Vermiculite Mine Research Articles

LUNG CANCER MORTALITY RISK AMONG AN OCCUPATIONAL COHORT EXPOSED TO A MIXTURE OF COMMERCIAL AND NON-COMMERCIAL AMPHIBOLE ASBESTOS FROM VERMICULITE MINING OPERATIONS IN LIBBY, MONTANA

Background and Aims: Vermiculite mining and milling in Libby, MT supplied the majority of North American demand for vermiculite during the 20th century. Libby vermiculite is contaminated with amphibole asbestos, including, winchite, richterite and tremolite. Libby workers and residents in the community were exposed to this mixture of largely non-commercial asbestos fibers for decades. Several million homes in North America may have vermiculite attic insulation from the Libby, MT mine. Methods: Our analysis is based on 880 workers hired after 1959 when fiber concentrations were lower and possibly more consistent with potential environmental exposure. We used Cox proportional hazards models with time-dependent covariates to assess the effects of Libby amphibole asbestos on lung cancer mortality. Multiple exposure metrics including cumulative exposure, residency-time weighted exposure, and metrics mathematically allowing for effects of fiber clearance or translocation were evaluated. We compared lung cancer potency results using slopes and their corresponding risks with the slopes and risks from other investigations of the Libby worker cohort. Results: There are four researchers who have recently published lung cancer potency results for the Libby amphibole asbestos worker cohort: Sullivan (2007), Berman and Crump (2008), Larson et al. (2010) and Moolgavkar et al. (2010). The adjusted lung cancer mortality slopes per calendar year from these four studies ranged from 1.6x10E-3 to 4.2 x10E-3 per fiber/cc-year. Risks derived through life tables based on age-specific cancer mortality rates from these four studies ranged from 0.01 to 0.08 per fiber/cc. Uncertainties in the application of these epidemiologic data are discussed. Conclusions: Libby amphibole asbestos, a mixture composed primarily of non-commercial amphibole asbestos, has repeatedly been shown to be associated with lung cancer mortality by different investigators using varying definitions of this occupational cohort. Disclaimer: This abstract does not necessarily reflect EPA policy.

Differences in Fe-redox for asbestiform and nonasbestiform amphiboles from the former vermiculite mine, near Libby, Montana, U.S.A.

We obtained oriented Fe-XANES spectra on three amphibole bundles of fiber (i.e., asbestiform morphology), two single-crystal fragments (i.e., nonasbestiform morphology), and one aggregate of thousands of smaller particles (termed herein a “puffball”) by use of a spindle stage mounted on beamline X26A, National Synchrotron Light Source, Brookhaven National Lab. These particles of differing morphology were obtained from a single rock sample collected at the former vermiculite mine near Libby, Montana, U.S.A. The XANES spectra vary as a function of orientation for the singlecrystal fragments and bundles of fibers and were the same regardless of orientation for the puffball. Differences in the spectra for the single-crystal fragments and bundles of fibers indicate variations in Fe oxidation states between the two differing morphologies. Comparison to a calibration line based on amphibole standards allowed determination of Fe 3+ /ΣFe values of 0.69 for the bundles of fibers and 0.60 for the single-crystal fragments, while the puffball had a Fe 3+ /ΣFe value of 0.66. These values agree well with our earlier Mossbauer data on a bulk sample of this same material, with Fe 3+

Lung function, radiological changes and exposure: analysis of ATSDR data from Libby, MT, USA

In 2000, the Agency for Toxic Substances and Disease Registry (ATSDR; Atlanta, GA, USA) investigated lung disease in those exposed to the tremolite-contaminated vermiculite mine in Libby, MT, USA. Previously unreported spirometric results are presented here in relation to exposure and radiographic findings. 4,524 study participants were assigned to one of seven mutually exclusive exposure categories. Associations among radiographic findings, spirometric results and exposure were investigated, along with the effect of a reduction in exposure potential when production was moved to a wet process mill in the mid 1970s. Spirometry data for the total population by smoking status and age were within the normal range. Prevalence of pleural plaque increased with age, but was lowest in the environmentally exposed group (0.42-12.74%) and greatest in the W.R. Grace & Co. mineworkers (20-45.68%). For males, there was a significant (4.5%) effect of pleural plaques on forced vital capacity. For W.R. Grace & Co. workers and household contacts, a reduction in plaque (0.11 versus 1.64%) and in diffuse pleural thickening or costophrenic angle obliteration (1.94 and 0.13%) was noted for those exposed after 1976. These analyses do not support a clinically important reduction in spirometry of this cohort. The 1976 reductions in exposure have led to decrease in radiographic changes.

The dispersion of fibrous amphiboles by glacial processes in the area surrounding Libby, Montana, USA

Mining operations began at a world-class vermiculite deposit at Vermiculite Mountain near Libby, Montana, circa 1920 and ended in 1990. Fibrous and asbestiform amphiboles intergrown with vermiculite ore are suspected to be a causative factor in an abnormally high number of cases of respiratory diseases in former mine and mill workers, and in residents of Libby. The question addressed in this report is whether some of the amphibole from Vermiculite Mountain could have been dispersed by Pleistocene glacial processes rather than by human activity after vermiculite mining began. The history of Pinedale glaciation in the Libby area provides a framework for estimating the presence and distribution of asbestiform amphiboles derived from Vermiculite Mountain and found in naturally occurring sediments of Glacial Lake Kootenai that underlie the Libby Valley area. There were two situations where sediments derived from Vermiculite Mountain were deposited into Glacial Lake Kootenai: (1) as lake-bottom sediments derived from meltwater flowing down Rainy Creek when the valley south of Vermiculite Mountain was free of ice but active ice still covered Vermiculite Mountain; and (2) as lake-bottom sediments eroded from the Rainy Creek outwash and re-deposited during a re-advance of the Purcell Trench Glacier lobe near Moyie Springs, Idaho.

Epidemiology of Malignant Mesothelioma—An Outline

In the 1960s and 1970s, well designed case-referent studies put beyond doubt that exposure to airborne asbestos fibres was a cause of malignant mesothelioma. Some 35 cohort mortality studies in a large variety of industries during the 20-year period, 1974-1994, showed a wide range of outcomes, but in general that the risk was higher in exposures which included amphiboles rather than chrysotile alone. Real progress began, however, with discoveries along several lines: the link between pleural changes and mineralogy, the concept and importance of biopersistence, the developments in counting and typing mineral fibres in lung tissue, and data on amphibole mining in South Africa and Australia for comparison with that on chrysotile in Canada and Italy. This led to the recognition of the potential contamination in North America of chrysotile with tremolite. A survey in Canada in 1980-1988 and other surveys demonstrated that crocidolite, amosite, and tremolite could explain almost all cases of mesothelioma. Effective confirmation of this was finally achieved with data on vermiculite miners in Libby, Montana, in the years 1983-1999, where exposure was to tremolite-actinolite and/or other amphibole fibres alone.

Potency Factors for Risk Assessment at Libby, Montana

We reanalyzed the Libby vermiculite miners' cohort assembled by Sullivan to estimate potency factors for lung cancer, mesothelioma, nonmalignant respiratory disease (NMRD), and all-cause mortality associated with exposure to Libby fibers. Our principal statistical tool for analyses of lung cancer, NMRD, and total mortality in the cohort was the time-dependent proportional hazards model. For mesothelioma, we used an extension of the Peto formula. For a cumulative exposure to Libby fiber of 100 f/mL-yr, our estimates of relative risk (RR) are as follows: lung cancer, RR = 1.12, 95% confidence interval (CI) =[1.06, 1.17]; NMRD, RR = 1.14, 95% CI =[1.09, 1.18]; total mortality, RR = 1.06, 95% CI =[1.04, 1.08]. These estimates were virtually identical when analyses were restricted to the subcohort of workers who were employed for at least one year. For mesothelioma, our estimate of potency is K(M) = 0.5 x 10(-8), 95% CI =[0.3 x 10(-8), 0.8 x 10(-8)]. Finally, we estimated the mortality ratios standardized against the U.S. population for lung cancer, NMRD, and total mortality and obtained estimates that were in good agreement with those reported by Sullivan. The estimated potency factors form the basis for a quantitative risk assessment at Libby.

Laboratory Tests to Compare Airborne Respirable Mass and Fibre Concentrations from Soil Samples from Libby, Montana

Soil samples from sites around Libby, Montana, where tailings or waste from the vermiculite mine formerly operated by W.R. Grace & Co. were thought to have been deposited, have been tested. The tests examined airborne concentrations of fibre and respirable dust generated from the soils after drying and dispersion. Results showed that the concentration of all fibres, normalised to the mass concentration of respirable airborne dust, ranged from 0.01 fibres mL-1 per mg · m-3 to 6.8 fibres mL-1 per mg · m -3. The normalised asbestos fibre concentrations ranged from 0.00 to 1.41 asbestos fibres mL-1. The relative levels appeared to be consistent with the amount of fine fibre in the bulk soil samples. A synthetic soil sample tested under the same conditions gave results consistent with these and samples from previous work. The effect of moisture was examined with the synthetic soil and showed that 5% moisture substantially reduced dust and fibre emissions, and at 10% moisture content the dust and fibre emission was almost completely suppressed, reducing the airborne fibre levels by several orders of magnitude. The addition of 10% moisture to the Libby soil reduced airborne asbestos fibre concentrations to ∼4% of that from the dried dust. Finally, ambient airborne particulate dust concentrations in Libby are several orders of magnitude lower than the particulate (soil) dust levels observed in these tests. The data from these tests suggest that ambient air concentrations in Libby resulting from disturbance of soils will typically be more than 1000 times lower than those reported in this study.

Evaluation of Alternative Exposure Metrics for Assessing Mortality Risk from Lung Cancer and Mesothelioma among an Occupational Cohort Exposed to Amphibole Asbestos from Vermiculite Mining Operations in Libby, Montana

ISEE-0744 Background and Objective: In Libby, MT, the mining and milling of vermiculite, which was contaminated with amphibole asbestos, exposed workers and residents to asbestos fibers for decades. Several million homes in the United States and Canada may have vermiculite attic insulation from the Libby, MT mine. USEPA is conducting its cancer risk assessment based on epidemiologic analysis of the occupational cohort assembled by NIOSH. Methods: This analysis was based on 880 workers hired in 1960 or later when fiber concentrations were lower and believed to be more consistent with potential environmental exposure. We used extended Cox proportional hazards models to assess the effects of Libby amphibole asbestos on lung cancer mortality and a Poisson model of absolute risk of mesothelioma mortality. Sensitivity analyses used multiple lags for cancer latency and multiple exposure metrics including cumulative exposure, residency-time weighted exposure, and metrics allowing for fiber clearance, translocation or biologic sequestration. We compared results using AIC weights which assign probabilities of each model being the best fit. Results: The best fitting class of models in these analyses for lung cancer mortality used 10-year lags and allowed for simulated clearance over time. These models had 2.6–3.8 times the probability of being the best model compared to cumulative exposure. The best fitting model for mesothelioma mortality had 15-year lag and also allowed for clearance. The two best fitting metrics had 56–167 times the probability of being the best model compared to cumulative exposure. We found that residency-time weighted exposure models had low relative probability for both lung cancer and mesothelioma mortality. Conclusions: The adverse effects of Libby amphibole exposure on cancer mortality in this cohort are clear. Models that mathematically allow for fiber clearance over time provide clearly superior fit to these cancer mortality data. Disclaimer: This abstract does not necessarily reflect EPA policy.

Amphibole forensics: Using the composition of amphiboles to determine their source, the Libby, Montana, example

As part of a larger on-going research project, we found amphiboles in soils in Libby, Montana, that were derived from both the former vermiculite mine near Libby and other sources. SEM-EDS spectra of the soil amphiboles from these locations are used to ascertain if the amphiboles came from the vermiculite deposit near Libby, Montana, or some other source. This distinction is possible because amphiboles known to originate from the vermiculite deposit all contain Na and K that can be observed in the EDS spectra. We make this statement with confidence because multiple workers have performed EPMA characterization on over 40 amphiboles collected from the deposit and all are found to contain measureable amounts of Na and K. In this brief communication, we show examples from four locations that contain amphiboles consistent in composition and others that have inconsistent composition when compared to reference amphiboles collected from the vermiculite deposit. Also, we demonstrate the presence of amphiboles consistent in composition with amphiboles known to originate from the deposit in sediment that pre-date mining activity. A full-length publication is in preparation detailing the quantity, distribution, and composition of amphiboles in the Libby Valley, if they are consistent in composition with amphiboles known to originate from the deposit or not, and if they pre- or post-date mining operations.

Collagen accumulation over time in the murine lung after exposure to crocidolite asbestos or Libby amphibole

Libby, MT is the site of a closed vermiculite mine that produced ore contaminated with asbestos-like amphiboles. Worldwide distribution of the material and the long latency period for manifestation of asbestos-related diseases (ARDs) has created a significant health threat for many years to come. The composition of the Libby material [termed the Libby amphibole (LA)] differs from other well-studied types of asbestos in that it is a mixture of several amphibole fibers. The purpose of this study was to determine the fibrotic effects of LA exposure in a mouse model and to compare these effects to those of a well-characterized amphibole fiber, crocidolite asbestos. We exposed C57Bl/6 mice to LA or crocidolite and analyzed lung RNA, protein, and morphology at 1 week, 1 month, and 3 months post instillation. Our results indicate that both forms of amphibole studied induced increased collagen types I and III mRNA expression and collagen protein deposition in exposed murine lungs compared to the PBS-instilled control lungs, and that these collagen increases were the most significant at 1 month after exposure. However, crocidolite-exposed mice demonstrated greater increases in collagen deposition than those exposed to LA, indicating that the fibrotic effects of LA exposure, although not as severe as those of crocidolite in this model system, were still able to induce collagen deposition.

An Evaluation of Potential Occupational Exposure to Asbestiform Amphiboles near a Former Vermiculite Mine

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/cm2). 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.

The landscape of asbestos: Libby and beyond

This article discusses risk assessment and decision‐making in the rural community of Libby, Montana. The community faces serious health risks caused by decades of exposure to asbestos from an asbestos‐contaminated vermiculite mining operation. The risks from the exposure are not only occupational but involve the greater community as well. The article explores issues that are increasingly evident and problematic in this community: the willingness or ability to recognize a problem, like serious environmental contamination, can be influenced by one's perceived willingness or ability to accept or manage the consequences of such recognition. This finding has emerged not only in this community but in a number of other studies conducted by the authors. Findings suggest that if the consequences of recognition are not deemed manageable or possible (e.g. perceived ability to move to a safer location or the willingness to change occupations), residents will minimize the scope of a problem and remain in a situation that continues to expose them to serious health risk. This tension needs to be recognized and managed when trying to find innovative solutions and effective policies for risk assessment.

Characterization of historical amphibole samples from the former vermiculite mine near Libby, Montana, U.S.A

To determine if the composition or morphology of amphiboles occurring in the Rainy Creek vermiculite deposit near Libby, Montana, has changed as the deposit has been mined over the last 70 years, we assembled amphiboles collected in the ~1920s, ~1960s, and 1999. Amphibole morphologies range from ~20–50 μm prismatic single crystals to massive aggregates of fine-grained, asbestiform amphiboles only a few micrometers in size. Compositional data obtained by electron-microprobe analysis (EPMA) using wavelength-dispersive spectrometry and Fe 3+ /∑Fe ratio determined by Mossbauer spectroscopy showed that calcic, sodic–calcic, and sodic group amphiboles are present in each suite. The samples all have Mg numbers > 0.90. According to the current nomenclature for amphibole species, the majority of amphiboles analyzed are winchite or richterite. There are a few exceptions: one of the 1960 samples was tremolite, while one of the ~1920s samples was fluoro-magnesio-arfvedsonite; the latter was characterized optically. Compositions generally concur with previous characterizations of the amphiboles originating from the Rainy Creek Complex, but composition variations occur both within each temporally-related sample suite and among suites. Therefore, suggestions that the bulk composition of amphiboles in the deposit has changed over the 70 years of mining are unsupported. Overall, there is no correlation between major and minor element composition and morphology. However, fibrous and prismatic crystal trace element concentrations from a sample from the 1920s suite were analyzed using laser ablation mass spectrometry. Based on the trace elements the separate morphologies could be distinguished by cluster analysis, but only within this single sample.

Autoantibodies from mice exposed to Libby amphibole asbestos bind SSA/Ro52-enriched apoptotic blebs of murine macrophages

Asbestos exposure is associated with increased autoimmune responses in humans. For example, in Libby, MT where significant asbestos exposure has occurred due to an asbestos-contaminated vermiculite mine near the community, residents have developed increased autoimmune responses compared to an unexposed population. However, the exact mechanism by which Libby amphibole asbestos generates autoimmune responses is unclear. A murine model of amphibole asbestos-induced autoimmunity was recently established, and one of the targets of the autoantibodies (AAs) was the SSA/Ro52 autoantigen. The purpose of this study was to determine whether the SSA/Ro52 autoantigen is exposed at the surface of cells as a result of asbestos exposure as a possible mechanism leading to antigenicity. Our results indicate that Libby asbestos induces apoptosis in murine macrophages as determined by phosphatidylserine exposure, cleavage of poly(ADP-ribose) polymerase and morphological changes such as nuclear condensation. Moreover, asbestos-induced apoptosis results in the formation of apoptotic cell surface blebs enriched in SSA/Ro52 as determined by confocal microscopy. Most importantly, apoptotic cell surface blebs are recognized by AAs from mice exposed to amphibole asbestos suggesting that these cell surface structures may be antigenic when presented in a pro-inflammatory context. This study supports the hypothesis that the induction of apoptosis plays a key role in environmentally induced autoimmunity through cell surface exposure of a known autoantigen.

Separation and Characterization of Respirable Amphibole Fibers from Libby, Montana

The vermiculite mine in Libby, Montana, was in operation for over 70 yr and was contaminated with asbestos-like amphibole fibers. The mining, processing, and shipping of this vermiculite led to significant fiber inhalation exposure throughout the community, and residents of Libby have developed numerous pulmonary diseases such as lung cancer and mesothelioma. The present study describes the separation of Libby 6-mix into respirable and nonrespirable size fractions by means of aqueous elutriation. The elutriator, designed to separate fibers with aerodynamic diameters smaller than 2.5 μ m (respirable) from larger fibers, used an upward flow rate of 3.4 × 10− 4 cm s−1. The resultant respirable fraction constituted only 13% of the raw Libby 6-mix mass, and less than 2% of the fibers in the elutriated fraction had aerodynamic diameters exceeding 2.5 μ m. Surface area of the elutriated fibers was 5.3 m− 2 g−1, compared to 0.53 m−2 g−1 for the raw fibers. There were no detectable differences in chemical composition between the larger and smaller fibers. Such harvesting of respirable fractions will allow toxicological studies to be conducted within a controlled laboratory setting, utilizing fiber sizes that may more accurately simulate historical exposure of Libby residents' lungs. Importantly, this work describes a method that allows the use of material enriched in more uniform respirable material than raw Libby 6-mix, making comparisons with other known fiber preparations more valid on a mass basis.

Directions and Needs in Asbestos Research: New Insights: Conference Summary

The Center for Environmental Health Sciences (CEHS) Conference, entitled “Directions and Needs in Asbestos Research: New Insights,” was held at the University of Montana in Missoula. Researchers, physicians, health care workers, and federal agency representatives from around the country met for a cross-disciplinary exploration of many issues related to asbestos research. Topics included community and psychosocial issues in biomedical research, asbestos exposure assessment, assessment and mechanisms of asbestos related diseases, and new research directions. This meeting report is a summary of the conference presentations, and of the topics identified for future research directions. This conference was a follow-up to one hosted by the CEHS in June 2002, and continued to take advantage of opportunities to work with a unique population in Libby MT, where significant asbestos exposures have occurred due to the mining of asbestos-contaminated vermiculite. The goals of this conference were to bring together experts from diverse fields to identify progress made since the last conference and to develop new research avenues that would allow us to address the research needs in emerging asbestos-exposed populations. Participants indicated that these objectives were met, and expressed enthusiasm for follow-up conferences to maintain the dialog that has been established regarding directions and needs in asbestos research. Selected papers from the conference are presented here.

Gene Expression Changes after Exposure to Six-Mix in a Mouse Model

The exposure of Libby MT residents to amphibole-contaminated vermiculite is well known. To explore the gene-environment interactions in the development of asbestos-related diseases (ARD), a mouse model of asbestos exposure using Six-mix (a combination of amphibole fibers gathered from six sites at the Libby vermiculite mine), crocidolite asbestos, or saline as a negative control was used to determine both gene expression responses by using mouse 10,000 oligonucleotide array and to visualize these changes histologically. Mice were sacrificed and whole lungs harvested for histology and microarray analysis six months following exposure via intratracheal instillation. Using an arbitrary cutoff of 1.25-fold change, genes whose RNA expression levels were specifically altered in response to the different amphibole exposures were grouped into categories by a gene ontology analysis program, GoMiner. Our hypothesis was that assessment of asbestos-responsive genes would provide a better understanding of response mechanisms. These experiments have provided new candidates for genes involved in the asbestos response pathways.

Evaluation of Asbestos Exposures during Firewood-Harvesting Simulations in Libby, MT, USA—Preliminary Data

Research was conducted in order to assess potential exposure to asbestos while harvesting firewood 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 personal protective clothing were measured by TEM. The mean (n = 12) PBZ PCM sample time-weighted average (TWA) concentration was 0.29 fibers per milliliter, standard deviation (SD = 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 = 12) PBZ TEM sample TWA concentration for amphibole fibers <5-microm long was 0.15 fibers per milliliter (SD = 0.21) and the mean (n = 12) PBZ TEM concentration for amphibole fibers >5-microm long was 0.07 fibers per milliliter (SD = 0.08). Substantial amphibole fiber concentrations were revealed on Tyvek clothing wipe samples. The mean concentration (n = 12) was 29 826 fibers per square centimeter (SD = 37 555), with 91% (27 192 fibers per square centimeter) comprised fibers <5-microm long. There were no significant differences in PBZ and wipe sample concentrations among the tasks performed by four investigators. Each of these three simulations were consistent in demonstrating that amphibole fibers are released from tree reservoirs during firewood-harvesting activities in asbestos-contaminated areas and that the potential for exposure exists during such activities.

The Database Solution to Particle-by-Particle Analysis of Mixed Mineral Dusts

Abstract This work involves the development and application of a database for the morphological, crystallographic, and chemical characterization of amphibole particles that occur as accessory minerals in the former vermiculite mine in Libby, Montana. The data in the database were collected using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) techniques for particle-by-particle characterization of mixed mineral dust samples. In the fall of 1999, public attention was focused on the small town of Libby due to health concerns over potential amphibole asbestos exposure that occurred in the now closed vermiculite mine. The vermiculite deposit, located in the Rainy Creek Igneous Complex, about seven miles northeast of Libby, was discovered in 1913 and commercial production of vermiculite began in 1923.

Exposure to airborne amphibole structures and health risks: Libby, Montana

Libby, Montana is the site of a large vermiculite deposit that was mined between 1920 and 1990 to extract vermiculite for commercial applications such as insulation, gardening products, and construction materials. The Libby vermiculite deposit also contains amphibole minerals including tremolite, actinolite, richterite, and winchite. Historically, Libby mine workers experienced high exposures to amphibole structures, and, as a group, have experienced the health consequences of those occupational exposures. It has been suggested that Libby residents also have been and continue to be exposed to amphibole structures released during the vermiculite mining operations and therefore are at increased risk for disease. The Agency for Toxic Substance and Disease Registry (ATSDR) conducted two epidemiological-type studies of residents living in Libby and the surrounding areas to assess these risks. The Environmental Protection Agency (EPA) collected and analyzed exposure data in Libby and used those data to project risks of asbestos-associated disease for Libby residents. The EPA has placed the Libby Asbestos Site, which includes the mine and the town of Libby, on its National Priority List of hazardous waste sites in need of clean up. This article presents a review of the exposure studies conducted in Libby and an analysis of health risks based on the data collected in those studies. Libby mine workers have experienced elevated levels of asbestos-associated disease as a consequence of their occupational exposures to amphibole structures. Libby residents’ exposures typically are substantially lower than mine workers’ historical exposures, and the health risk projections for residents are, accordingly, substantially lower.