DRAFT – DO NOT CITE OR QUOTE

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Fact Sheet on the Draft IRIS Toxicological Review of Libby Amphibole Asbestos for Interagency 

Science Consultation

May 13, 2011

Background: In Libby, Montana, the mining and milling of vermiculite, which was contaminated with 

amphibole asbestos, exposed workers and the residents to asbestos fibers. Several million homes in the 

United States and Canada may have vermiculite attic insulation from the Libby mine. For the purposes of 

this document, the material from the mine near Libby, MT will be called Libby Amphibole asbestos. 

Amphiboles are a complex group of minerals. 

INHALATION REFERENCE CONCENTRATION (RfC):

Version Principal Study / Critical Effect POD

(fibers-yr/cc) UFs RfC(fibers/cc)

Current RfC 

on IRIS

N/A N/A N/A N/A

Interagency 

Science 

Consultation

Draft (May

2011)

Marysville, OH worker cohort

exposure to Libby Amphibole 

asbestos (Lockey et al., 1984; 

Rohs et al., 2008). 

Sub-cohort (n=119) of total 

worker population (n=434), 

exposure data based on analytical 

measurements (post-1972) and 

recent follow-up was selected due 

to limitations in exposure 

estimates (no measurement pre1972), and preference for use of 

radiographic data from the 2002-

2005 update.

Critical effect selected is localized 

pleural thickening as identified by 

radiographic analysis.

BMCL10 = 0.076 100

 10 – intraspecies

 10 – database 

uncertainties

1 x 10-5

*See footnote 

for details

* The POD is based on cumulative exposure. To obtain the RfC, the POD is divided by 60 years (lifetime of 70 years 

minus 10 years to account for lagged exposures) and then divided by the UF of 100.
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CANCER ASSESSMENT:

Version Cancer 

Descriptor

Study used for Quantitation Extrapolation 

Method

Inhalation 

Unit Risk 

(fibersyr/cc)

Oral Slope

Factor

(mg/kg-day)-1

Current assessment 

on IRIS1

N/A N/A N/A N/A N/A

Interagency Science 

Consultation Draft 

(May 2011)

“Carcinogenic 

to human.”

Based on strong 

human evidence 

of an 

association 

between 

exposure to 

Libby 

Amphibole 

asbestos and the 

occurrence of 

mesothelioma 

and lung cancer

in the 

workplace and

a finding of 

mesothelioma 

risk among

nonoccupationally 

exposed 

residents of 

Libby, MT. 

Libby worker cohort:

studied by NIOSH researchers 

with several updates (Amandus 

and Wheeler, 1987; Amandus 

et al., 1987a; Sullivan, 2007). 

NIOSH conducted two 

epidemiologic investigations, 

resulting in several publications 

(listed above). EPA 

collaborated with NIOSH to 

update the mortality data 

through 2006. It is this update 

which has been modeled by 

EPA to derive the IUR.

Workers were employed at the 

Libby, MT vermiculite mine, 

mill and processing plant. Subcohort (n=880), hired after 

1959 or later, of total worker 

population (n=1871) selected 

for derivation of IUR due to 

limitations in earlier exposure 

estimates, and resulting issues 

with exposure-response 

modeling of full cohort data. 

Linear 

extrapolation 

from PODs 

determined by 

lifetable 

analysis.

0.17 None 

established

1 Although there is currently no health assessment specific to Libby Amphibole asbestos, there is a previous 

inhalation health assessment for asbestos in general that EPA has previously used in assessing risks from Libby 

Amphibole asbestos. That assessment (U.S. EPA, 1988) determined that asbestos is a human carcinogen (Class A) 

based on an synthesis of 14 human studies (including chrysotile, amosite and asbestos mixtures) demonstration lung 

cancer and/or mesothelioma following exposure to various forms of asbestos (AAHUA, 1986). That assessment 

derived an IUR of 0.23 fibers/cc.