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

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.