A comparison of fibre counting stopping rules in the application of the membrane filter method

Our plain-English summary of A comparison of fibre counting stopping rules in the application of the membrane filter method, by N.P. Crawford, W.M. Maclaren, P. Brown, et al (The Annals of Occupational Hygiene, 1987).

The study

• Authors: N.P. Crawford, W.M. Maclaren, P. Brown, A.D. Jones
• Journal: The Annals of Occupational Hygiene
• Published: 1987;31(1):53-66
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher

In plain English

This study examined microscope counting of airborne asbestos fibres by the membrane filter method, not health effects. The European Reference Method (ERM) stops after 100 fibres or 100 fields of view (minimum 20 fields), so more effort goes into low-level samples. The authors proposed two alternative 'stopping rules' for judging compliance with the European control limit of 1.0 fibre per ml for chrysotile and amosite. In a simulation on existing counts, a shorter count disagreed with the full ERM count on compliance for three of 422 evaluations (less than 1%). The authors said most samples were outside the concentration range where compliance decisions tend to disagree. Two microscopists counted 15 chrysotile samples under all three rules, twice each. There was no evidence that the rules gave different fibre density estimates. Geometric mean counting times were 6.2 minutes (ERM rule), 2.9 minutes and 2.1 minutes (new rules). At low concentrations the new rules took as little as one-fifth of the ERM time. The authors called the experiment limited and noted the new rules are less precise well below the limit.

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: Methods study of asbestos fibre counting. It combined computer simulations on existing count data, theoretical (Poisson-based) calculations, and a small experimental trial comparing two candidate stopping rules with the ERM rule.
• Who was studied: Simulations used existing counts: 425 counts of 164 samples made with the ERM procedure, drawn from UK factory surveys, the UK Central Reference Scheme and a CEC/Canadian international study. The simulations were run on 422 of these evaluations. The trial used 15 chrysotile samples, with densities from 0 to 600 fibres per square millimetre. Each sample was counted by two microscopists using each of the three rules, twice.
• Exposure: Airborne asbestos fibre counting by phase contrast microscopy of membrane filters. The rules were designed for the European control limit of 1.0 fibre per ml for chrysotile and amosite. Assumed sampling conditions were a 240 litre sample volume, a 415 square millimetre filter area and a 0.00785 square millimetre graticule area.
• Main results:
  • In the simulation on 422 existing evaluations, a shortened count (randomly sampling until 20 fibres or 20 fields) disagreed with the full ERM count about compliance with 1.0 fibre per ml for only three evaluations (less than 1%). The authors said this low disagreement reflected that most sample densities were outside the range where disagreement on compliance would be expected.
  • Of 164 existing samples, field-to-field fibre counts did not differ significantly from a Poisson distribution for 72%. They were more variable for 21% and less variable for 7% (p less than 0.05).
  • In theoretical calculations (Operating Characteristic curves), both candidate rules performed marginally better than the ERM rule at making the correct compliance decision for samples close to the 1.0 fibre per ml control limit.
  • In the experimental trial, the only statistically significant source of variation in fibre density was the sample. There was no evidence that the three stopping rules gave different fibre density estimates.
  • Averaged over samples and microscopists, geometric mean counting times were 6.2 minutes for Rule 1 (ERM), 2.9 minutes for Rule 2 and 2.1 minutes for Rule 3.
  • Below about 0.8 fibres per ml, the time needed for the candidate rules fell progressively as concentration fell. At concentrations below about 0.2 fibre per ml they could take as little as about one-fifth of the time of the ERM rule.
• The authors' conclusion: The authors concluded that, although the experiment was limited, the candidate rules and the ERM rule gave fibre densities that did not differ systematically. They also concluded that the candidate rules produced estimates in less time below about 0.8 fibre per ml.
• Limitations the authors state:
  • The authors described the experiment as limited.
  • The rules apply to occupational monitoring, not to measurements made after asbestos removal from public buildings.
  • The candidate rules give less precise fibre density estimates than the ERM rule well below the compliance level. For this reason they are not intended for individual cumulative exposure estimates.
  • The time-saving results depend on the mix of densities in the trial samples. A laboratory's net time saving will depend on its own routine mix of densities.
  • For different control limits or sampling conditions, the numbers of fibres and fields in the candidate rules would have to change.

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.

Related studies

• Comparison of a fibrous aerosol monitor (FAM) with the membrane filter method for measuring airborne asbestos concentrations — P.J. Iles, T. Shenton-Taylor (1986)
• 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)
• Inter-laboratory comparisons of the counting of asbestos fibres sampled on membrane filters — S.T. Beckett, M.D. Attfield (1974)
• Comparison of NIOSH and AIA methods for evaluating asbestos fibres: effects of asbestos type, mounting medium, graticule type and counting rules — E. Gonzalez-Fernandez, F.R. Martín (1986)
• 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)
• Void-counting in assessing membrane filter samples of asbestos fibre — M.D. Attfield, S.T. Beckett (1983)

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