Evaluation of asbestos exposures during firewood-harvesting simulations in Libby, MT, USA--preliminary data

Our plain-English summary of Evaluation of asbestos exposures during firewood-harvesting simulations in Libby, MT, USA--preliminary data, by J.F. Hart, T.J. Ward, T.M. Spear, et al (The Annals of Occupational Hygiene, 2007).

The study

• Authors: J.F. Hart, T.J. Ward, T.M. Spear, K. Crispen, T.R. Zolnikov
• Journal: The Annals of Occupational Hygiene
• Published: 2007;51(8):717-23
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Research was conducted in order to assess potential exposure to asbestos while harvesting firewood from amphibole-contaminated trees near Libby, MT, USA. Three firewood-harvesting simulations took place in the summer and fall of 2006 in the Kootenai Forest inside the US Environmental Protection Agency (EPA) restricted zone surrounding the former W.R. Grace vermiculite mine. Another simulation was conducted near Missoula, MT, USA, which served as the control. The work practices following each simulation were consistent throughout each trial. Personal breathing zone (PBZ) asbestos concentrations were measured by phase contrast microscopy (PCM) and transmission electron microscopy (TEM). Surface wipe samples of personal protective clothing were measured by TEM. The mean (n = 12) PBZ PCM sample time-weighted average (TWA) concentration was 0.29 fibers per milliliter, standard deviation (SD = 0.54). A substantial portion (more than five fibers per sample) of non-asbestos fibers (cellulose) was reported on all PBZ samples (excluding field blanks) when analyzed by TEM. The mean (n = 12) PBZ TEM sample TWA concentration for amphibole fibers <5-microm long was 0.15 fibers per milliliter (SD = 0.21) and the mean (n = 12) PBZ TEM concentration for amphibole fibers >5-microm long was 0.07 fibers per milliliter (SD = 0.08). Substantial amphibole fiber concentrations were revealed on Tyvek clothing wipe samples. The mean concentration (n = 12) was 29 826 fibers per square centimeter (SD = 37 555), with 91% (27 192 fibers per square centimeter) comprised fibers <5-microm long. There were no significant differences in PBZ and wipe sample concentrations among the tasks performed by four investigators. Each of these three simulations were consistent in demonstrating that amphibole fibers are released from tree reservoirs during firewood-harvesting activities in asbestos-contaminated areas and that the potential for exposure exists during such activities.

In plain English

This study, reported as preliminary data, measured asbestos exposure during simulated firewood harvesting near Libby, Montana. In summer and fall 2006, four to five investigators felled, cut and stacked trees in three trials inside the EPA-restricted zone around the former W.R. Grace vermiculite mine. There was also one control trial near Missoula. Bark at the Libby site held 7 to 97 million amphibole fibers per square centimeter, and none were detected in Missoula bark. Across 12 Libby breathing-zone air samples, the mean was 0.29 fibers per milliliter by phase contrast microscopy. By electron microscopy, amphibole fibers averaged 0.15 fibers per milliliter for fibers shorter than 5 micrometers and 0.07 for longer fibers. Control air samples showed no asbestos above detection limits. Three of 12 electron microscopy results for longer fibers exceeded the OSHA limit of 0.1 fiber per milliliter, assuming an 8-hour exposure. Wipes from the chest and upper thigh of outer Tyvek suits averaged 29 826 fibers per square centimeter after harvesting. Exposures did not differ significantly between tasks. The authors concluded that amphibole fibers can be released from trees during harvesting, and called the simulations near worst-case.

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: Field exposure simulation study: three firewood-harvesting simulations inside the Libby EPA-restricted zone plus one control simulation near Missoula, with personal breathing zone air sampling, clothing wipe sampling and bark sampling
• Who was studied: Four to five study investigators per trial, working as chain saw operator, operator assistant, two wood stackers and (trials 2 and 3) a data recorder. Three trials took place in summer and fall 2006 on US Forest Service land in the Kootenai Forest inside the EPA-restricted zone around the former W.R. Grace vermiculite mine near Libby, MT. One control trial took place near Missoula, MT.
• Exposure: Amphibole fibers typical of the Libby vermiculite contaminants, released from contaminated tree bark during chain-saw felling, branch removal, sawing logs into 30-cm blocks, and stacking. Fibers classified as actinolite/tremolite included winchite/richterite. Harvest work lasted 45 to 50 minutes per trial.
• Main results:
  • Eight bark samples from the Libby restricted zone had amphibole fiber concentrations of 7 to 97 million fibers per square centimeter of bark surface. No amphibole fibers were detected in Missoula bark samples.
  • In the Missoula control trial, PCM fiber concentrations ranged from 0.01 to 0.02 fibers per milliliter. TEM showed these fibers to be non-asbestos (cellulose), with no asbestos above the analytical sensitivity.
  • Libby restricted-zone breathing zone samples (n = 12): mean PCM TWA 0.29 f/ml (SD 0.54). Mean TEM TWAs were 0.15 f/ml (SD 0.21) for fibers shorter than 5 µm, 0.07 f/ml (SD 0.08) for fibers longer than 5 µm, and 0.22 f/ml (SD 0.29) for total asbestos.
  • In the Libby breathing zone samples, shorter fibers (less than 5 µm) were compared with longer fibers (more than 5 µm); the P value printed was 0.055. By laboratory fiber counts, 69% of fibers on these samples were shorter than 5 µm.
  • No significant differences in breathing zone asbestos concentrations were found between tasks. The operator assistant had the highest mean total TEM asbestos concentration, 0.40 f/ml (SD 0.51).
  • 21 of 24 breathing zone sample results from the Libby restricted-zone trials were above analytical sensitivity. Control-trial samples did not detect asbestos above TEM analytical sensitivity.
  • By PCM, two of three samples each from the chain saw operator and the operator assistant, and one of three samples each from the two wood stackers, exceeded the OSHA limit of 0.1 fiber per milliliter, assuming an 8-hour exposure.
  • By TEM, 3 of 12 samples for asbestos fibers longer than 5 µm exceeded the OSHA PEL, assuming an 8-hour exposure. All three came from harvest trial 3 (operator assistant and wood stackers 1 and 2).
  • Post-harvest outer Tyvek wipe samples (n = 12, each a composite of one chest and one upper-thigh wipe per investigator) had these means: 27 192 f/cm2 (SD 36 749) for fibers shorter than 5 µm, 2634 f/cm2 (SD 1983) for fibers longer than 5 µm, and 29 826 f/cm2 (SD 37 555) for total asbestos. The shorter-versus-longer comparison gave P = 0.038.
  • Field blank, inner-layer Tyvek and pre-harvest outer-layer Tyvek wipes showed no asbestos above the analytical sensitivity of 878 structures per square centimeter. Post-harvest wipes from the chest and thigh showed asbestos above analytical sensitivity in 23 of 24 samples.
  • There were no statistically significant differences in wipe asbestos concentrations between the four investigators.
• The authors' conclusion: The authors concluded that amphibole fibers can become liberated from trees when harvesting firewood in asbestos-contaminated areas. They also stated that clothing contamination may serve as a secondary source of exposure.
• Limitations the authors state:
  • The authors stated that the simulations represent near worst-case scenarios.
  • The study area is secured and not currently open to the public for firewood harvesting.
  • The comparisons with occupational limits used only fibers longer than 5 µm. The authors noted that fibers shorter than 5 µm may also contribute to health risks.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Amphibole Group, W.R. Grace, vermiculite, phase contrast microscopy (PCM), Transmission Electron Microscopy (TEM), Wipe Sampling, Time-Weighted Average (TWA).

Related studies

• Amphibole asbestos in tree bark--a review of findings for this inhalational exposure source in Libby, Montana — T.J. Ward, T.M. Spear, et al. (2012)
• Trees as reservoirs for amphibole fibers in Libby, Montana — T.J. Ward, T. Spear, et al. (2006)
• An evaluation of potential occupational exposure to asbestiform amphiboles near a former vermiculite mine — J.F. Hart, T.M. Spear, et al. (2009)
• Where occupation and environment overlap: US Forest Service worker exposure to Libby Amphibole fibers — M. Harper, C. Butler, et al. (2015)
• Comparative long-term toxicity of Libby amphibole and amosite asbestos in rats after single or multiple intratracheal exposures — J.M. Cyphert, D.J. Carlin, et al. (2015)
• Airborne asbestos exposures associated with gasket and packing replacement: a simulation study of flange and valve repair work and an assessment of exposure variables — A.K. Madl, K.D. Devlin, et al. (2015)

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