Vermiculite was discovered in the mountains near Libby in 1919, and vermiculite mining quickly became the dominant industry in town. The mineral, which has many commercial uses, from acoustic tiling to insulation, is perhaps most widely known today as the small white bits mixed into potting soil. When WR Grace & Company acquired the mines in 1963, they were producing 80% of the world's vermiculite. However, the strain of vermiculite being mined was heavily contaminated with a number of asbestiform minerals that have come to be known as Libby amphibole mix.

The term asbestos refers to several naturally occurring fibrous silicate minerals that share similar physical characteristics. Exposure to asbestos can cause several serious illnesses in people, from lung cancer and mesothelioma to pulmonary fibrosis and asbestosis. Chrysotile fibres account for 95% of the common commercial uses of asbestos in the USA, but Libby amphibole mix is made of five different forms of amphibole, or needle-like asbestos. Its principal components are winchite (84%) and richterite (11%), which are both unregulated asbestiform fibres, and tremolite (5%), a regulated form of asbestos.

Although mining activities at the Libby mine ceased in 1990, in 2005, the Department of Justice began criminal proceedings against WR Grace, accusing them of concealing knowledge of the negative health effects of the vermiculite mine as far back as the 1970s, as well as wire fraud and obstruction of justice. WR Grace and its executives were ultimately acquitted of the charges, but the corporation agreed to a US$250 million settlement to be used for future clean-up activities as part of a bankruptcy settlement in 2008.

Although most people are familiar with the effects of occupational exposure to asbestos, such as in workers at shipyards and asbestos miners, the situation in Libby was slightly different. Workers at the vermiculite mines were heavily exposed to asbestiform contamination, but many other people in the town were also exposed. Contaminated material from the mines was used throughout the community. “It was used in insulation, it was used to fill in low spots in people's yards, it was used to line the track at the schools, it was used as playground sand, it was used at the baseball field at the community park. It just became ubiquitous throughout both communities, so it was a very widespread problem”, said Rebecca Thomas, Environmental Protection Agency's (EPA) Lead Project Manager for the Libby clean-up.

Black says that the pattern of disease progression seen from exposure to the Libby asbestiform types differs significantly from exposure patterns to commercial, chrysotile asbestos. While that type of exposure is typified by progressive interstitial disease, with a limited amount of plaque or scarring of the pleura, in Libby, the opposite is true, with less interstitial disease and more progressive pleural scarring. “Low-level exposure can lead to scarring”, Black said of the Libby asbestos mix. “A much lower rate of exposure than what we currently allow in the workplace is causing pleural plaquing.”

Because asbestos is a particle, chemical testing that might be used to test for benzene or other hazardous material won't work to assess asbestos concentrations in a particular area. Determining that level requires microscopic observation of samples to count the number of asbestos particles, and different areas and materials require different actions to determine exposure risk. The EPA calls this activity-based sampling.

“If it's a residential area, we mow the lawn at a place where we know that there's asbestos in the soil”, said Deborah McKean, EPA senior toxicologist for the Libby clean-up. “Or in a house we vacuum or do housework and try to generate dust in that environment and then measure the air in that vicinity and the breathing space of that person conducting that activity to determine how many fibres are liberated during that dust-generation activity, to determine how much would be inhaled by that person.” The EPA evaluated more than 150 different activities and collected more than 5000 activity-based air samples throughout Libby to understand what the asbestos exposures could be. Top soil with an asbestos concentration of 1% or greater was removed down to a level of 1 m; soil with concentrations as low as 0·2% was removed to a depth of 0·5 m. Asbestos-contaminated material was taken to the former mine.

According to Thomas, removing asbestos-containing products in the home is contingent on the risk of exposure. If people were unlikely to disturb the material, it was left in place. “Anywhere where there was an exposure pathway inside a home, we would remove it”, Thomas said. “So if it was in the attic and someone could access the attic and it was filled with this vermiculite insulation contaminated with asbestos, we would remove it. If the insulation, on the other hand, was sealed behind a wall and there were no plans to do any remodelling, no deterioration of the wall, we might leave some insulation in place, again where it didn't pose an exposure pathway.” However, concerns about future exposure still remain, as people choose to remodel their homes or a home is destroyed to make way for a new development.

The EPA is also looking at asbestos levels in the surrounding forested areas of Libby. The Libby amphibole mix is a natural occurring component of the vermiculite found throughout the area, and over the years, asbestos particles both from the mine and other natural deposits have become integrated into the forest duff, soil, and bark of the trees. As natural occurrences, forest fires can be expected to happen eventually, and the EPA is looking closely at how a fire might impact surrounding communities. “We have done a number of studies to look at the release of asbestos fibres during different types of fire, and where the asbestos goes”, said McKean. “We've done some laboratory-based experiments where we've burned wood and duff material to understand how much is released in the smoke and how much may stay behind in the ash.”

Libby has also benefited from an unusual provision in the Affordable Care Act. Montana Democratic Senator Max Baucus, a strong opponent of the inclusion of a public option in the Act generally, nevertheless saw the advantages of providing immediate access to single-payer health care through Medicare for residents of Lincoln County, Montana, who had been diagnosed with an asbestos-related ailment, regardless of age. But the expansion of health care to those affected by the asbestos situation goes much further, according to Black. “It's remarkable how many people have complex medical issues like diabetes and [other conditions], that didn't have health insurance but, because of their asbestos-related disease, were able to enroll in Medicare and get access to care for their chronic diseases”, he said. “It's been very helpful outside of the asbestos problems, to also bring them some health care earlier, and more intensely to improve their outcomes for other diseases, as well.”