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.