Where occupation and environment overlap: US Forest Service worker exposure to Libby Amphibole fibers

Our plain-English summary of Where occupation and environment overlap: US Forest Service worker exposure to Libby Amphibole fibers, by M. Harper, C. Butler, D. Berry, et al (Journal of Occupational and Environmental Hygiene, 2015).

The study

• Authors: M. Harper, C. Butler, D. Berry, J. Wroble
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2015;12(5):D47-53
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The National Institute for Occupational Safety and Health (NIOSH) conducted an evaluation of exposures to asbestiform amphibole, known as Libby Amphibole (LA), to personnel from the US Department of Agriculture-Forest Service (USFS) working in the Kootenai National Forest near a former vermiculite mine close to Libby, Montana. LA is associated with vermiculite that was obtained from this mine; mining and processing over many years have resulted in the spread of LA into the surrounding Kootenai Forest where it has been found in tree bark, soil, and forest floor litter. As a result of this and other contamination, Libby and surrounding areas have been designated a "Superfund" site by the US Environmental Protection Agency (EPA). This article describes the application of EPA methods for assessing cancer risks to NIOSH sampling results. Phase-contrast microscopy for airborne asbestos fiber evaluation was found to be less useful than transmission electron microscopy in the presence of interfering organic (plant) fibers. NIOSH Method 7402 was extended by examination of larger areas of the filter, but fiber counts remained low. There are differences between counting rules in NIOSH 7402 and the ISO method used by EPA but these are minor in the context of the uncertainty in concentration estimates from the low counts. Estimates for cancer risk are generally compatible with those previously estimated by the EPA. However, there are limitations to extrapolating these findings of low risk throughout the entire area and to tasks that were not evaluated.

In plain English

In summer 2012, the National Institute for Occupational Safety and Health (NIOSH) sampled the air breathed by US Forest Service workers in the Kootenai National Forest, within 6.5 miles of the former vermiculite mine near Libby, Montana. It collected one hundred thirty-six personal breathing-zone samples from 23 workers doing 10 tasks. Under phase-contrast microscopy every sample showed fibers, and six fireline-construction samples exceeded the 0.1 fibers per cubic centimeter limit. The authors suspected that plant fibers or fiberglass were being counted. Of 33 samples re-examined by transmission electron microscopy, most had no Libby Amphibole fibers detected. The article's table lists three samples with three or more fibers, although its text says two. Twelve 8-hour averages were calculated. Using the EPA's risk method (8 hours a day, 30 days a year), the highest estimated excess lifetime cancer risk in each work category was below 1 in a million. The authors concluded that cancer risks from this specific evaluation were low. They cautioned that the findings may not represent exposures in other areas. Because no wildland fires or prescribed burns were sampled, the results cannot be extrapolated to firefighting.

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: Occupational air-sampling exposure assessment by NIOSH, with cancer risk estimates calculated by the EPA method and compared with earlier EPA simulated-activity sampling
• Who was studied: 23 US Forest Service workers doing 10 different tasks in the Kootenai National Forest within a 6.5-mile radius of the former vermiculite mine near Libby, Montana, over a four-day period in summer 2012. One hundred thirty-six personal breathing-zone samples were collected.
• Exposure: Airborne Libby Amphibole (LA), described as primarily winchite with lesser richterite, tremolite and magnesioriebeckite. Tasks included administrative work, facilities maintenance, trail maintenance, tree stand evaluation, thinning/sawing and piling, rock raking, hydrology lab work with stream gravels, and fireline construction.
• Main results:
  • By phase-contrast microscopy, all samples had detectable fibers. Six samples exceeded the NIOSH REL/OSHA PEL of 0.1 fibers per cubic centimeter, and all six were from fireline construction.
  • The authors found these phase-contrast values unexpectedly high and suspected that non-asbestos fibers were being counted.
  • Of the 33 samples analyzed by transmission electron microscopy, most had no LA fibers detected. Table I lists three samples with three or more fibers: a fireline construction sample with 3, a swamper sample with 4, and a swamper (sawing) sample with 3.
  • The article's text states that only two samples had three or more fibers, enough to be above the ISO method's limit of detection. This conflicts with the three such samples listed in Table I.
  • Twelve 8-hour time-weighted averages were calculated: six for sawyers in fireline construction, three for swampers, two for tree stand measurement, and one for fire cache clean-up.
  • Using EPA assumptions of 8 hours a day for 30 days a year, the highest single excess lifetime cancer risk estimate in each category (fireline and non-fireline work) was less than 1 E-06.
  • The authors combined all twelve results, assigning one-half fiber to samples with no fibers detected. This gave a conservatively high mean risk estimate of 7 E-7, a 95% upper confidence limit of 8 E-7, and a 95% upper tolerance limit on the 95th percentile of 2 E-6.
  • A confirmatory EPA contract laboratory analysis of 11 structures identified them as actinolite (1), winchite (6) and richterite (4). That laboratory concluded the fibers most likely came from the former Libby vermiculite mine.
  • Earlier EPA risk estimates from simulated activities were somewhat higher than this study's. The authors considered both studies to support the conclusion of low risk.
• The authors' conclusion: The authors concluded that cancer risks from this specific evaluation are low. They also stated there are limitations to extending this conclusion to all work in the Kootenai National Forest.
• Limitations the authors state:
  • The findings may not represent exposures in other areas of the forest.
  • No sampling was done during a wildland fire or prescribed burn, so the results cannot be extrapolated to firefighting.
  • Workers are generally local residents who may have LA exposure from other sources, and the estimates do not include non-work exposure.
  • The low fiber counts make the results uncertain. Long-term exposure is hard to estimate from few samples when most are below the limit of detection.
  • If the inhalation unit risk value changes, recalculating the risks may change the conclusion.
  • Non-cancer effects were not evaluated.

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: Libby amphiboles, NIOSH, Asbestiform, Amphibole Group, vermiculite, Superfund, Transmission Electron Microscopy (TEM), NIOSH 7402.

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)
• Separation and characterization of respirable amphibole fibers from Libby, Montana — J.S. Webber, D.J. Blake, et al. (2008)
• Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole — T.C. Larson, V.C. Antao, et al. (2010)
• An evaluation of potential occupational exposure to asbestiform amphiboles near a former vermiculite mine — J.F. Hart, T.M. Spear, et al. (2009)
• Exposure to airborne amphibole structures and health risks: Libby, Montana — B. Price (2008)
• Low levels of exposure to libby amphibole asbestos and localized pleural thickening — K.Y. Christensen, T.F. Bateson, et al. (2013)

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