Trees as reservoirs for amphibole fibers in Libby, Montana

Our plain-English summary of Trees as reservoirs for amphibole fibers in Libby, Montana, by T.J. Ward, T. Spear, J. Hart, et al (The Science of the Total Environment, 2006).

The study

• Authors: T.J. Ward, T. Spear, J. Hart, C. Noonan, A. Holian, M. Getman, J.S. Webber
• Journal: The Science of the Total Environment
• Published: 2006;367(1):460-5
• Cited by: 15 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Tree bark and core samples were collected from areas surrounding the asbestos-contaminated vermiculite mine in Libby, MT. These samples were collected to provide preliminary data in support of a proposed study to determine if trees can serve as reservoirs for amphibole fibers and to determine if there is a potential for exposure to those that harvest contaminated wood in the Libby mine area, specifically during firewood harvesting and commercial logging. Initially, three sets of samples were taken both within and directly outside of the EPA restricted area surrounding the mine site. Based on the results of the initial samples, a follow-up sampling program was conducted both in the town of Libby and directly outside the city limits. Gravimetric reduction of a tree core sample did not indicate the presence of amphibole fibers. However, transmission electron microscopy analysis of bark samples collected near the vermiculite mine yielded substantial amphibole fiber concentrations ranging from 41 million to 530 million fibers/g of bark. In addition, a bark sample collected approximately 7 miles west of the town next to a railroad line had concentrations of 19 million fibers/g. A conversion of these mass-based concentrations to areal concentrations (to reflect surface area contamination) revealed concentrations in excess of 100 million amphibole fibers/cm(2). These preliminary results suggest that trees in the Libby valley and along vermiculite shipping corridors can serve as reservoirs for amphibole fibers, and that a potential for exposure exists for those who harvest contaminated wood.

In plain English

This short communication reported preliminary sampling of tree bark and tree cores around the former W.R. Grace vermiculite mine near Libby, Montana, to support a proposed study of exposure during firewood harvesting and commercial logging. In November 2004 the authors sampled coniferous trees at three sites at and just outside the mine area, plus a control pine in Albany, New York. In June 2005 they added samples along the railroad line west of Libby and inside the town; only some of these had been analyzed. Electron microscopy found 41 million to 530 million amphibole fibers per gram in bark from the mine-area sites, and 19 million per gram in bark about 7 miles west of Libby beside the railroad. One in-town sample showed 0.13 million per gram and another showed none detected. The Albany control showed none detected. One tree core left too little residue to analyze further. The authors read this as showing that fibers were not taken up through the roots. The fibers sat on the bark surface, mostly in crevices. The authors concluded that trees can serve as reservoirs for amphibole fibers, which points to potential exposure for people who harvest timber or firewood. They noted that a repeat analysis of the railroad sample gave 3 million fibers per gram.

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: Short communication reporting preliminary environmental sampling of tree bark and tree cores, analyzed by electron microscopy
• Who was studied: Coniferous trees (lodgepole pine, ponderosa pine, larch, Douglas fir). Sites: three locations at and near the former W.R. Grace vermiculite mine near Libby, Montana, sampled November 2, 2004; follow-up sites along the railroad west of Libby and within the town, sampled June 2005, of which only a subset were analyzed; one control pine in Albany, NY. Table 1 lists 10 bark samples.
• Exposure: Amphibole fibers from the Libby vermiculite deposit, deposited on trees. Fibers were identified by transmission electron microscopy following the AHERA protocol, and the authors described them as typical of the Libby vermiculite amphibole contaminants.
• Main results:
  • Location 1, about 100 yards from the former pump house at the mine: bark from two lodgepole pines had 530 million and 330 million amphibole fibers per gram; bark from a larch at the same site had 140 million per gram.
  • Location 2, immediately outside the mine property: bark from a lodgepole pine had 160 million amphibole fibers per gram.
  • Location 3, near the decontamination trailer and access gate, directly outside the EPA restricted area: two bark samples had 41 million and 95 million amphibole fibers per gram.
  • The Albany, NY control bark sample showed no fibers detected, at an analytical sensitivity of 19 million fibers per gram.
  • June 2005 railroad sample, about 7 miles west of Libby: 19 million amphibole fibers per gram of bark.
  • Two samples within the Libby city limits: 0.13 million amphibole fibers per gram, a figure equal to the analytical sensitivity, which Table 1 notes is based on one fiber detected; and no fibers detected at a sensitivity of 0.42 million fibers per gram.
  • Per-gram counts were converted to fibers per square centimeter by assuming about 2 cm2 of exposed bark per subsample. Mine-area bark was in the hundred million fibers per square centimeter range (Table 1: up to 260 million per cm2).
  • The one tree core sample left residue below 0.5% of its original mass, so it was not analyzed further. The authors interpreted the lack of fibers in the core as meaning fibers were not taken up by the root system.
  • Scanning electron microscopy showed the fibers lay on the bark surface rather than through its depth, mostly in crevices and wrinkles.
  • The fibers identified in bark had a mean length of 4.9 micrometers and a mean aspect ratio of 17.
• The authors' conclusion: The authors concluded that trees can serve as reservoirs for amphibole fibers, likely deposited by wind-blown dust, and that this points to potential fiber exposure for people who harvest timber or firewood from contaminated areas of the Libby valley. They added that the railroad result raises the possibility that vermiculite transport corridors may also be contaminated.
• Limitations the authors state:
  • A second preparation and analysis of the railroad sample gave a much lower result than the first, because the second subsample was thicker and included unexposed bark that diluted the per-gram concentration.
  • The per-square-centimeter figures rest on an assumed nominal 2 cm2 of exposed surface, but the wrinkled bark surface is much larger than that.
  • The authors cautioned that comparisons with asbestos in settled dust on smooth surfaces should be made carefully.
  • Only a subset of the June 2005 follow-up samples had been analyzed at the time of writing.

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 Group, vermiculite, Gravimetric Reduction, Transmission Electron Microscopy (TEM).

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)
• 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)
• Exposure to airborne amphibole structures and health risks: Libby, Montana — B. Price (2008)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• 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)

Browse more research in The Science of the Total Environment, or return to medical research.