Sampling Methods

Our plain-English summary of Sampling Methods, by S. Holmes (1973).

The study

• Author: S. Holmes
• Published: BIOLOGICAL EFFECTS OF ASBESTOS (J.C. WAGNER, EDITOR), LYON: IARC SCIENTIFIC PUBLICATIONS, VOLUME 8, PP. 109-112 (1973)
• Type of article: Conference paper
• 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 paper by S. Holmes was a review of methods for sampling and measuring airborne asbestos dust. It was not a study of people. The author explained that fibre shape matters. Straight amphibole fibres (amosite, crocidolite and anthophyllite) behave differently in the airways from "curly" chrysotile, so pre-filters that sort particles only by how fast they fall do not account for fibre length or shape. For workplaces, the paper described the membrane filter method, in which fibres are counted under a phase contrast microscope. For this count, a fibre was defined as at least 5 micrometres long and at least three times as long as it is wide. The author noted that very fine fibres may not be seen this way. In general outdoor air, asbestos, if present at all, was found in amounts too small for standard instruments to measure. Very large volumes of air were therefore collected and analysed by X-ray diffraction or electron microscopy. The author concluded that membrane filter counting was still the best system for monitoring asbestos at work, and that mass estimation was the only available method for general environmental air.

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 dust sampling and analysis methods for asbestos (no human or animal subjects)
• Who was studied: No people were studied. The paper discusses sampling of asbestos dust in factory (occupational) atmospheres and in general environmental air, including a UK asbestos industry programme of measurements near factories and in selected urban and rural locations.
• Exposure: Airborne asbestos dust: chrysotile and the amphiboles amosite, crocidolite and anthophyllite
• Main results:
  • In the membrane filter method, a particle counted as a fibre if its length was at least three times its diameter and it was at least 5 micrometres long. In the UK, an upper diameter limit of 3 micrometres was customary.
  • The 5 micrometre lower length limit was chosen somewhat arbitrarily in the early development of the method.
  • Earlier samplers were discarded. Instruments based on impingement tended to break up fibre bundles, and thermal precipitators (except the long-running precipitator) appeared to be very inefficient collectors of longer fibres.
  • The Royco Automatic Particle Counter was used by the UK industry for routine measurements where the dust was almost entirely asbestos. The author stated it was not suitable for biological studies, which need personal sampling.
  • Among instruments examined for automatically counting fibres on microscope slides, the Quantimet 720 was described as showing particular promise.
  • The membrane filter method can be criticised because very fine asbestos fibres may not be seen in the light microscope.
  • Among mass-based methods, X-ray diffraction was described as the most promising. It can measure as little as 10 micrograms of chrysotile; the rest of this sentence is damaged in the text.
  • In UK measurements of general air, asbestos, if present at all, was in such small quantities that standard sampling instruments and methods were completely inadequate. A high-speed electrostatic sampler was used to collect samples of a million litres (1000 cubic metres).
  • In many of these large-volume samples, no asbestos was detected by X-ray diffraction. Those samples were treated by ultrasound and the asbestos mass estimated by transmission electron microscopy.
• The authors' conclusion: The author concluded that membrane filter sampling with optical microscope analysis was still considered the best system for monitoring asbestos dust in occupational situations. Mass estimation methods could not be adopted with confidence, particularly for chrysotile, until a reliable elutriation system was available. For general environmental atmospheres, mass estimation by X-ray diffraction or electron microscopy was the only available method.

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Group, Amosite, Crocidolite, Anthophyllite, Chrysotile, Membrane Filter Method, X-Ray Diffraction (XRD).

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