An evaluation of potential occupational exposure to asbestiform amphiboles near a former vermiculite mine

Our plain-English summary of An evaluation of potential occupational exposure to asbestiform amphiboles near a former vermiculite mine, by J.F. Hart, T.M. Spear, T.J. Ward, et al (Journal of Environmental and Public Health, 2009).

The study

• Authors: J.F. Hart, T.M. Spear, T.J. Ward, C.E. Baldwin, M.N. Salo, M.I. Elashheb
• Journal: Journal of Environmental and Public Health
• Published: 2009;2009:189509
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Amphibole asbestos (AA) has been detected on the surface of tree bark in forests neighboring an abandoned vermiculite mine near Libby, Montana. In the present study, simulations were performed to assess potential AA exposure associated with United States Department of Agriculture Forest Service (FS) occupational activities. Bark samples were collected prior, and personal breathing zone (PBZ) and Tyvek clothing wipe samples were collected during and immediately after trials that simulated FS activities. Transmission electron microscopy (TEM) analyses revealed AA bark concentrations up to 15 million structures per square centimeter (s/cm(2)). AA was detected in 25% of the PBZ TEM samples. AA was detected on wipe samples collected from all activities evaluated. This research demonstrates the potential for airborne exposure and transport of AA in the Kootenai National Forest. These findings are especially relevant to those that work in the area and to the general public who may conduct recreational activities.

In plain English

In this pilot field study, done mainly in July 2008 on Forest Service land within 4.8 km of the former vermiculite mine near Libby, Montana, researchers wore protective suits and respirators while simulating Forest Service work: driving forest roads, walking, measuring trees, building fire line and maintaining trail. Seven tree bark samples from the area carried amphibole asbestos at 37 thousand to 15 million structures per square centimeter. Electron microscopy found amphibole asbestos above the detection level in 25% of breathing-zone air samples, all from fire line and tree measurement work. None exceeded the OSHA limit for fibers longer than 5 µm, assuming an 8-hour day. A light-microscope count of all fibers, which the authors expected to include plant fibers, put 10 of 24 samples, all from fire line work, above that limit. Amphibole asbestos was found on outer clothing after each of the four activities evaluated (52% of post-activity wipes) and on the chainsaw bar after all three sampled trials. The authors concluded that exposure may exist when working in this forest. They noted that few samples were taken over a small area.

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: Pilot exposure-assessment study using field simulations of Forest Service tasks, with tree bark, personal breathing zone air, clothing wipe, equipment wipe and vehicle wipe sampling analyzed by PCM and TEM
• Who was studied: The research team's investigators simulated U.S. Forest Service tasks, not Forest Service employees themselves. Simulations took place on Forest Service land north and east of the former vermiculite mine and the EPA-restricted zone near Libby, Montana (Kootenai National Forest), within 4.8 km of the mine. They were done in July 2008, plus one driving simulation in November 2007. Investigators worked in teams of two to five depending on the task.
• Exposure: Amphibole asbestos (AA) from the Libby vermiculite deposit, present on tree bark and possibly soil. The activities were driving on Forest Service roads, walking through forested areas, tree measurement, fire line construction (two trials each) and trail maintenance (one trial). Trials lasted 31 to 42 minutes for fire line construction and 70 to 90 minutes for the other tasks.
• Main results:
  • Seven bark samples from trees in the simulation area had amphibole asbestos ranging from 37 thousand to 15 million structures per square centimeter of bark surface. Most fibers were shorter than 5 µm.
  • The control bark sample from a tree in Missoula, Montana, had no amphibole fibers detected.
  • By phase contrast microscopy (PCM), which counts all fibers, 10 of 24 personal breathing zone samples (42%) exceeded the OSHA limit of 0.1 fiber per mL, assuming an eight-hour exposure. All 10 were from fire line construction. The authors expected a substantial portion of the PCM fibers to be cellulose from forest vegetation.
  • The 10 PCM samples above the OSHA limit were all collected during fire line construction.
  • By AHERA TEM, 25% of personal breathing zone samples had concentrations above the analytical sensitivity. These samples came from fire line construction and tree measurement. Trail maintenance and walking samples had no detectable amphibole asbestos.
  • By TEM, no personal breathing zone sample exceeded the OSHA PEL for asbestos fibers longer than 5 µm, assuming an 8-hour exposure duration.
  • One walking-activity air sample contained chrysotile asbestos. The authors said this may have come from sources other than the vermiculite mine.
  • All personal breathing zone samples with detectable amphibole asbestos showed measurable sodium and potassium. The authors cite other research linking this to amphiboles from the vermiculite deposit.
  • 52% of post-activity Tyvek clothing wipe samples had amphibole asbestos above the analytical sensitivity of 448 structures per square centimeter. Field blanks and pre-activity wipes showed none.
  • Amphibole asbestos was detected on clothing wipe samples from every activity evaluated: fire line construction, tree measurement, trail maintenance and walking.
  • Chainsaw bar wipe samples taken after three trials had amphibole asbestos ranging from 896 to 11 825 structures per square centimeter.
  • Vehicle bumper wipes for Forest Service Road 4872 were below the analytical sensitivity before and after travel. The post-travel wipe for Road 401 showed one amphibole fiber, a concentration of 17 917 structures per square centimeter.
• The authors' conclusion: The authors concluded that exposure to amphibole asbestos may exist when work is performed in the Kootenai National Forest near the former vermiculite mine. They added that contaminated clothing and equipment may be a secondary source of exposure for Forest Service personnel.
• Limitations the authors state:
  • A limited number of samples was collected within a relatively small geographical area.
  • Mean concentrations were calculated using zero for nondetects, which may give an uncertain estimate of the true mean; actual risks may be higher or lower.
  • For the Pulaski tool task, which clears vegetation down to mineral soil, it is unclear whether exposure came from vegetation or soil.
  • The authors said future research is needed across a range of weather conditions and seasons, other activities such as fire fighting, and wider distances from the mine.

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

The National Library of Medicine indexes this study under subjects our site covers: Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Asbestiform, Amphibole Group, vermiculite, Amphibole Asbestos, Wipe Sampling, Transmission Electron Microscopy (TEM).

Related studies

• Exposure to asbestos-containing vermiculite ore and respiratory symptoms among individuals who were children while the mine was active in Libby, Montana — L.C. Vinikoor, T.C. Larson, et al. (2010)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole — T.C. Larson, V.C. Antao, et al. (2010)
• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates — H.E. Amandus, R. Wheeler, et al. (1987)

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