Evaluation of asbestos exposures during firewood harvesting simulations in Libby

Research was conducted in order to assess potential exposure to asbestos while harvesting fire-wood 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 per-sonal protective clothing were measured by TEM. The mean (n 5 12) PBZ PCM sample time-weighted average (TWA) concentration was 0.29 fibers per milliliter, standard deviation (SD 5 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 5 12) PBZ TEM sample TWA concentration for amphibole fibers <5-mm long was 0.15 fibers per milliliter (SD 5 0.21) and the mean (n 5 12) PBZ TEM concentration for amphibole fibers>5-mm long was 0.07 fibers per milliliter (SD 5 0.08). Substantial amphi