Airborne fiber size characterization in exposure estimation: Evaluation of a modified transmission electron microcopy protocol for asbestos and potential use for carbon nanotubes and nanofibers

Our plain-English summary of Airborne fiber size characterization in exposure estimation: Evaluation of a modified transmission electron microcopy protocol for asbestos and potential use for carbon nanotubes and nanofibers, by J.M. Dement, E.D. Kuempel, R.D. Zumwalde, et al (American Journal of Industrial Medicine, 2015).

The study

• Authors: J.M. Dement, E.D. Kuempel, R.D. Zumwalde, A.M. Ristich, J.E. Fernback, R.J. Smith
• Journal: American Journal of Industrial Medicine
• Published: 2015;58(5):494-508
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Airborne fiber size has been shown to be an important factor relative to adverse lung effects of asbestos and suggested in animal studies of carbon nanotubes and nanofibers (CNT/CNF).

Materials And Methods: The International Standards Organization (ISO) transmission electron microscopy (TEM) method for asbestos was modified to increase the statistical precision of fiber size determinations, improve efficiency, and reduce analysis costs. Comparisons of the fiber size distributions and exposure indices by laboratory and counting method were performed.

Results: No significant differences in size distributions by the ISO and modified ISO methods were observed. Small but statistically-significant inter-lab differences in the proportion of fibers in some size bins were found, but these differences had little impact on the summary exposure indices. The modified ISO method produced slightly more precise estimates of the long fiber fraction (>15 μm).

Conclusions: The modified ISO method may be useful for estimating size-specific structure exposures, including CNT/CNF, for risk assessment research.

© 2015 Wiley Periodicals, Inc.

In plain English

This study tested a modified international standard (ISO) transmission electron microscopy (TEM) method for counting and sizing airborne asbestos fibers. The authors had used it in US chrysotile textile worker research. Two laboratories analyzed nine archived air filters from two textile plant zones (fiber preparation and light weaving). The second laboratory also used the standard ISO method. Repeat counts in the primary laboratory showed no statistically significant differences. However, they used the same grid openings, so the authors said independence could not be assumed and the analysis had substantial limitations. The two laboratories' fiber size distributions differed significantly overall (p-value 0.0008), mainly for fibers shorter than 5 micrometres, though most differences were small. The modified and standard methods did not differ significantly overall (p-value 0.2759). Three fiber exposure indices differed between zones, but within each zone their 95% confidence intervals overlapped across laboratories and methods. The authors noted few samples, new grid preparations and the second laboratory's lesser experience. They concluded it gives results comparable to the ISO method and may be a candidate method for carbon nanotube and nanofiber research.

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 study (within-laboratory, between-laboratory and between-method comparison of TEM fiber counting protocols), with a bootstrap sensitivity analysis of fiber exposure indices
• Who was studied: Nine archived membrane air filter samples. They were a subset of the 86 filters used to build a fiber size-specific job-exposure matrix for an epidemiological study of US asbestos textile workers: four from Zone 1 (fiber preparation) and five from Zone 9 (light weaving). Three of the samples in the original 10% stratified random selection were excluded beforehand. Primary laboratory quality control samples were used for the within-laboratory comparison.
• Exposure: Airborne chrysotile asbestos fibers in textile plant exposure zones, sized by diameter and length with the modified ISO 10312 TEM protocol and with the standard ISO 10312 protocol. Exposure indices were calculated for a phase contrast microscopy (PCM) exposure of 1.0 f/cc.
• Main results:
  • Within the primary laboratory, repeat quality-control analyses showed no statistically significant differences in fiber size distributions from the original analyses (chi-square 45.15, 90 degrees of freedom). The recounts used the same grid openings, so statistical independence could not be assumed.
  • Between the two laboratories using the modified method, fiber size distributions differed by a statistically significant amount (overall likelihood ratio chi-square 66.2498, 34 degrees of freedom, p-value 0.0008). The difference was largely driven by fibers shorter than 5 micrometres; in Zone 1 this stratum had chi-square 28.6325, p-value 0.0002.
  • In the detailed between-laboratory comparison, no size categories differed significantly in Zone 9. In Zone 1, six of 20 size categories differed significantly.
  • The overall pattern suggested that, for fibers less than 1.0 micrometre in diameter, the second laboratory estimated a smaller proportion of fibers longer than 3 micrometres than the primary laboratory did.
  • In the second laboratory, fiber size distributions from the modified and standard ISO methods were not statistically different overall (chi-square 37.3502, 33 degrees of freedom, p-value 0.2759). The stratum of fibers shorter than 5 micrometres did differ significantly in Zone 9 (p-value 0.0336).
  • Size-specific concentrations for each index varied by exposure zone. Within each zone, the 95% confidence intervals for each index (Stanton, Pott and modified Lippmann approximations) overlapped between laboratories and between methods. Bootstrap comparisons found no statistically significant differences in mean index values (p greater than 0.05).
  • In the second laboratory's data, standard deviations of the summary exposure indices were generally slightly larger with the modified ISO method.
  • The modified method counted more long fibers than the ISO method. Its estimates of the fraction of very long fibers had slightly reduced variability.
• The authors' conclusion: The authors concluded that the modified ISO method produces fiber size data comparable to the ISO method at reduced analytical cost. They also concluded that it may be a useful candidate method for quantitative exposure assessment of carbon nanotubes and nanofibers, especially in research supporting toxicology or epidemiology studies.
• Limitations the authors state:
  • The study was based on a limited number of samples.
  • The second laboratory had to prepare new TEM grids for all samples because the original preparations were unstable, which added a source of variability.
  • The second laboratory had much less experience with the modified protocol.
  • The within-laboratory quality-control recounts used the same grid openings, so statistical independence could not be assumed. The authors said this may explain the surprisingly small test statistic, and they described this analysis as subject to substantial limitations.
  • Data for fibers longer than 15 micrometres could not be used in the within-laboratory evaluation.
  • Some exposure indices could only be approximated, because their size cut-offs did not match the available size categories.

Topics

Medical terms in this study, explained in our Lexicon: Transmission Electron Microscopy (TEM).

Related studies

• Factoring-in agglomeration of carbon nanotubes and nanofibers for better prediction of their toxicity versus asbestos — A.R. Murray, E.R. Kisin, et al. (2012)
• Development of a fibre size-specific job-exposure matrix for airborne asbestos fibres — J.M. Dement, E.D. Kuempel, et al. (2008)
• Size distributions of occupational airborne asbestos textile fibres as determined by transmission electron microscopy — A.P. Rood, R.R. Streeter (1984)
• High-temperature calcined fullerene nanowhiskers as well as long needle-like multi-wall carbon nanotubes have abilities to induce NLRP3-mediated IL-1β secretion — H. Cui, W. Wu, et al. (2014)
• A comparison of light microscopy and transmission electron microscopy results in the evaluation of the occupational exposure to airborne chrysotile fibres — A. Marconi, E. Menichini, et al. (1984)
• Exposure assessment of carbon nanotube manufacturing workplaces — J.H. Lee, S.B. Lee, et al. (2010)

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