elongated dust particles of aspect ratio>3:1. Discrimination between these and true asbestos particles is extremely difficult in individual cases, although overall size distributions differ. Recent codes for the MF/OM method indicate that discrimination is not to be attempted, but present practice in the U.K. is variable with marked effect on counting levels. Increasing the aspect ratio limit to >5:1 or preferably >10:1 would exclude many of the non-asbestiform particles but have relatively little effect on the true asbestos count. This supplements the biological evidence for a change from the present aspect ratio limit of 3:1 which was arbitrarily chosen by the ARC in 1958.

Another problem for which there is little scientific guidance is the evaluation of clumps and other entities. The use of aerodynamic size selectors in sampling together with an upper size limit of length+width <100 µm for 'thistle-down' particles should help. Other technical changes to the MF/OM method are suggested.

The novel suggestion is made that total fibre length per unit volume (m/m3) might reflect the hazard of asbestos exposure better than fibre number concentration and be easier to assess by man or machine—by counting crossing-points between fibres and a line drawn across the sample.

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

In plain English

This 1982 review examined the nature, hazards and measurement of workplace airborne asbestos dust. Only the abstract, introduction and contents were provided; this summary rests on the abstract. The author concluded that the membrane filter/phase contrast optical microscope method had no practicable alternative for routine use, despite many limitations, but that some features, including fibre size limits, should be revised. On present evidence, fibre counts seemed a better hazard index than respirable mass. The author suggested counting fibres longer than 5–10 micrometres but shorter than 100, thinner than 1.5–2 micrometres, with length-to-width ratio above 5:1 to 10:1 rather than the existing 3:1. The method's main drawback was that fibres thinner than about 0.4–0.2 micrometres could not be seen. That limit depended on the microscope, the mounting and the observer. Yet these fibres contributed substantially to the number, to an extent varying with asbestos variety. In animal experiments chrysotile had generally been more damaging than the amphiboles, unlike human epidemiological experience. The author considered this probably because chrysotile is less durable in the body. The author also suggested total fibre length per unit volume might better reflect hazard.

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: Review of the literature on the nature, hazards and measurement of occupational exposure to airborne asbestos dust, begun at the instigation of the Asbestos International Association
• Who was studied: No study population. The review covered published evidence on hygiene measurement of airborne asbestos at the workplace, from mineralogical and biological standpoints.
• Exposure: Airborne asbestos dust at the workplace, including chrysotile and the amphiboles crocidolite and amosite. The review also covered non-asbestiform amphiboles and other minerals that form elongated dust particles.
• Main results:
  • The author found no practicable alternative to the membrane filter/phase contrast optical microscope (MF/OM) method for routine use, despite its many limitations.
  • On present evidence, fibre counts appeared to provide a better index of hazard than respirable mass concentration.
  • The author suggested counting fibres longer than 5–10 micrometres but shorter than 100 micrometres, with width less than 1.5–2 micrometres (no minimum specified) and aspect ratio greater than 5:1 to 10:1. The existing definition was length greater than 5 micrometres, diameter less than 3 micrometres and aspect ratio greater than 3:1.
  • The main drawback of the MF/OM method was that fibres thinner than about 0.4–0.2 micrometres cannot be seen. The figure depends on the microscope, the sample mounting and the observer's eyesight. These thin fibres contribute substantially to the fibre number, to an extent that varies with asbestos variety.
  • In animal experiments chrysotile had generally been more damaging than the amphiboles, in contrast to human epidemiological experience. The author considered this probably due to the lesser durability of chrysotile retained in the body.
  • Raising the aspect ratio limit to more than 5:1, or preferably more than 10:1, would exclude many non-asbestiform particles but have relatively little effect on the true asbestos count. The existing 3:1 limit had been arbitrarily chosen by the ARC in 1958.
  • The author suggested that total fibre length per unit volume might reflect the hazard of asbestos exposure better than fibre number concentration.
• The authors' conclusion: The author concluded that the MF/OM method should remain in routine use but that some features, including the fibre size limits, should be revised.

Topics

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Aspect Ratio (Length-to-diameter ratio), Amphibole Group, Crocidolite, Amosite, Deposition, Chrysotile, Autopsy.

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• An electron microscopy study of the effect of asbestos dust on the lung — J.M. Davis (1963)
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