Estimates of historical exposures by phase contrast and transmission electron microscopy in North Carolina USA asbestos textile plants

Our plain-English summary of Estimates of historical exposures by phase contrast and transmission electron microscopy in North Carolina USA asbestos textile plants, by J.M. Dement, D. Myers, D. Loomis, et al (Occupational and Environmental Medicine, 2009).

The study

• Authors: J.M. Dement, D. Myers, D. Loomis, D. Richardson, S. Wolf
• Journal: Occupational and Environmental Medicine
• Published: 2009;66(9):574-83
• Cited by: 29 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objectives: To develop a job-exposure matrix (JEM) for fibre exposures in three asbestos textile plants and to develop estimates of fibre size-specific exposures.

Methods: Historical dust samples from three North Carolina, USA asbestos textile plants were obtained. Plant specific samples were used to express impinger dust concentrations as fibre concentrations by phase contract microscopy (PCM). Mixed models were used to estimate PCM exposures by plant, department, job and calendar time. Archived membrane filter samples were analysed by transmission electron microscopy (TEM) to determine the bivariate diameter/length distribution of airborne fibres by plant and operation.

Results: PCM fibre levels estimated from the models were very high in the 1930s, with some operations having in excess of 200 fibres/ml, and decreased appreciably over time. TEM results for 77 airborne dust samples found that only a small proportion of airborne fibres were measured by PCM (>0.25 microm in diameter and >5 microm in length) and the proportion varied considerably by plant and operation (range 2.9% to 10.0%). The bivariate diameter/length distribution of airborne fibres demonstrated a relatively high degree of variability by plant and operation. PCM adjustment factors also varied substantially across plants and operations.

Conclusions: These data provide new information concerning airborne fibre levels and characteristics in three historically important asbestos textile plants. PCM concentrations were high in the early years and TEM data demonstrate that the vast majority of airborne fibres inhaled by the workers were shorter than 5 microm in length, and thus not included in the PCM-based fibre counts.

In plain English

The authors built a job-exposure matrix, a table of estimated airborne asbestos fibre levels, for three asbestos textile plants in western North Carolina, USA, covering workers employed between 1950 and 1973. The plants made mainly chrysotile textiles; one also had a small amosite operation. The authors used 3420 historical air samples. They converted older dust counts into fibre counts by phase contrast microscopy (PCM), the standard method, which counts only fibres longer than 5 micrometres and wider than about 0.25 micrometres. Statistical models then estimated levels by plant, department, job group and year. The authors also examined 77 archived air filters from 1964 to 1971 by transmission electron microscopy (TEM), which can see smaller fibres. Estimated levels were very high in the early years and fell substantially over time. Depending on plant and department, only 2.9% to 10.0% of airborne fibres were of a size PCM counts. The authors concluded that, although the plants used longer-fibre grades of chrysotile, the air workers breathed was mostly short fibres, and that fibre sizes varied considerably across plants and operations.

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: development of a historical job-exposure matrix from industrial hygiene records, with transmission electron microscopy analysis of archived air filters to estimate fibre size-specific exposures
• Who was studied: Three asbestos textile plants (plants 1, 3 and 4) in western North Carolina, USA, with workers employed 1950 to 1973. The study used 3420 industrial hygiene samples collected between 1935 and 1986. Of 333 archived membrane filters from 1964 to 1971, 77 were analysed by TEM. A fourth plant (plant 2), which made brake bands, was left out of the matrix.
• Exposure: Airborne asbestos in textile manufacturing (fibre preparation, carding, spinning, twisting, weaving, finishing and related jobs). The plants mainly made chrysotile textile products, with about 90% of the asbestos from Canada. Plant 3 also had a small amosite insulation operation.
• Main results:
  • Across all three plants and all departments, estimated PCM fibre levels were very high in the early years of operation and fell substantially over time.
  • Factors for converting old dust counts to PCM fibre counts differed significantly by plant and by exposure zone (department group).
  • The final statistical model accounted for 64% of the variability in the (log-transformed) PCM measurements.
  • In plant 1, workers handling raw unspun fibre in preparation and carding had very high predicted exposure levels. Similar patterns were seen in plants 3 and 4.
  • Fibre levels fell throughout the period studied. The rate of decline changed significantly after the first OSHA standards, which the results section dates to 1971.
  • TEM analysis of the 77 filters counted and sized 22 776 fibres or fibre bundles.
  • Fibres both short (under 1.5 micrometres long) and thin (under 0.25 micrometres wide) made up 47.4% of airborne fibres in plant 4 and 55.8% in plant 3, the lowest and highest plant values.
  • The proportion of airborne fibres of a size PCM can detect ranged from 2.9% (95% CI 1.9% to 4.0%) in winding at plant 4 to 10.0% (95% CI 8.0% to 12.3%) in carding at plant 3.
  • In most plant zones, only a small proportion of airborne fibres were longer than 15 micrometres. This ranged from 1.6% (95% CI 1.1% to 2.2%) in zone 1 (labelled carding) of plant 4 to 6.7% (95% CI 4.8% to 8.8%) in finishing and shipping at plant 1.
  • Adjustment factors varied within plants. In plant 1, the factor for very thin, very long fibres (under 0.25 micrometres wide, over 40 micrometres long) was 0.0056 in preparation and 0.0422 in weaving.
  • In plant 1, ranking jobs by estimated exposure to long thin fibres (under 0.25 micrometres wide, over 5 micrometres long) changed the order. Twisting machine operators ranked third by TEM-based estimates but fifth by PCM.
• The authors' conclusion: The authors concluded that, although the textile plants typically used longer-fibre grades of chrysotile, the airborne dust workers were exposed to was mostly short fibres, the vast majority shorter than 5 micrometres and so not counted by PCM. They added that fibre size distributions and PCM adjustment factors varied considerably across plants and operations.
• Limitations the authors state:
  • Converting old dust-count samples to PCM fibre counts may introduce error into the PCM exposure estimates.
  • Variation between and within individual workers could not be modelled because the historical records did not identify workers.
  • Predicted exposures and time trends depend largely on model assumptions. For example, the model assumed the same relative decrease over time for all plants, departments and job groups, because detailed data on process changes were lacking.
  • There were not enough samples or resources to study fibre size differences between job groups within each department.
  • The TEM samples came only from 1964 to 1971, so the authors assumed fibre size distributions stayed stable over time. They could not completely rule out changes.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestos textiles.

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

Related studies

• Estimates of historical exposures by phase contrast and transmission electron microscopy for pooled exposure--response analyses of North Carolina and South Carolina, USA asbestos textile cohorts — J.M. Dement, D. Loomis, et al. (2011)
• Assessment of occupational exposure to asbestos fibers: Contribution of analytical transmission electron microscopy analysis and comparison with phase-contrast microscopy — C. Eypert-Blaison, A. Romero-Hariot, et al. (2018)
• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)
• Lung cancer mortality and fibre exposures among North Carolina asbestos textile workers — D. Loomis, J.M. Dement, et al. (2009)
• A technical comparison of evaluating asbestos concentration by phase-contrast microscopy (PCM), scanning electron microscopy (SEM), and analytical transmission electron microscopy (ATEM) as illustrated from data generated from a case report — R.F. Dodson, S.P. Hammar, et al. (2008)
• Comments on Dodson et al., A technical comparison of evaluating asbestos concentration by phase contrast microscopy (PCM), scanning electron microscopy (SEM), and analytical transmission electron microscopy (ATEM) as illustrated from data generated from a case report — V.L. Roggli, T.A. Sporn, et al. (2008)

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