Development of a fibre size-specific job-exposure matrix for airborne asbestos fibres

Our plain-English summary of Development of a fibre size-specific job-exposure matrix for airborne asbestos fibres, by J.M. Dement, E.D. Kuempel, R.D. Zumwalde, et al (Occupational and Environmental Medicine, 2008).

The study

• Authors: J.M. Dement, E.D. Kuempel, R.D. Zumwalde, R.J. Smith, L.T. Stayner, D. Loomis
• Journal: Occupational and Environmental Medicine
• Published: 2008;65(9):605-12
• Cited by: 39 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objective: To develop a method for estimating fibre size-specific exposures to airborne asbestos dust for use in epidemiological investigations of exposure-response relations.

Methods: Archived membrane filter samples collected at a Charleston, South Carolina asbestos textile plant during 1964-8 were analysed by transmission electron microscopy (TEM) to determine the bivariate diameter/length distribution of airborne fibres by plant operation. The protocol used for these analyses was based on the direct transfer method published by the International Standards Organization (ISO), modified to enhance fibre size determinations, especially for long fibres. Procedures to adjust standard phase contrast microscopy (PCM) fibre concentration measures using the TEM data in a job-exposure matrix (JEM) were developed in order to estimate fibre size-specific exposures.

Results: A total of 84 airborne dust samples were used to measure diameter and length for over 18,000 fibres or fibre bundles. Consistent with previous studies, a small proportion of airborne fibres were longer than >5 microm in length, but the proportion varied considerably by plant operation (range 6.9% to 20.8%). The bivariate diameter/length distribution of airborne fibres was expressed as the proportion of fibres in 20 size-specific cells and this distribution demonstrated a relatively high degree of variability by plant operation. PCM adjustment factors also varied substantially across plant operations.

Conclusions: These data provide new information concerning the airborne fibre characteristics for a previously studied textile facility. The TEM data demonstrate that the vast majority of airborne fibres inhaled by the workers were shorter than 5 mum in length, and thus not included in the PCM-based fibre counts. The TEM data were used to develop a new fibre size-specific JEM for use in an updated cohort mortality study to investigate the role of fibre dimension in the development of asbestos-related lung diseases.

In plain English

This study built a method to estimate exposure to asbestos fibres of different sizes at a Charleston, South Carolina asbestos textile plant, for use in an updated mortality study of its workers. The authors used transmission electron microscopy to analyse archived air filters collected there by the US Public Health Service in 1964–8. Usable data came from 84 filters in 10 exposure zones, and 18,824 fibres or fibre bundles were measured. The estimated share of airborne fibres longer than 5 micrometres ranged from 6.9% (95% confidence interval 3.9% to 9.8%) in finishing to 20.8% (18.7% to 22.8%) in universal winding. Fibres thinner than 0.25 micrometres and longer than 5 micrometres, which standard phase contrast microscopy (PCM) generally does not detect, ranged from an estimated 4.5% in finishing (its printed interval does not contain it) to 11.4% (10.1% to 12.5%) in mule spinning. Factors for converting PCM counts into size-specific counts varied considerably between departments. The authors concluded that, although the plant used longer-fibre chrysotile grades, the airborne dust workers were exposed to was mostly short fibres. They noted they could not study job differences within zones or rule out changes in fibre size over time.

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: Exposure assessment study. The authors used laboratory electron microscopy (TEM) to analyse archived air samples, then used the results to build a fibre size-specific job-exposure matrix by adjusting earlier phase contrast microscopy (PCM) estimates.
• Who was studied: Archived membrane filter air samples collected by the US Public Health Service in 1964–8 at a Charleston, South Carolina asbestos textile plant. The authors located 203 archived samples and selected a stratified random sample of 86 across 10 exposure zones. Two were unusable, so 84 filters were analysed (3 to 11 per zone), and 18,824 fibres or fibre bundles were measured.
• Exposure: Airborne chrysotile asbestos fibres (major source Canada) in asbestos textile operations, classified by fibre diameter and length. Operations included preparation, carding, spinning, winding, twisting, weaving and finishing.
• Main results:
  • Across the 10 exposure zones, a total of 18,824 fibres or fibre bundles were counted and sized.
  • The estimated proportion of airborne fibres longer than 5 micrometres ranged from 6.9% (95% CI 3.9% to 9.8%) in zone 10 (finishing) to 20.8% (95% CI 18.7% to 22.8%) in zone 7 (universal winding).
  • Fibres thinner than 0.25 micrometres and longer than 5 micrometres are not generally detectable by PCM. Their estimated proportion ranged from 4.5% in zone 10 (finishing) to 11.4% (95% CI 10.1% to 12.5%) in zone 4 (mule spinning). The article prints the 4.5% interval as 6.7% to 12.8%, which does not contain the estimate.
  • Only a small proportion of airborne fibres were longer than 15 micrometres. The range was 1.7% (95% CI 0.8% to 2.7%) in finishing to 6.2% (95% CI 5.3% to 7.2%) in universal winding.
  • In an example for general area personnel in 1965, PCM adjustment factors for long, thin fibres varied substantially by department. They were smaller for the initial operations (preparation 1.02, carding 1.15) than for others such as twisting (2.17).
• The authors' conclusion: The authors concluded that, although asbestos textile operations typically used longer-fibre grades of chrysotile, the airborne dust workers were exposed to was mainly short fibres. They also concluded that fibre size distributions and PCM adjustment factors varied considerably across plant operations. They added that the method could be applied elsewhere where archived samples or TEM data exist.
• Limitations the authors state:
  • The authors lacked the samples and resources to study differences between job groups within each exposure zone.
  • Samples came only from 1964–8, so fibre size distributions were assumed to be stable over time. The authors could not completely rule out changes.
  • The surface-area measure is relative, not absolute, because fibres were assumed to be simple cylinders.
  • Precise measurement of chrysotile fibre diameter is difficult, especially for bundles and for fibres within complex structures.

Topics

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

Related studies

• 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 — J.M. Dement, E.D. Kuempel, et al. (2015)
• An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers — L. Stayner, E. Kuempel, et al. (2008)
• Examining the association of lung cancer and highly correlated fibre size-specific asbestos exposures with a hierarchical Bayesian model — G.B. Hamra, D. Loomis, et al. (2014)
• Occupational exposure to asbestos in New South Wales, Australia (1970-1989): development of an asbestos task exposure matrix — R.A. Hyland, D.H. Yates, et al. (2010)
• Letter to the editor: "Comparing milled fiber, Quebec ore, and textile factory dust: has another piece of the asbestos puzzle fallen into place?" by D. Wayne Berman — J.M. Dement, L.T. Stayner (2010)
• A study of the size distributions of airborne amosite fibres in the manufacture of asbestos insulation boards — S.T. Beckett, J.L. Jarvis (1979)

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