Void-counting in assessing membrane filter samples of asbestos fibre

Our plain-English summary of Void-counting in assessing membrane filter samples of asbestos fibre, by M.D. Attfield, S.T. Beckett (The Annals of Occupational Hygiene, 1983).

The study

• Authors: M.D. Attfield, S.T. Beckett
• Journal: The Annals of Occupational Hygiene
• Published: 1983;27(3):273-82
• Read it: PubMed · Publisher

In plain English

This laboratory study tested a quicker way of counting asbestos fibres on membrane filter samples under a phase-contrast microscope. In the 'void-count' method, the counter notes only whether each grid area is empty of fibres, stopping after 25 empty areas. The authors compared it with the standard Asbestosis Research Council method. In a first experiment, two experienced counters assessed five chrysotile slides. Void counting gave lower estimates (160 versus 192 fibres per square millimetre, a statistically significant difference), cut counting time by 30% overall, and was less reliable. In a second experiment, four counters assessed chrysotile and amosite slides. Void counting saved about 70% of time but was less reliable at all densities, and differences between counters were larger with it. The authors concluded that time savings were generally offset by lost reliability, except within a narrow fibre density range. In a small trial, counting 30 rather than 100 grid areas by the standard method gave equivalent counts with similar reproducibility, though the authors said further work was needed.

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 comparison: two planned fibre-counting experiments, plus a small third trial, comparing a modified void-count (blank field) method with the standard fibre-counting method
• Who was studied: Membrane filter slides, not people. Experiment I: five chrysotile slides, each counted 16 times by two experienced counters. Experiment II: slides counted by four counters with two replicates; the text says ten slides, but Table 2 lists 20. Third trial: one counter examined two amosite and two chrysotile slides four times each.
• Exposure: Chrysotile fibres (Experiment I), and chrysotile and amosite asbestos (Experiment II), on membrane filter samples examined by phase-contrast microscopy at 600x
• Main results:
  • Experiment I: void counting gave lower estimates than the usual method, 160 versus 192 fibres per square millimetre (P less than 0.05)
  • Experiment I: the void method saved 30% of counting time overall, from 9.4 to 6.4 minutes
  • Experiment I: the void-count was less reliable than the standard method, and agreement between the two counters was no better with it
  • Experiment II: void counts gave higher estimates overall, but this excess was due almost entirely to one counter (C), whose void counts were twice their usual counts
  • Experiment II: the void method saved about 70% of counting time, 3.1 versus 7.9 minutes
  • Experiment II: observed and theoretical coefficients of variation both showed the void method was less reliable at all fibre densities
  • Third trial: with the standard method, counting 30 graticule areas instead of 100 gave equivalent counts with similar reproducibility
  • For slides near the authors' suggested optimal density (0.5 to 1.5 fibres per graticule area), the average coefficient of variation was 20 for the void method and 22 for the standard method
• The authors' conclusion: The authors concluded that the void method's time savings were generally offset by a corresponding loss of reliability. It could save time without losing precision only if the count per graticule area fell within close limits.
• Limitations the authors state:
  • The authors reserved judgement on inter-counter variation because experience with the void-count method had been extremely limited
  • The authors said further work was needed before the finding on the reduced (30-field) standard method could be generally accepted

Related studies

• A comparison of fibre counting stopping rules in the application of the membrane filter method — N.P. Crawford, W.M. Maclaren, et al. (1987)
• Inter-laboratory comparisons of the counting of asbestos fibres sampled on membrane filters — S.T. Beckett, M.D. Attfield (1974)
• Industrial trials with the Vickers M88 rapid asbestos fibre counter — A.D. Jones, R.W. Gale (1982)
• The counting of small numbers of asbestos fibres on membrane filters: a comparison of results from some commercial laboratories — M.J. Duggan, E.W. Culley (1978)
• In situ identification of asbestos fibres collected on membrane filters for counting — N.P. Vaughan, S.J. Rooker, et al. (1981)
• 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)

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