Amphibole asbestos in tree bark--a review of findings for this inhalational exposure source in Libby, Montana

Our plain-English summary of Amphibole asbestos in tree bark--a review of findings for this inhalational exposure source in Libby, Montana, by T.J. Ward, T.M. Spear, J.F. Hart, et al (Journal of Occupational and Environmental Hygiene, 2012).

The study

• Authors: T.J. Ward, T.M. Spear, J.F. Hart, J.S. Webber, M.I. Elashheb
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2012;9(6):387-97
• Type of article: Review
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In June 2009, the U.S. Environmental Protection Agency (EPA) designated the town of Libby, Montana, a public health emergency--the first and only time the EPA has made such a determination under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). From about 1920 until 1990, the leading source of vermiculite ore for the United States and the world was from a mine near Libby. This vermiculite ore was contaminated with fibrous and asbestiform amphibole in veins throughout the deposit. Today, areas surrounding the abandoned vermiculite processing/mining facilities and much of the town of Libby are contaminated with these asbestos fibers, contributing to an outbreak of asbestos-related diseases in the Libby population. Trees in Libby and in forested areas surrounding the abandoned mine have accumulated amphibole asbestos fibers on their bark surface, providing for inhalational exposures. Several studies have been conducted to further understand this exposure pathway. To address exposures to the public, Libby amphibole (LA) was measured in personal breathing zone and Tyvek surface wipe samples collected during firewood harvesting simulations, as well as in the ash and emissions of woodstoves when amphibole-contaminated firewood was combusted. Occupational studies simulating wildland firefighting and routine U.S. Department of Agriculture (USDA) Forest Service activities have also been conducted in the forested areas surrounding the abandoned mine, demonstrating the potential for inhalational exposures during common regional workplace activities. We present a review of the findings of this emerging environmental health concern impacting not only the residents of Libby but applicable to other populations living near asbestos-contaminated areas.

In plain English

This review summarized several studies of Libby amphibole asbestos on tree bark near the former vermiculite mine at Libby, Montana, and in the town. Bark taken near the mine in November 2004 held 14 million to 110 million asbestos structures per square centimeter. No fibers were found in a tree core sample. The fibers included tremolite, winchite and richterite. In 2006 firewood-cutting simulations, all 14 breathing-zone samples had asbestos detectable by electron microscopy, and clothing wipes averaged 30,000 structures per square centimeter. When contaminated wood was burned in woodstoves, most fibers were found in the ash (137, 84 and 18 million structures per gram in three trials), and some reached the ductwork. In simulated Forest Service tasks, assuming an 8-hour exposure, 10 of 24 air samples exceeded the OSHA limit by optical microscopy, which, unlike electron microscopy, did not exclude cellulose fibers. Under the same assumption, none did when fibers longer than 5 µm were counted by electron microscopy. The authors concluded that these studies demonstrate the potential for inhalational exposures from common activities that disturb contaminated trees.

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: Review summarizing several studies: tree bark sampling plus activity-based exposure simulations (firewood harvesting, woodstove combustion, Forest Service work tasks, a small controlled burn)
• Who was studied: Trees (lodgepole pine, ponderosa pine, larch, Douglas fir) near the former W.R. Grace vermiculite mine, along the railroad and in the town of Libby, Montana, sampled 2004 to 2005. Simulations with investigators: firewood harvesting in 2006 (three trials in the Kootenai Forest inside the EPA restricted zone, plus a control near Missoula), Forest Service tasks in summer 2008, and a controlled burn in July 2009.
• Exposure: Libby amphibole (LA) on tree bark, including regulated tremolite and unregulated winchite and richterite. It was disturbed by firewood cutting, burning contaminated wood in EPA-certified woodstoves, Forest Service tasks (walking, tree measurement, fireline construction, trail maintenance), and a controlled burn.
• Main results:
  • Bark samples collected near the abandoned mine in November 2004 ranged from 14 million to 110 million asbestos structures per square centimeter of bark surface.
  • No amphibole fibers were found in a tree core sample. The authors took this to mean the fibers were not taken up through the roots.
  • Firewood harvesting simulations (breathing-zone samples, n = 14): mean time-weighted average was 0.15 structures/mL (SD 0.21) for fibers shorter than 5 µm and 0.07 structures/mL (SD 0.08) for fibers longer than 5 µm.
  • Firewood harvesting simulations: Tyvek clothing wipes (n = 14) averaged 30,000 structures/cm2 (SD 38,000), with 91% of these fibers shorter than 5 µm.
  • Woodstove combustion study: most LA fibers were found in the ash, at 137, 84 and 18 million structures per gram in the three trials.
  • Woodstove combustion study: after burning, wipes from inside the ductwork and the collection tote ranged from none detected to 20,476 structures/cm2 (fibers shorter than 5 µm).
  • Forest Service occupational trials: assuming an 8-hour exposure duration, 10 of 24 breathing-zone samples (42%) exceeded the OSHA permissible exposure limit of 0.1 f/mL when counted by phase contrast microscopy, a count that did not exclude cellulose fibers. All 10 came from fireline construction.
  • Forest Service occupational trials: by transmission electron microscopy (which excludes cellulose fibers), none of the samples exceeded the OSHA limit for LA fibers longer than 5 µm, again assuming an 8-hour exposure duration.
  • Controlled burn: 9 of 12 breathing-zone samples (75%) were above the analytical sensitivity by electron microscopy. Ash from the burn plot held 8 to 19 million structures per gram.
  • Firewood harvesting: clothing wipe levels were much higher in the hot, dry July trial than in the two cooler, wetter October trials.
• The authors' conclusion: The authors concluded that, taken together, these studies demonstrate the potential for inhalational exposures from common activities that disturb LA-contaminated trees in Libby. They called for more activity-based sampling and risk assessment.
• Limitations the authors state:
  • In the woodstove study, smoke samples could not be collected because heavy loading caused pump failures, so no air (smoke) results were reported.
  • Some controlled burn air samples needed indirect preparation and may underestimate concentrations.
  • No fibers were found in ductwork wipes from the final combustion trial, possibly because it lasted only 29 minutes.
  • The authors could not tell whether the higher firewood harvesting levels came from the activity itself or from higher bark contamination at those sites.

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: Amphibole Asbestos, Superfund, vermiculite, Asbestiform, Amphibole Group, Libby amphiboles, Wipe Sampling.

Related studies

• Trees as reservoirs for amphibole fibers in Libby, Montana — T.J. Ward, T. Spear, et al. (2006)
• Separation and characterization of respirable amphibole fibers from Libby, Montana — J.S. Webber, D.J. Blake, et al. (2008)
• Where occupation and environment overlap: US Forest Service worker exposure to Libby Amphibole fibers — M. Harper, C. Butler, et al. (2015)
• Evaluation of asbestos exposures during firewood-harvesting simulations in Libby, MT, USA--preliminary data — J.F. Hart, T.J. Ward, et al. (2007)
• A review of scientific literature examining the mining history, geology, mineralogy, and amphibole asbestos health effects of the Rainy Creek igneous complex, Libby, Montana, USA — B.R. Bandli, M.E. Gunter (2006)
• An evaluation of potential occupational exposure to asbestiform amphiboles near a former vermiculite mine — J.F. Hart, T.M. Spear, et al. (2009)

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