A comparison of airborne asbestos fibre counting with and without an eyepiece graticule

Our plain-English summary of A comparison of airborne asbestos fibre counting with and without an eyepiece graticule, by S.T. Beckett, R.K. Hey, R. Hirst, et al (The Annals of Occupational Hygiene, 1976).

The study

• Authors: S.T. Beckett, R.K. Hey, R. Hirst, R.D. Hunt, J.L. Jarvis, A.L. Rickards
• Journal: The Annals of Occupational Hygiene
• Published: 1976;19(1):69-76
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Previous results have shown that when asbestos fibres on membrane filters are counted over the whole field of view of the microscope, lower concentration values are obtained than when the counting is confined to a central graticule grid area. Further studies have been carried out to verify this grid effect, to determine its magnitude for amosite and chrysotile asbestos fibres, and to ascertain its causes. The effects of various physical and human factors were investigated. With small graticule grids, fibre counts were increased by a factor of about 1.5 for amosite and 2.5 for chrysotile as compared with full-field counting. The effect was attributed mainly to human errors associated with full-field counting. It was concluded that more reliable and accurate counts are obtained in the routine evaluation of airborne asbestos dust concentrations by using an eyepiece graticule method of counting than by the full field method. It is therefore recommended that this method of counting be adopted and that steps be taken to reach national or, preferably, international agreement on a standard grid counting procedure.

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

In plain English

This was a laboratory methods study, not a study of health effects. It compared two ways of counting asbestos fibres on membrane filters under a microscope: over the whole field of view, or only inside a small eyepiece grid (a graticule). Five experienced laboratories counted factory dust samples both ways. Three counted chrysotile and two counted amosite. The 'grid factor' was the fibres per unit area counted in the grid divided by those counted over the full field. Average grid factors ranged from 1.01 to 2.48 for chrysotile (mean 1.65), with a mean of 1.37 for amosite. The effect was largest with small grids and smaller with grids covering 40% of the field. Raising the magnification from x600 to x900 appeared to have little or no effect on this ratio. When one experienced observer carefully re-counted 16 chrysotile samples over the full field, the average grid factor fell from 3.26 to 1.14. Repeat counts varied less with the grid. The authors concluded that the difference came mostly from human errors in full-field counting, and they recommended grid counting.

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: Inter-laboratory comparison of two microscope counting methods (eyepiece graticule grid versus full field of view), followed by further tests of possible causes
• Who was studied: Factory dust samples on membrane filters, counted by five experienced laboratories (A to E). Three laboratories counted chrysotile samples (10, 87, 40 and 41 samples in the four data sets) and two counted amosite samples (94 and 30 samples).
• Exposure: Chrysotile and amosite fibres on membrane filters, counted by full-field and graticule-grid methods. Grid sizes ranged from 5% to 40% of the full field.
• Main results:
  • In the five-laboratory comparison, average grid factors (grid count in fibres per unit area divided by full-field count) were 1.01 to 2.48 (mean 1.65) for chrysotile and averaged 1.37 for amosite.
  • The grid effect was most apparent with small graticule areas (5 to 10% of the full field) and smaller with larger (40%) graticule areas.
  • In further chrysotile tests by laboratory A, grids of 5, 10, 14 and 20% of the full field all gave grid factors close to 2, with no statistically significant difference between them at the 5% level. The 33% Miller square grid gave a grid factor of 1.06.
  • On 13 slides counted with a 5% grid, average grid factors were 1.79 at x600 magnification and 1.72 at x900. Increasing the magnification appeared to have little or no effect on the ratio of grid to full-field counts.
  • Effects of fibre density were not consistent between laboratories and were small compared with the grid factor.
  • When an experienced observer rigorously re-evaluated 16 selected chrysotile samples by the full-field method, the average grid factor fell from 3.26 to 1.14.
  • In repeat counts, coefficients of variation were 12.4% (grid) and 17.4% (full field) for chrysotile, and 14.3% (grid) and 18.4% (full field) for amosite.
• The authors' conclusion: The authors concluded that, for small grids, the grid factor is approximately 1.5 for amosite and 2.5 for chrysotile. They attributed it mostly to human errors in full-field counting, found grid counts more reliable and accurate, and recommended graticule counting and a standard grid procedure.
• Limitations the authors state:
  • The authors stated that the many differences in methods between laboratories made it difficult to quantify the errors.
  • The authors described the repeatability results as limited.

Topics

Medical terms in this study, explained in our Lexicon: Amosite, Chrysotile.

Related studies

• 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)
• A summary of asbestos fibre counting experience in seven countries — G.W. Gibbs, P. Baron, et al. (1977)
• Inter-laboratory comparisons of the counting of asbestos fibres sampled on membrane filters — S.T. Beckett, M.D. Attfield (1974)
• Comparison of direct and indirect methods of measuring airborne chrysotile fibre concentration — C. Eypert-Blaison, S. Veissiere, et al. (2010)
• A comparison of fibre counting across three European national proficiency testing schemes — A.D. Jones, M.C. Arroyo, et al. (2005)

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