-
EPA/635/R-11/002F www.epa.gov/iris
TOXICOLOGICAL REVIEW
OF
LIBBY AMPHIBOLE ASBESTOS
In Support of Summary Information on the Integrated Risk
Information System (IRIS)
December 2014
(Note: This document is an assessment of the noncancer and
cancer health effects
associated with the inhalation route of exposure only)
Integrated Risk Information System National Center for
Environmental Assessment
Office of Research and Development U.S. Environmental Protection
Agency
Washington, DC
-
DISCLAIMER
This document has been reviewed in accordance with U.S.
Environmental Protection Agency policy and approved for
publication. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
ii
-
CONTENTS―TOXICOLOGICAL REVIEW OF LIBBY AMPHIBOLE ASBESTOS
LIST OF TABLES
.........................................................................................................................
ix LIST OF FIGURES
.....................................................................................................................
xvi FOREWORD
..............................................................................................................................
xxii AUTHORS, CONTRIBUTORS, AND REVIEWERS
............................................................. xxiii
1. INTRODUCTION
..............................................................................................................
1-1
1.1. RELATED
ASSESSMENTS.....................................................................................
1-2 1.1.1. Integrated Risk Information System (IRIS) Assessment for
Asbestos
(U.S. EPA, 1988a)
.........................................................................................
1-2 1.1.2. EPA Health Assessment for Vermiculite (U.S. EPA, 1991b)
....................... 1-4
1.2. LIBBY AMPHIBOLE ASBESTOS-SPECIFIC HUMAN HEALTH ASSESSMENT
..........................................................................................................
1-4
2. LIBBY AMPHIBOLE ASBESTOS: GEOLOGY AND EXPOSURE POTENTIAL
...... 2-1
2.1. INTRODUCTION
.....................................................................................................
2-1 2.2. GEOLOGY AND MINERALOGY OF AMPHIBOLES
.......................................... 2-3
2.2.1. Overview
........................................................................................................
2-3 2.2.2. Mineralogy of Amphibole Asbestos and Related Amphibole
Minerals ........ 2-3 2.2.3. Morphology of Amphibole Minerals
.............................................................
2-6
2.3. METHODS FOR ANALYSIS OF ASBESTOS
....................................................... 2-9 2.3.1.
Methods for Air Samples
...............................................................................
2-9 2.3.2. Methods for Solid Materials
........................................................................
2-10
2.4. CHARACTERISTICS OF LIBBY AMPHIBOLE ASBESTOS
............................. 2-11 2.4.1. Mineralogy of Libby
Amphibole
Asbestos.................................................. 2-11
2.4.2. Morphology of Libby Amphibole Asbestos
................................................ 2-16
2.5. HUMAN EXPOSURE POTENTIAL
......................................................................
2-20 2.5.1. Exposures Pathways in the Libby Community
............................................ 2-20 2.5.2. Exposure
Pathways in Communities with Vermiculite Expansion and
Processing Plants
.........................................................................................
2-21 2.5.3. Libby Amphibole Asbestos Exposure Pathways in Other
Communities .... 2-23
3. FIBER TOXICOKINETICS
...............................................................................................
3-1
3.1. DEPOSITION OF FIBERS IN THE RESPIRATORY TRACT
............................... 3-2 3.2. CLEARANCE MECHANISMS
................................................................................
3-8
3.2.1. Physical and Physicochemical Clearance of Fibers
....................................... 3-9 3.2.1.1. Mechanical
Reflex Mechanisms
..................................................... 3-9 3.2.1.2.
Mucociliary Clearance
....................................................................
3-9 3.2.1.3. Phagocytosis by Alveolar Macrophages
....................................... 3-10 3.2.1.4. Epithelial
Transcytosis
..................................................................
3-11 3.2.1.5.
Translocation.................................................................................
3-11 3.2.1.6. Dissolution and Fiber Breakage
.................................................... 3-13
3.3. DETERMINANTS OF TOXICITY
........................................................................
3-13 3.3.1. Dosimetry and Biopersistence
.....................................................................
3-13 3.3.2. Biological Response Mechanisms
...............................................................
3-14
3.3.2.1. Inflammation and Reactive Oxygen Species (ROS)
Production
.....................................................................................
3-16
iii
-
CONTENTS (continued)
3.3.2.2. Genotoxicity
..................................................................................
3-16 3.3.2.3. Carcinogenicity
.............................................................................
3-16
3.4. FIBER DOSIMETRY MODELS
............................................................................
3-18 3.5. SUMMARY
.............................................................................................................
3-18
4. HAZARD IDENTIFICATION OF LIBBY AMPHIBOLE ASBESTOS
.......................... 4-1
4.1. STUDIES IN HUMANS―EPIDEMIOLOGY
......................................................... 4-1
4.1.1. Overview of Primary Studies
.........................................................................
4-3
4.1.1.1. Studies of Libby, MT Vermiculite Mining and Milling
Operations Workers
........................................................................
4-3
4.1.1.2. Studies of O.M. Scott, Marysville, OH Plant Workers
................... 4-9 4.1.1.3. Community-Based Studies Around
Libby, MT Conducted by
Agency for Toxic Substances and Disease Registry (ATSDR) ....
4-12 4.1.2. Respiratory Effects, Noncancer
...................................................................
4-14
4.1.2.1. Asbestosis and Other Nonmalignant Respiratory Disease
Mortality
.......................................................................................
4-14
4.1.3. Other Effects, Noncancer
.............................................................................
4-37 4.1.3.1. Cardiovascular Disease
.................................................................
4-37 4.1.3.2. Autoimmune Disease and Autoantibodies
.................................... 4-38
4.1.4. Cancer Effects
..............................................................................................
4-41 4.1.4.1. Lung Cancer
..................................................................................
4-41 4.1.4.2. Mesothelioma
................................................................................
4-46 4.1.4.3. Other
Cancers................................................................................
4-50 4.1.4.4. Summary of Cancer Mortality Risk in Populations
Exposed
to Libby Amphibole Asbestos
...................................................... 4-50 4.1.5.
Comparison With Other Asbestos Studies―Environmental Exposure
Settings
.........................................................................................................
4-51 4.2. SUBCHRONIC- AND CHONIC-DURATION STUDIES AND CANCER
BIOASSAYS IN ANIMALS―ORAL, INHALATION, AND OTHER ROUTES OF
EXPOSURE
......................................................................................
4-53 4.2.1. Inhalation
.....................................................................................................
4-62 4.2.2. Intratracheal Instillation Studies
..................................................................
4-63 4.2.3. Injection/Implantation Studies
.....................................................................
4-65 4.2.4. Oral
..............................................................................................................
4-66 4.2.5. Summary of Animal Studies for Libby Amphibole and
Tremolite
Asbestos
.......................................................................................................
4-67 4.3. OTHER DURATION- OR ENDPOINT-SPECIFIC
STUDIES.............................. 4-69
4.3.1. Immunological
.............................................................................................
4-69 4.4. MECHANISTIC DATA AND OTHER STUDIES IN SUPPORT OF THE
MODE OF ACTION
...............................................................................................
4-71 4.4.1. Inflammation and Immune Function
........................................................... 4-78
4.4.2. Genotoxicity
.................................................................................................
4-81 4.4.3. Cytotoxicity and Cellular Proliferation
........................................................ 4-83
4.5. SYNTHESIS OF MAJOR NONCANCER EFFECTS
............................................ 4-84 4.5.1. Pulmonary
Effects
........................................................................................
4-85
4.5.1.1. Pulmonary Fibrosis (Asbestosis)
.................................................. 4-85
iv
-
CONTENTS (continued)
4.5.1.2. Other Nonmalignant Respiratory Diseases
................................... 4-86 4.5.2. Pleural Effects
..............................................................................................
4-86 4.5.3. Other Noncancer Health Effects (Cardiovascular
Toxicity,
Autoimmune Effects)
...................................................................................
4-87 4.5.4. Summary of Noncancer Health Effects of Exposure to
Libby
Amphibole Asbestos
....................................................................................
4-88 4.5.5. Mode-of-Action Information (Noncancer)
.................................................. 4-88
4.6. EVALUATION OF
CARCINOGENICITY............................................................
4-90 4.6.1. Summary of Overall Weight of Evidence
.................................................... 4-90
4.6.1.1. Synthesis of Human, Animal, and Other Supporting
Evidence.... 4-90 4.6.2. Mode-of-Action Information (Cancer)
........................................................ 4-92
4.6.2.1. Description of the Mode-of-Action Information
.......................... 4-92 4.6.2.2. Evidence Supporting a
Mutagenic Mode of Action ..................... 4-92 4.6.2.3.
Evidence Supporting Mechanisms of Action of Chronic
Inflammation, Cytotoxicity, and Cellular Proliferation
................ 4-93 4.6.2.4. Conclusions About the Hypothesized
Modes of Action ............... 4-96 4.6.2.5. Application of the
Age-Dependent Adjustment Factors ............. 4-101
4.7. SUSCEPTIBLE POPULATIONS
.........................................................................
4-102 4.7.1. Influence of Different Life Stages on Susceptibility
................................. 4-102
4.7.1.1. Life-Stage Susceptibility
............................................................. 4-103
4.7.2. Influence of Gender on Susceptibility
....................................................... 4-107
4.7.3. Influence of Race or Ethnicity on Susceptibility
....................................... 4-107 4.7.4. Influence of
Genetic Polymorphisms on Susceptibility
............................. 4-108 4.7.5. Influence of Health
Status on Susceptibility
.............................................. 4-109 4.7.6.
Influence of Lifestyle Factors on Susceptibility
........................................ 4-110 4.7.7. Susceptible
Populations Summary
.............................................................
4-110
5. EXPOSURE-RESPONSE ASSESSMENT
........................................................................
5-1
5.1. ORAL REFERENCE DOSE (RfD)
...........................................................................
5-1 5.2. INHALATION REFERENCE CONCENTRATION (RfC)
..................................... 5-1
5.2.1. Choice of Principal Study
..............................................................................
5-3 5.2.1.1. Candidate
Studies............................................................................
5-3 5.2.1.2. Evaluation of Candidate Studies and Selection of
Principal
Study
...............................................................................................
5-7 5.2.2. Methods of Analysis
....................................................................................
5-10
5.2.2.1. Exposure
Assessment....................................................................
5-10 5.2.2.2. Data Sets for Modeling Analyses
................................................. 5-11 5.2.2.3.
Selection of Critical Effect
............................................................ 5-14
5.2.2.4. Selection of Explanatory Variables to Include in the
Modeling
.......................................................................................
5-19 5.2.2.5. Selection of the Benchmark Response
.......................................... 5-21 5.2.2.6.
Exposure-Response Modeling
...................................................... 5-22
5.2.3. Derivation of a Reference Concentration (RfC) for the
Critical Effect of Localized Pleural Thickening (LPT) in the
Marysville Workers Who Underwent Health Evaluations in 2002−2005
and Were Hired in 1972 or Later―Including Application of
Uncertainty Factors (UFs) ......... 5-40
v
-
CONTENTS (continued)
5.2.3.1. Derivation of a Reference Concentration (RfC) for the
Alternative Endpoint of Any Pleural Thickening (APT) in the
Marysville Workers Who Underwent Health Evaluations in 2002−2005
and Were Hired in 1972 or Later ...............................
5-44
5.2.3.2. Derivation of a Reference Concentration (RfC) for the
Alternative Endpoint of Any Radiographic Change (ARC) in the
Marysville Workers Who Underwent Health Evaluations in 2002−2005
and Were Hired in 1972 or Later ...........................
5-45
5.2.4. Derivation of a Reference Concentration (RfC) for
Localized Pleural Thickening (LPT) in the Marysville Workers Who
Underwent Health Evaluations in 2002−2005 and Were Hired in 1972 or
Later Based on the Cumulative Exposure Model
.................................................................
5-46
5.2.5. Derivation of a Reference Concentration (RfC) for the
Alternative Endpoint of Any Pleural Thickening (APT) in the
Marysville Cohort with Combined X-Ray Results from 1980 and
2002−2005 Regardless of Date of Hire
.............................................................................................
5-46
5.2.6. Summary of Reference Concentration Values (RfCs) for the
Different Health Endpoints and Different Sets of Workers in the
Marysville Cohort
..........................................................................................................
5-48
5.3. UNCERTAINTIES IN THE INHALATION REFERENCE CONCENTRATION
(RfC)
.....................................................................................
5-51 5.3.1. Uncertainty in the Exposure Reconstruction
............................................... 5-51 5.3.2.
Uncertainty in the Radiographic Assessment of Localized Pleural
Thickening
(LPT).........................................................................................
5-56 5.3.3. Uncertainty Due to Potential
Confounding.................................................. 5-57
5.3.4. Uncertainty Due to Time Since First Exposure (TSFE)
.............................. 5-61 5.3.5. Uncertainty in the
Endpoint
Definition........................................................
5-65 5.3.6. Summary of Sensitivity Analyses
................................................................
5-69
5.4. CANCER EXPOSURE-RESPONSE ASSESSMENT
............................................ 5-70 5.4.1. Overview
of Methodological Approach
...................................................... 5-70 5.4.2.
Choice of Study/Data—with Rationale and Justification
............................ 5-72
5.4.2.1. Description of the Libby Worker Cohort
...................................... 5-73 5.4.2.2. Description of
Cancer Endpoints ..................................................
5-75 5.4.2.3. Description of Libby Worker Cohort Work Histories
.................. 5-77 5.4.2.4. Description of Libby Amphibole
Asbestos Exposures ................. 5-78 5.4.2.5. Estimated
Exposures Based on Job-Exposure Matrix (JEM)
and Work Histories
.......................................................................
5-85 5.4.3. Exposure-Response Modeling
.....................................................................
5-90
5.4.3.1. Modeling of Mesothelioma Exposure Response in the Libby
Worker Cohort
..............................................................................
5-91
5.4.3.2. Results of the Analysis of Mesothelioma Mortality in
the Full Libby Worker
Cohort....................................................................
5-93
5.4.3.3. Modeling and Results of Lung Cancer Exposure Response
in the Full Libby Worker Cohort
...................................................... 5-96
5.4.3.4. Rationale for Analyzing the Subcohort of Libby Workers
After 1959
...................................................................................
5-101
vi
-
CONTENTS (continued)
5.4.3.5. Results of the Analysis of Mesothelioma Mortality in
the Subcohort
....................................................................................
5-103
5.4.3.6. Results of the Analysis of the Lung Cancer Mortality in
the Subcohort
....................................................................................
5-113
5.4.3.7. Sensitivity Analysis of the Influence of High Exposures
in Early 1960s on the Model Fit in the Subcohort
.......................... 5-124
5.4.3.8. Additional Analysis of the Potential for Confounding of
Lung Cancer Results by Smoking in the Subcohort ...................
5-126
5.4.4. Exposure Adjustments and Extrapolation Methods
................................... 5-127 5.4.5. Inhalation Unit
Risk (IUR) of Cancer Mortality
........................................ 5-128
5.4.5.1. Unit Risk Estimates for Mesothelioma Mortality
....................... 5-128 5.4.5.2. Unit Risk Estimates for Lung
Cancer Mortality ......................... 5-131 5.4.5.3.
Inhalation Unit Risk (IUR) Derivation for Combined
Mesothelioma and Lung Cancer Mortality
................................. 5-132 5.4.6. Uncertainties in the
Cancer Risk Values
................................................... 5-140
5.4.6.1. Sources of Uncertainty
................................................................
5-140 5.4.6.2. Summary
.....................................................................................
5-154
6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD AND
EXPOSURE RESPONSE
...................................................................................................
6-1 6.1. HUMAN HAZARD POTENTIAL
............................................................................
6-1
6.1.1. Exposure
........................................................................................................
6-1 6.1.2. Fiber Toxicokinetics
......................................................................................
6-2 6.1.3. Noncancer Health Effects in Humans and Laboratory
Animals .................... 6-3 6.1.4. Carcinogenicity in Humans
and Laboratory Animals ................................... 6-5
6.1.5. Susceptible Populations
.................................................................................
6-6 6.1.6. Mode-of-Action Information
.........................................................................
6-7 6.1.7. Weight-of-Evidence Descriptor for Cancer Hazard
...................................... 6-7
6.2.
EXPOSURE-RESPONSE..........................................................................................
6-8 6.2.1. Noncancer/Inhalation
.....................................................................................
6-8
6.2.1.1. Uncertainty and Sensitivity Analyses for Reference
Concentration (RfC) Derivation
................................................... 6-12
6.2.2. Cancer/Inhalation
.........................................................................................
6-13 6.2.2.1. Background and Methods
.............................................................
6-13
6.2.3. Modeling of Mesothelioma Exposure Response
......................................... 6-15 6.2.4. Unit Risk
Estimates for Mesothelioma Mortality
........................................ 6-16 6.2.5. Modeling of
Lung Cancer Exposure Response
........................................... 6-17
6.2.5.1. Analysis of Potential Confounding of Lung Cancer
Results by Smoking in the
Subcohort........................................................
6-18
6.2.6. Unit Risk Estimates for Lung Cancer Mortality
.......................................... 6-18 6.2.7. Inhalation
Unit Risk (IUR) Derivation Based on Combined
Mesothelioma and Lung Cancer Mortality from Exposure to Libby
Amphibole Asbestos
....................................................................................
6-19 6.2.7.1. Comparison with Other Published Studies of Libby,
MT
Workers
Cohort.............................................................................
6-21 6.2.8. Uncertainty in the Cancer Risk Values
........................................................ 6-21
vii
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CONTENTS (continued)
7. REFERENCES
........................................................................................................................
7-1 APPENDIX A: EPA RESPONSE TO MAJOR EXTERNAL PEER-REVIEW AND
PUBLIC
COMMENTS....................................................................................
A-1 APPENDIX B: PARTICLE SIZE DISTRIBUTION DATA FOR LIBBY
AMPHIBOLE
STRUCTURES OBSERVED IN AIR AT THE LIBBY ASBESTOS SUPERFUND SITE
..........................................................................................B-1
APPENDIX C: CHARACTERIZATION OF AMPHIBOLE FIBERS FROM ORE
ORIGINATING FROM LIBBY, MONTANA, LOUISA COUNTY, VIRGINIA,
ENOREE, SOUTH CAROLINA, AND PALABORA, REPUBLIC OF SOUTH AFRICA
...................................................................C-1
APPENDIX D: ANALYSIS OF SUBCHRONIC- AND CHRONIC-DURATION
STUDIES AND CANCER BIOASSAYS IN ANIMALS AND MECHANISTIC STUDIES
.............................................................................
D-1
APPENDIX E: EVALUATION OF EXPOSURE-RESPONSE DATA FOR
RADIOGRAPHIC CHANGES IN WORKERS FROM THE MARYSVILLE, OH COHORT
COMBINING DATA FROM THE 1980 AND 2002-2005 HEALTH EXAMINATIONS
...................................... E-1
APPENDIX F: WORKER OCCUPATIONAL EXPOSURE RECONSTRUCTION FOR
THE MARYSVILLE COHORT
......................................................................
F-1 APPENDIX G: EXTRA RISK AND UNIT RISK CALCULATION
...................................... G-1 APPENDIX H: GLOSSARY OF
ASBESTOS TERMINOLOGY ...........................................
H-1 APPENDIX I: EVALUATION OF LOCALIZED PLEURAL THICKENING IN
RELATION TO PULMONARY FUNCTION MEASURES
.......................... I-1 APPENDIX J: DOCUMENTATION OF
IMPLEMENTATION OF THE 2011
NATIONAL RESEARCH COUNCIL RECOMMENDATIONS ....................
J-1
viii
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LIST OF TABLES
1-1. Derivation of the current Integrated Risk Information
System (IRIS) inhalation unit risk for asbestos from the lifetime
risk tables in the Airborne Asbestos Health Assessment Update
(AAHAU) at 0.01 fiber/cc ........................... 1-3
2-1. Optical and crystallographic properties of fibrous
amphiboles associated with Libby Amphibole asbestos
...........................................................................
2-14
2-2. Air sampling results for asbestos from Zonolite vermiculite
attic insulation (VAI) in three homes
............................................................................................
2-23
3-1. Factors influencing fiber deposition and clearance in the
respiratory system ........ 3-5
3-2. Determinants of fiber toxicity
...............................................................................
3-15
4-1. Population and exposure assessment methodologies used in
studies of Libby, MT vermiculite workers
..............................................................................
4-5
4-2. Source of primary samples for fiber measurements at the
Libby vermiculite mining and milling operations
................................................................................
4-6
4-3. Dimensional characteristics of fibers from air samples
collected in the vermiculite mill and screening plant, Libby, MT
................................................... 4-9
4-4. Population and methods used in studies of O.M. Scott,
Marysville, OH plant
workers..................................................................................................................
4-10
4-5. Summary of methods used in community-based studies of
Libby, MT residents conducted by Agency for Toxic Substances and
Disease Registry (ATSDR)
...............................................................................................................
4-13
4-6. Nonmalignant respiratory mortality studies of populations
exposed to Libby Amphibole asbestos
..............................................................................................
4-15
4-7. Chest radiographic studies of the Libby, MT vermiculite
mine workers ............. 4-22
4-8. Pulmonary function and chest radiographic studies of the
O.M. Scott, Marysville, OH plant workers
..............................................................................
4-24
4-9. Prevalence of pleural pathological alterations according to
quartiles of cumulative fiber exposure in 280 participants
...................................................... 4-25
ix
-
LIST OF TABLES (continued)
4-10. Prevalence of pleural thickening in 280 participants
according to various cofactors
................................................................................................................
4-26
4-11. Pathological alterations of lung parenchyma and pleura in
community-based studies
...................................................................................................................
4-30
4-12. Pulmonary function and respiratory symptoms and conditions
changes in the Libby, MT community
..........................................................................................
4-32
4-13. Analyses of pulmonary changes seen on radiographs in
relation to pulmonary function in the Libby, MT community
............................................... 4-34
4-14. Pulmonary function and respiratory system changes in the
Libby, MT community: clinic-based study
............................................................................
4-36
4-15. Autoimmune-related studies in the Libby, MT community
................................. 4-40
4-16. Respiratory (lung) cancer mortality and exposure-response
analyses based on related studies of the vermiculite mining and
milling workers in Libby, MT
........................................................................................................................
4-42
4-17. Mesothelioma mortality risk based on studies of the
vermiculite mine workers in Libby, MT
...........................................................................................
4-48
4-18. Exposure levels and health effects observed in communities
exposed to tremolite, chrysotile, and crocidolite asbestos
...................................................... 4-52
4-19. In vivo data following exposure to Libby Amphibole
asbestos ........................... 4-55
4-20. In vivo data following exposure to tremolite
asbestos.......................................... 4-61
4-21. In vitro data following exposure to Libby Amphibole
asbestos ........................... 4-74
4-22. In vitro data following exposure to tremolite asbestos
......................................... 4-76
4-23. Hypothesized modes of action for carcinogenicity of Libby
Amphibole asbestos in specific organs
..................................................................................
4-100
5-1. Summary of candidate principal studies on LAA for reference
concentration (RfC) derivation
......................................................................................................
5-6
x
-
LIST OF TABLES (continued)
5-2. Summary of rationale for identifying candidate principal
studies on LAA for reference concentration (RfC) development
........................................................... 5-8
5-3. Characteristics of workers at the O.M. Scott plant in
Marysville, OH ................. 5-12
5-4. Characteristics of workers at the O.M. Scott plant in
Marysville, OH, with health evaluations in 2002−2005 who did not
report any previous occupational exposure to asbestos
........................................................................
5-18
5-5. Modelsa considered to develop a point of departure (POD)
................................. 5-24
5-6. Evaluation of association between covariates and exposure,
and between covariates and LPT
...............................................................................................
5-27
5-7. Model features considered in exposure-response modeling to
develop a point of departure (POD)
......................................................................................
5-28
5-8. Univariate exposure-response modeling for any LPT in the
Marysville workers who underwent health evaluations in 2002−2005
and whose job start date was on or after 1/1/1972 (n = 119), using
a benchmark response (BMR) of 10% extra risk of any localized
pleural thickening (LPT) ................... 5-31
5-9. Estimated point of departure (POD) combining information
from the Marysville workers who underwent health evaluations in
2002−2005 and hired in 1972 or later (Primary), and from all
workers who underwent health evaluations in 2002−2005 (regardless
of hire date), using a benchmark response (BMR) of 10% extra risk
of LPT in the Dichotomous Hill model with plateau fixed at 85%
.....................................................................................
5-38
5-10. (Copy of Table E-11) Reference concentrations (RfCs) for
the alternative endpoint of any pleural thickening (APT) in the
Marysville cohort with combined x-ray results from 1980 and
2002−2005 regardless of date of hire ..... 5-48
5-11. Multiple derivations of a reference concentration from the
Marysville, OH cohort
....................................................................................................................
5-49
5-12. Exposure distribution among workers at the O.M. Scott
plant in Marysville, OH
.........................................................................................................................
5-53
xi
-
LIST OF TABLES (continued)
5-13. Effect of truncating exposures after 1980 and of using
arithmetic or geometric mean to summarize multiple fiber
measurements ............................... 5-55
5-14. Effect of including covariates into the final model
............................................... 5-60
5-15. Effect of different assumptions for the plateau parameter
.................................... 5-62
5-16. Exposure-response modeling for any localized pleural
thickening (LPT) in the Marysville workers who underwent health
evaluations in 2002−2005 and whose job start date was on or after
1/1/1972 (n = 119), using a benchmark response (BMR) of 10% extra
risk of any LPT, and RTW exposure
................................................................................................................
5-64
5-17. Effect of using different case/noncase definitions
................................................ 5-66
5-18. Exposure-response modeling for any localized pleural
thickening (LPT) in the Marysville workers who underwent health
evaluations in 2002−2005 (n = 252), comparing the multinomial model
and logistic model with different outcome group definitions
.....................................................................
5-68
5-19. Summary of sensitivity analyses. Exposure-response
modeling performed using mean exposure in the hybrid Dichotomous
Hill model with plateau fixed at 85%, Marysville workers who
underwent health evaluations in 2002−2005 and whose job start date
was on or after 1/1/1972 (n = 119) ............ 5-70
5-20. Demographic, mortality, and exposure characteristics of
the Libby worker cohort
....................................................................................................................
5-74
5-21. Exposure intensity (fibers/cc) for each location operation
from the beginning of operations through 1982
...................................................................................
5-79
5-22. Demographic, mortality, and exposure characteristics of
the subset of the Libby worker subcohort hired after 1959
.............................................................
5-83
5-23. Mesothelioma mortality rate shown by duration of exposure
(yr) in the full Libby worker cohort including all hires (n =
1,871) ............................................ 5-93
5-24. Mesothelioma mortality rate shown by age at first exposure
in the full Libby worker cohort including all hires (n = 1,871)
....................................................... 5-93
xii
-
LIST OF TABLES (continued)
5-25. Mesothelioma mortality rate shown by time since first
exposure (TSFE) in the full Libby worker cohort including all hires
(n = 1,871) ................................ 5-94
5-26. Comparison of model fit of various univariate exposure
metrics for mesothelioma mortality in the full Libby worker cohort
including all hires (n = 1,871)
............................................................................................................
5-95
5-27. Lung cancer mortality rate shown by duration of exposure
(yr) in the full Libby worker cohort including all hires (n =
1,871) ............................................ 5-96
5-28. Lung cancer mortality rate shown by age at first exposure
in the full Libby worker cohort including all hires (n = 1,871)
....................................................... 5-97
5-29. Lung cancer mortality rate shown by time since first
exposure (TSFE) in the full Libby worker cohort including all hires
(n = 1,871) ...................................... 5-97
5-30. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 shown by duration of exposure (yr)
....................................................................
5-104
5-31. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 shown by age at first exposure
............................................................................
5-104
5-32. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 shown by time since first exposure (TSFE)
........................................................ 5-104
5-33. Comparison of model fit of exposure metrics for
mesothelioma mortality in the subcohort hired after 1959
............................................................................
5-105
5-34. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 for the cumulative exposure (CE) with 15-year lag
and 5-year half-life ............ 5-107
5-35. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 for the cumulative exposure (CE) with 10-year lag
and 5-year half-life ............ 5-107
5-36. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 for the Peto model
...............................................................................................
5-107
5-37. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 for the Peto model with power k = 3.9 and decay λ
= 6.8%/yr .......................... 5-107
xiii
-
LIST OF TABLES (continued)
5-38. Mesothelioma mortality rate in the subcohort of employees
hired after 1959 for the Peto model with power k = 5.4 and decay λ
= 15%/yr ............................ 5-108
5-39. Mesothelioma mortality exposure metrics fits, slopes per
day, and credible intervals in the subcohort of employees hired
after 1959 ................................... 5-112
5-40. Peto model and Peto model with clearance fits, slopes per
year, and credible intervals in the subcohort of employees hired
after 1959 ................................... 5-113
5-41. Lung cancer mortality rate in the subcohort of employees
hired after 1959 shown by duration of exposure (yr)
....................................................................
5-114
5-42. Lung cancer mortality rate in the subcohort of employees
hired after 1959 shown by age at first exposure
............................................................................
5-114
5-43. Lung cancer mortality rate in the subcohort of employees
hired after 1959 shown by time since first exposure (TSFE)
........................................................ 5-114
5-44. Model fit comparison for different exposure metrics and
lung cancer mortality associated with LAA, controlling for age,
gender, race, and date of birth
.....................................................................................................................
5-116
5-45. Lung cancer mortality exposure metrics fits, slopes, and
confidence intervals (CI) for all retained metrics from Table 5-44
..................................................... 5-120
5-46. Sensitivity analysis of model fit comparison for different
exposure metrics and mesothelioma mortality associated with LAA
............................................. 5-125
5-47. Sensitivity analysis of model fit comparison for different
exposure metrics and lung cancer mortality associated with LAA,
controlling for age, gender, race, and date of birth
.........................................................................................
5-126
5-48. Unit risks for the Peto model and Peto model with
clearance ............................ 5-128
5-49. Mesothelioma mortality exposure metrics unit risks for the
subcohort hired after 1959
............................................................................................................
5-129
5-50. Mesothelioma unit risks for the subcohort hired after 1959
adjusted for underascertainment
.............................................................................................
5-130
xiv
-
LIST OF TABLES (continued)
5-51. Mesothelioma unit risks for the subcohort hired after 1959
based on the Peto model and the Peto model with clearance adjusted
for mesothelioma underascertainment
.............................................................................................
5-130
5-52. Unit risks for subset of lung cancer models with lagged
exposures that yielded statistically significant model fit (p <
0.05) and exposure metric fit (p < 0.05) to the epidemiologic
data
...................................................................
5-131
5-53. Estimates of the combined central estimate of the unit
risk for mesothelioma and lung cancer and the combined upper-bound
lifetime unit risks for mesothelioma and lung cancer risks (the
Inhalation Unit Risk for LAA) for different combination of
mesothelioma and lung cancer models .......................
5-133
5-54. Lung cancer regression results from different analyses of
cumulative exposure in the cohort of workers in Libby, MT
................................................ 5-136
5-55. Mesothelioma analysis results from different analyses of
cumulative exposure in the Libby workers cohort
................................................................
5-140
6-1. Estimates of the combined central estimate of the unit risk
for mesothelioma and lung cancer and the combined upper-bound
lifetime unit risks for mesothelioma and lung cancer risks (the
Inhalation Unit Risk) for different combination of mesothelioma and
lung cancer models ........................................
6-20
xv
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LIST OF FIGURES
2-1. Vermiculite mining operation on Zonolite Mountain, Libby,
MT ......................... 2-1
2-2. Unexpanded and expanded vermiculite
..................................................................
2-2
2-3. Structure of the silicate minerals, illustrating silicate
subclasses by the linking of the basic silicon tetrahedron (A) into
more complex structures (B, C, or D)
...................................................................................................................
2-4
2-4. Cross section of amphibole fibers showing the silicon
tetrahedrons (triangles with open circles at apex) that make up
each double-chain plate (shown along the fiber axis)
................................................................................................
2-5
2-5. Comparison of crystalline forms of amphibole minerals
........................................ 2-8
2-6. Mineralogy of LAA structures from samples taken from the
Zonolite Mountain site
........................................................................................................
2-12
2-7. Solution series linking tremolite, winchite, and richterite
amphibole fibers ........ 2-13
2-8. Scanning electron microscope image of amphibole mineral
structures from the Libby, MT mine
..............................................................................................
2-17
2-9. Fiber morphology of amphibole asbestos from the Libby, MT
mine viewed under a scanning electron microscope
..................................................................
2-18
2-10. Particle size (length, width, aspect ratio) of fibers in
Libby ore and Libby air .... 2-19
2-11. Nationwide distribution of Libby ore by county (in tons)
.................................... 2-22
3-1. General scheme for fiber deposition, clearance, and
translocation of fibers from the lung and gastrointestinal tract
..................................................................
3-3
3-2. Architecture of the human respiratory tract and schematic
of major mechanisms of fiber deposition
..............................................................................
3-4
4-1. Investigations of populations exposed to LAA
....................................................... 4-2
xvi
-
LIST OF FIGURES (continued)
4-2. A (left). Gross appearance at autopsy of
asbestos-associated pleural plaques overlying the lateral thoracic
wall [(ATS, 2004) Figure 12]. Figure 4-2. B (right). Gross
appearance of large asbestos-related pleural plaque over the dome
of the diaphragm [(ATS, 2004) Figure 13].
................................................ 4-19
4-3. Lung cancer mortality risk among workers in the Libby, MT
vermiculite mine and mill workers
..........................................................................................
4-45
4-4. Proposed mechanistic events for carcinogenicity of asbestos
fibers .................... 4-72
5-1. Candidate studies for derivation of the reference
concentration (RfC) in three different study populations, with the
most recent study of each population circled
...................................................................................................
5-5
5-2. Radiographic outcomes among Marysville, OH workers
..................................... 5-13
5-3. Plot of exposure-response models for probability of
localized pleural thickening (LPT) as a function of mean
concentration of occupational exposure in the subcohort
.....................................................................................
5-34
5-4. Predicted risk of localized pleural thickening (LPT) at the
benchmark concentration (BMC) and the lower limit of the BMC
(BMCL), using the hybrid Dichotomous Hill model with plateau fixed
at 85% ................................. 5-40
5-5. Plot of the National Institute for Occupational Safety and
Health (NIOSH) job-exposure matrix for different job categories
over time .................................. 5-84
5-6. Distribution of values of the Peto metric and Peto metric
values of mesothelioma deaths (shown as inverted triangles) in the
subcohort of employees hired after 1959
.................................................................................
5-109
5-7. Distribution of observed values of cumulative exposure (CE)
with 15-year lag and 5-year half-life and CE with 15-yr lag and
5-yr half-life values of mesothelioma deaths (shown as inverted
triangles) in the subcohort of employees hired after 1959
.................................................................................
5-110
xvii
-
LIST OF FIGURES (continued)
5-8. Distribution of observed values of cumulative exposure (CE)
with 10-year lag and 5-year half-life and CE with 10-yr lag and
5-yr half-life values of mesothelioma deaths (shown as inverted
triangles) in the subcohort of employees hired after 1959
.................................................................................
5-111
5-9. Regression diagnostics showing model fit based on the
Schoenfeld residuals with two levels of nonparametric smoothing
(using cubic splines) to show any patterns of departures from the
model predicted values .............................. 5-122
xviii
-
LIST OF ABBREVIATIONS AND ACRONYMS
AAHAU Airborne Asbestos Health Assessment Update AIC Akaike
Information Criterion ADAF age-dependent adjustment factor ANA
antinuclear antibody APC antigen-presenting cells APT any pleural
thickening ARC any radiographic change ATS American Thoracic
Society ATSDR Agency for Toxic Substances and Disease Registry BALF
bronchoalveolar lavage fluid BGL β-glucuronidase BMI body mass
index BMC benchmark concentration BMCL lower limit of the BMC BMR
benchmark response C mean exposure CAO costophrenic angle
obliteration CDF cumulative distribution frequency CE cumulative
exposure CHEEC cumulative human equivalent exposure concentration
CI confidence interval COPD chronic obstructive pulmonary disease
COX-2 cyclooxygenase-2 CVD cardiovascular disease DEF deferoxamine
deq aerodynamic equivalent diameter DIC Deviance Information
Criterion DLCO single-breath carbon monoxide diffusing capacity DPT
diffuse pleural thickening dsDNA double-stranded DNA EcSOD
extracellular superoxide dismutase ED El Dorado tremolite EDS
energy-dispersive spectroscopy EPA U.S. Environmental Protection
Agency EPMA electron probe microanalysis FEV forced expiratory
volume FVC forced vital capacity GOF goodness of fit GSH
glutathione GST glutathione-S-transferase HAEC human airway
epithelial cell HO heme oxygenase HTE hamster tracheal epithelial
IARC International Agency for Research on Cancer ICD International
Classification of Diseases
xix
-
LIST OF ABBREVIATIONS AND ACRONYMS (continued)
IFN interferon Ig immunoglobulin IH industrial hygiene IL
interleukin ILO International Labour Organization IQR interquartile
range IRIS Integrated Risk Information System IUR inhalation unit
risk JEM job-exposure matrix KL lung cancer slope factor KM
mesothelioma slope factor LAA Libby Amphibole asbestos LDH lactate
dehydrogenase LEC01 95% lower confidence limit of the exposure
concentration associated with 1%
increased risk LPT localized pleural thickening MCAA
antimesothelial cell antibodies MCMC Monte Carlo Markov Chain MMP
matrix metalloproteinase MOA mode of action Mppcf million particles
per cubic foot MSHA U.S. Mine Safety and Health Administration NRC
National Research Council NDI National Death Index Nf2
neurofibromatosis 2 NIEHS National Institute of Environmental
Health Sciences NIOSH National Institute for Occupational Safety
and Health ON Ontario ferroactinolite OR odds ratio PBS
phosphate-buffered saline PCM phase contrast microscopy PCMe phase
contrast microscopy equivalent PG-PS peptidoglycan-polysaccharide
PLM polarized light microscopy PM2.5 particulate matter 2.5 μm
diameter or less POD point of departure RCF-1 refractory ceramic
fibers RfC reference concentration RfD reference dose RNP
ribonucleoprotein RNS reactive nitrogen species ROS reactive oxygen
species RPM rat pleural mesothelial RR relative risk RTW residence
time-weighted SAED selected area electron diffraction
xx
-
LIST OF ABBREVIATIONS AND ACRONYMS (continued)
SAID systemic autoimmune disease SD standard deviation SE
standard error SH spontaneously hypertensive SHE Syrian hamster
embryo SHHF spontaneously hypertensive-heart failure SIR
standardized incidence ratio SM Sumas Mountain chrysotile SMR
standardized mortality ratio SOD superoxide dismutase SRR
standardized rate ratio SSA/Ro52 autoantibody marker for apoptosis
SSB/La autoantibody marker SV40 Simian virus 40 TEM transmission
electron microscopy TSFE time since first exposure TWA
time-weighted average UCL upper confidence limit UF uncertainty
factor UICC Union for International Cancer Control USGS United
States Geological Survey VAI vermiculite attic insulation WDS
wavelength-dispersive x-ray spectroscopy WKY Wistar-Kyoto rat XRCC1
x-ray repair cross-complementing protein 1
xxi
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FOREWORD
The purpose of this Toxicological Review is to provide
scientific support and rationale for the hazard and dose-response
assessment in the Integrated Risk Information System (IRIS)
pertaining to chronic inhalation exposure to Libby Amphibole
asbestos, a mixture of amphibole fibers identified in the Rainy
Creek complex and present in ore from the vermiculite mine near
Libby, MT. It is not intended to be an assessment of the toxicity
of asbestos generally (nor a comprehensive treatise on the agent or
toxicological nature of Libby Amphibole asbestos). The purpose of
this document is to establish a Libby Amphibole asbestos-specific
reference concentration to address noncancer health effects and to
characterize the carcinogenic potential and establish an inhalation
unit risk for Libby Amphibole asbestos-related lung cancer and
mesothelioma mortality.
The intent of Section 6, Major Conclusions in the
Characterization of Hazard and Exposure Response, is to present the
significant conclusions reached in the derivation of the reference
dose, reference concentration, and cancer assessment where
applicable, and to characterize the overall confidence in the
quantitative and qualitative aspects of hazard and dose-response by
addressing the quality of data and related uncertainties. The
discussion is intended to convey the limitations of the assessment
and to aid and guide the risk assessor in the ensuing steps of the
risk assessment process.
The intent of Appendix J, Documentation of Implementation of the
2011 National Research Council Recommendations, is to present the
IRIS Program’s implementation of the NRC recommendations.
Implementation is following a phased approach that is consistent
with the NRC’s “Roadmap for Revision” as described in Chapter 7 of
the formaldehyde review report.
For other general information about this assessment or other
questions relating to IRIS, the reader is referred to U.S.
Environmental Protection Agency’s (EPA’s) IRIS Hotline at (202)
566-1676 (phone), (202) 566-1749 (fax), or [email protected]
(email address).
xxii
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AUTHORS, CONTRIBUTORS, AND REVIEWERS
CHEMICAL MANAGERS/AUTHORS Thomas F. Bateson, ScD, MPH National
Center for Environmental Assessment U.S. Environmental Protection
Agency Washington, DC Robert Benson, PhD Region 8 Office of
Partnerships and Regulatory Assistance U.S. Environmental
Protection Agency Denver, CO AUTHORS Krista Yorita Christensen, PhD
Formerly with the National Center for Environmental Assessment U.S.
Environmental Protection Agency Washington, DC Glinda Cooper, PhD
National Center for Environmental Assessment U.S. Environmental
Protection Agency Washington, DC Danielle DeVoney, PhD, DABT, PE
Captain in the U.S. Public Health Service Formerly with the
National Center for Environmental Assessment U.S. Environmental
Protection Agency Washington, DC Maureen R. Gwinn, PhD, DABT, ATS
National Center for Environmental Assessment U.S. Environmental
Protection Agency Washington, DC Leonid Kopylev, PhD National
Center for Environmental Assessment U.S. Environmental Protection
Agency Washington, DC
xxiii
-
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
CONTRIBUTING AUTHORS Rebecca Dzubow, MPH, MEM Formerly with the
National Center for Environmental Assessment U.S. Environmental
Protection Agency Washington, DC David Berry, PhD Region 8 U.S.
Environmental Protection Agency Denver, CO Malcolm Field, PhD
National Center for Environmental Assessment U.S. Environmental
Protection Agency Washington, DC Annie M. Jarabek National Center
for Environmental Assessment U.S. Environmental Protection Agency
Research Triangle Park, NC Keith Salazar, PhD National Center for
Environmental Assessment U.S. Environmental Protection Agency
Washington, DC Patricia Sullivan, ScD Division of Respiratory
Disease Studies National Institute for Occupational Safety and
Health Morgantown, WV CONTRIBUTORS David Bussard National Center
for Environmental Assessment U.S. Environmental Protection Agency
Washington, DC Samantha J. Jones, PhD National Center for
Environmental Assessment U.S. Environmental Protection Agency
Washington, DC
xxiv
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AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
CONTRIBUTORS (continued) Babasaheb Sonawane, PhD National Center
for Environmental Assessment U.S. Environmental Protection Agency
Washington, DC Paul White National Center for Environmental
Assessment U.S. Environmental Protection Agency Washington, DC
CONTRACTOR SUPPORT William Brattin, PhD Syracuse Research
Corporation Denver, CO Highlight Technologies, LLC, Fairfax, VA
Dan Heing Debbie Kleiser Sandra Moore Ashley Price Kathleen
Secor
CACI International, Inc, Arlington, VA
Thomas Schaffner Linda Tackett Lisa Walker
ECFlex, Inc., Fairborn, OH
Heidi Glick Crystal Lewis Carman Parker-Lawler Lana Wood
IntelliTech Systems, Inc., Fairborn, OH
Cris Broyles
xxv
-
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
REVIEWERS This document was provided for review to EPA
scientists, interagency reviewers from
other federal agencies and the Executive Office of the
President, and the public, and peer reviewed by independent
scientists external to EPA. A summary and EPA’s disposition of the
comments received from the independent external peer reviewers and
the public is included in Appendix A.
Science Advisory Board (SAB) Panel for Review of EPA’s Draft
Toxicological Review of Libby Amphibole Asbestos CHAIR Dr. Agnes
Kane Professor and Chair Department of Pathology and Laboratory
Medicine Brown University Providence, RI MEMBERS Dr. John R. Balmes
Professor Department of Medicine, Division of Occupational and
Environmental Medicine University of California San Francisco, CA
Dr. James Bonner Associate Professor Toxicology North Carolina
State University Raleigh, NC Dr. Jeffrey Everitt Director
Department of Laboratory Animal Science GlaxoSmithKline
Pharmaceuticals Research Triangle Park, NC Dr. Scott Ferson Senior
Scientist Applied Biomathematics Setauket, NY
xxvi
-
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
MEMBERS (continued) Dr. George Guthrie Focus Area Leader
Geological and Environmental Sciences National Energy Technology
Laboratory, U.S. Department of Energy Pittsburgh, PA Mr. John
Harris Principal LabCor Portland, Inc. Portland, OR Dr. Tom Hei
Professor and Vice-Chairman Radiation Oncology, College of
Physicians and Surgeons Columbia University Medical Center New
York, NY Dr. David Kriebel Professor and Chair Department of Work
Environment School of Health & Environment, University of
Massachusetts Lowell, MA Dr. Morton Lippmann Professor Nelson
Institute of Environmental Medicine New York University School of
Medicine Tuxedo, NY Dr. John Neuberger Professor Preventive
Medicine and Public Health, School of Medicine University of Kansas
Kansas City, KS Dr. Lee Newman Professor of Medicine Division of
Environmental and Occupational Health Sciences School of Public
Health, University of Colorado Aurora, CO
xxvii
-
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
MEMBERS (continued) Dr. Michael Pennell Assistant Professor
Division of Biostatistics College of Public Health, Ohio State
University Columbus, OH Dr. Julian Peto Professor Department of
Epidemiology and Population Health London School of Hygiene and
Tropical Medicine London, UK Dr. Carrie Redlich Professor of
Medicine Internal Medicine School of Medicine, Yale University New
Haven, CT Dr. Andrew G. Salmon Senior Toxicologist Office of
Environmental Health Hazard Assessment California Environmental
Protection Agency Oakland, CA Dr. Elizabeth A. (Lianne) Sheppard
Professor Biostatistics and Environmental & Occupational Health
Sciences School of Public Health, University of Washington Seattle,
WA Dr. Randal Southard Professor of Soils AES Dean's Office
University of California at Davis Davis, CA Dr. Katherine Walker
Senior Staff Scientist Health Effects Institute Boston, MA
xxviii
-
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)
MEMBERS (continued) Dr. James Webber Research Scientist
Wadsworth Center New York State Department of Health Albany, NY Dr.
Susan Woskie Professor Work Environment, Health and Environment
University of Massachusetts Lowell Lowell, MA SCIENCE ADVISORY
BOARD STAFF Dr. Diana Wong Designated Federal Officer U.S.
Environmental Protection Agency Washington, DC
xxix
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1. INTRODUCTION
This document presents background information and justification
for the Integrated Risk Information System (IRIS) summary of the
hazard and exposure-response assessment of Libby Amphibole asbestos
(LAA),1 a mixture of amphibole fibers identified in the Rainy Creek
complex and present in ore from the vermiculite mine near Libby,
MT. IRIS summaries may include oral reference dose (RfD) and
inhalation reference concentration (RfC) values for chronic
exposure durations, and a carcinogenicity assessment. This
assessment reviews the potential hazards, both cancer and noncancer
health effects, from exposure to LAA and provides quantitative
information for use in risk assessments: an RfC for noncancer
health effects and an inhalation unit risk (IUR) addressing cancer
risk. LAA-specific data are not available to support RfD or cancer
slope factor derivations for oral exposures.
An RfC is defined as “an estimate (with uncertainty spanning
perhaps an order of magnitude) of an exposure (including sensitive
subgroups) that is likely to be without an appreciable risk of
adverse health effects over a lifetime.” (U.S. EPA, 2002). In the
case of LAA, the RfC is expressed in terms of the lifetime exposure
in units of fibers per cubic centimeter of air (fibers/cc) in units
of the fibers as measured by phase contrast microscopy (PCM). The
inhalation RfC for LAA considers toxic effects for both the
respiratory system (portal of entry) and for effects peripheral to
the respiratory system (extrarespiratory or systemic effects) that
may arise after inhalation of LAA.
The carcinogenicity assessment provides information on the
carcinogenic hazard potential of the substance in question, and
quantitative estimates of risk from inhalation exposures are
derived. The information includes a weight-of-evidence judgment of
the likelihood that the agent is a human carcinogen and the
conditions under which the carcinogenic effects may be expressed.
Quantitative risk estimates are derived from the application of a
low-dose extrapolation procedure from human data. An inhalation
unit risk (IUR) is typically defined as a plausible upper bound on
the estimate of cancer risk per μg/m3 air breathed for 70 years.
For LAA, the RfC is expressed as a lifetime daily exposure in
fibers/cc (in units of the fibers as measured by PCM), and the IUR
is expressed as cancer risk per fibers/cc (in units of the fibers
as measured by PCM).
Development of these hazard identification and exposure-response
assessments for LAA has followed the general guidelines for risk
assessment as set forth by the National Research Council (NRC,
1983). U.S. Environmental Protection Agency (EPA) Guidelines and
Risk Assessment Forum technical panel reports that may have been
used in the development of this assessment include the following:
Guidelines for the Health Risk Assessment of Chemical
1The term “Libby Amphibole asbestos” is used in this document to
identify the mixture of amphibole mineral fibers of varying
elemental composition (e.g., winchite, richterite, tremolite, etc.)
that have been identified in the Rainy Creek complex near Libby,
MT. It is further described in Section 2.2.
1-1
http://hero.epa.gov/index.cfm?action=search.view&reference_id=88824http://hero.epa.gov/index.cfm?action=search.view&reference_id=194806
-
Mixtures (U.S. EPA, 1986c), Guidelines for Mutagenicity Risk
Assessment (U.S. EPA, 1986b), Recommendations for and Documentation
of Biological Values for Use in Risk Assessment (U.S. EPA, 1988b),
Guidelines for Developmental Toxicity Risk Assessment (U.S. EPA,
1991a), Interim Policy for Particle Size and Limit Concentration
Issues in Inhalation Toxicity (U.S. EPA, 1994a), Methods for
Derivation of Inhalation Reference Concentrations and Application
of Inhalation Dosimetry (U.S. EPA, 1994b), Use of the Benchmark
Dose Approach in Health Risk Assessment (U.S. EPA, 1995),
Guidelines for Reproductive Toxicity Risk Assessment (U.S. EPA,
1996), Guidelines for Neurotoxicity Risk Assessment (U.S. EPA,
1998), Science Policy Council Handbook: Risk Characterization (U.S.
EPA, 2000b), Benchmark Dose Technical Guidance Document (U.S. EPA,
2012), Supplementary Guidance for Conducting Health Risk Assessment
of Chemical Mixtures (U.S. EPA, 2000c), A Review of the Reference
Dose and Reference Concentration Processes (U.S. EPA, 2002),
Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005a),
Supplemental Guidance for Assessing Susceptibility from Early-Life
Exposure to Carcinogens (U.S. EPA, 2005b), Science Policy Council
Handbook: Peer Review (U.S. EPA, 2006c), and A Framework for
Assessing Health Risks of Environmental Exposures to Children (U.S.
EPA, 2006b).
The literature search strategy employed for this assessment is
based on EPA’s National Center for Environmental Assessment’s
Health and Environmental Research Online database tool (which
includes PubMed, MEDLINE, Web of Science, JSTOR, and other
literature sources). The key search terms included the following:
Libby Amphibole, tremolite, asbestos, richterite, winchite,
amphibole, and Libby, MT. The relevant literature was reviewed
through July 2011. Any pertinent scientific information submitted
by the public to the IRIS Submission Desk was also considered in
the development of this document. Note that references have been
added to the Toxicological Review after the external peer review
SAB (2013) in response to peer reviewers’ comments and for the sake
of completeness. 1.1. RELATED ASSESSMENTS 1.1.1. Integrated Risk
Information System (IRIS) Assessment for Asbestos (U.S. EPA,
1988a) The IRIS assessment for asbestos was posted online in
IRIS in 1988 and includes an IUR
of 0.23 excess cancers per 1 fiber/cc (U.S. EPA, 1988a); this
unit risk is given in units of the fibers as measured by PCM. The
IRIS IUR2 for general asbestos (CAS Number 1332-21-4) is derived by
estimating excess cancers for a continuous lifetime exposure and is
based on the central tendency―not the upper bound―of the risk
estimates (U.S. EPA, 1988a) and is applicable to exposures across a
range of exposure environments and types of asbestos. Although
other cancers have been associated with asbestos [e.g., laryngeal,
stomach, ovarian
2For purposes of this document, termed “IRIS IUR.”
1-2
http://hero.epa.gov/index.cfm?action=search.view&reference_id=1468http://hero.epa.gov/index.cfm?action=search.view&reference_id=1466http://hero.epa.gov/index.cfm?action=search.view&reference_id=64560http://hero.epa.gov/index.cfm?action=search.view&reference_id=64560http://hero.epa.gov/index.cfm?action=search.view&reference_id=8567http://hero.epa.gov/index.cfm?action=search.view&reference_id=76133http://hero.epa.gov/index.cfm?action=search.view&reference_id=76133http://hero.epa.gov/index.cfm?action=search.view&reference_id=6488http://hero.epa.gov/index.cfm?action=search.view&reference_id=5992http://hero.epa.gov/index.cfm?action=search.view&reference_id=30019http://hero.epa.gov/index.cfm?action=search.view&reference_id=30019http://hero.epa.gov/index.cfm?action=search.view&reference_id=30021http://hero.epa.gov/index.cfm?action=search.view&reference_id=52149http://hero.epa.gov/index.cfm?action=search.view&reference_id=1239433http://hero.epa.gov/index.cfm?action=search.view&reference_id=1065850http://hero.epa.gov/index.cfm?action=search.view&reference_id=88824http://hero.epa.gov/index.cfm?action=search.view&reference_id=86237http://hero.epa.gov/index.cfm?action=search.view&reference_id=86237http://hero.epa.gov/index.cfm?action=search.view&reference_id=88823http://hero.epa.gov/index.cfm?action=search.view&reference_id=1104578http://hero.epa.gov/index.cfm?action=search.view&reference_id=1104578http://hero.epa.gov/index.cfm?action=search.view&reference_id=194567http://hero.epa.gov/index.cfm?action=search.view&reference_id=2325151http://hero.epa.gov/index.cfm?action=search.view&reference_id=783514http://hero.epa.gov/index.cfm?action=search.view&reference_id=783514http://hero.epa.gov/index.cfm?action=search.view&reference_id=783514http://hero.epa.gov/index.cfm?action=search.view&reference_id=783514brendabuckHighlight
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(Straif et al., 2009)], the IRIS IUR for asbestos accounts for
only lung cancer and mesothelioma. Additionally, pleural and
pulmonary effects from asbestos exposure (e.g., pleural thickening,
asbestosis, and reduced lung function) are well documented,
although there is no RfC for these noncancer health effects on the
IRIS database (U.S. EPA, 1988a).
The derivation of the unit risk for general asbestos is based on
the Airborne Asbestos Health Assessment Update [AAHAU (U.S. EPA,
1986a)]. The AAHAU provides various cancer potency factors and
mathematical models of lung cancer and mesothelioma mortality based
on synthesis of data from occupational studies and presents
estimates of lifetime cancer risk for continuous environmental
exposures [0.0001 fiber/cc and 0.01 fiber/cc; (see Table 6-3 of
(U.S. EPA, 1986a)]. For both lung cancer and mesothelioma,
life-table analysis was used to generate risk estimates based on
the number of years of exposure and the age at onset of exposure.
Although various exposure scenarios were presented, the unit risk
is based on a lifetime continuous exposure from birth. The final
asbestos IUR is 0.23 excess cancers per 1 fiber/cc continuous
exposure3 and was posted on the IRIS database in 1988 [(U.S. EPA,
1988a) see Table 1-1 below].
Table 1-1. Derivation of the current Integrated Risk Information
System (IRIS) inhalation unit risk for asbestos from the lifetime
risk tables in the Airborne Asbestos Health Assessment Update
(AAHAU) at 0.01 fiber/cc
Gender
Excess deaths per 100,000a
Risk Unit risk
(per fiber/cc) Mesothelioma Lung cancer Total
Female 183 35 218.5 2.18 × 10-1
Male 129 114 242.2 2.42 × 10-1
All 156 74 230.3 2.30 × 10-1 0.23
aData are for exposure at 0.01 fiber/cc for a lifetime. Source:
U.S. EPA (1988a).
The IRIS database has an IUR for asbestos based on 14
epidemiologic studies that
included occupational exposure to chrysotile, amosite, or
mixed-mineral exposures [chrysotile, amosite, crocidolite (U.S.
EPA, 1988a, 1986a)]. Some uncertainty remains in applying the
resulting IUR for asbestos to exposure environments and minerals
different from those analyzed
3An IUR of 0.23 for general asbestos can be interpreted as 0.23
excess risk of death from mesothelioma or lung cancer per person
for each 1 fiber/cc increase in continuous lifetime exposure. Thus,
as shown in Table 1-1, for 100,000 people exposed at a
concentration of 0.01 fiber/cc, 230 excess deaths would be expected
[IUR × Concentration × Number of people = (0.23 excess cancer
deaths per fiber/cc per person) × (0.01 fiber/cc) × (100,000
people) = 230 excess cancer deaths]. “Fiber/cc” is a commonly used
measure; it is equivalent to 1 million fibers per cubic meter of
air while 0.01 fiber/cc is 10,000 fibers per cubic meter of
air.
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in the AAHAU (U.S. EPA, 1986a). No RfC, RfD, or oral slope
factor are derived for asbestos on the IRIS database (U.S. EPA,
1988a).
1.1.2. EPA Health Assessment for Vermiculite (U.S. EPA,
1991b)
An EPA health assessment for vermiculite reviewed available
health data, including studies on workers who mined and processed
ore with no significant amphibole fiber content. The cancer and
noncancer health effects observed in the Libby, MT worker cohort
were not seen in studies of workers exposed to mines with similar
exposure to vermiculite but much lower exposures to asbestos
fibers. Therefore, it was concluded that the health effects
observed from the materials mined from Zonolite Mountain near
Libby, MT, were most likely due to amphibole fibers and not the
vermiculite itself (U.S. EPA, 1991b). At the time, EPA recommended
the application of the IRIS IUR for asbestos fibers (0.23 per
fiber/cc) in addressing potential risk of the amphibole fibers
entrained in vermiculite mined in Libby, MT.
1.2. LIBBY AMPHIBOLE ASBESTOS-SPECIFIC HUMAN HEALTH
ASSESSMENT
LAA is a complex mixture of amphibole fibers―both
mineralogically and morphologically (see Section 2.3). The mixture
primarily includes winchite, richterite, and tremolite fibers with
trace amounts of magnesio-riebeckite, edenite, and
magnesio-arfvedsonite. These fibers exhibit a complete range of
morphologies from prismatic crystals to asbestiform fibers (Meeker
et al., 2003). Epidemiologic studies of workers exposed to LAA
fibers indicate increased lung cancer and mesothelioma, as well as
asbestosis and other nonmalignant respiratory diseases (Larson et
al., 2010b; Larson et al., 2010a; Moolgavkar et al., 2010; Rohs et
al., 2008; Sullivan, 2007; McDonald et al., 2004, 2002; Amandus et
al., 1988; Amandus et al., 1987b; Amandus and Wheeler, 1987;
Amandus et al., 1987a; McDonald et al., 1986a; McDonald et al.,
1986b; Lockey et al., 1984).
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2. LIBBY AMPHIBOLE ASBESTOS: GEOLOGY AND EXPOSURE POTENTIAL
2.1. INTRODUCTION Libby is a community in northwestern Montana
that is located near a large open-pit
vermiculite mine that operated from the mid 1920s to 1990 (see
Figure 2-1). Vermiculite is a silicate mineral that exhibits a
sheet-like structure similar to mica (see Figure 2-2, Panel A).
When heated to approximately 870°C, water molecules between the
sheets change to vapor and cause the vermiculite to expand like
popcorn into a light, porous material (see Figure 2-2, Panel B).
This process of expanding vermiculite is termed “exfoliation” or
“popping.” Both unexpanded and expanded vermiculite have found a
range of commercial applications, the most common of which include
packing material, attic and wall insulation, various garden and
agricultural products, and various cement and building
products.
Figure 2-1. Vermiculite mining operation on Zonolite Mountain,
Libby, MT.
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Panel A: Vermiculite ore sample. Vermiculite ore sample,
Zonolite Mountain, Rainy Creek complex, Libby, MT.
Source: USGS Field Collection, Meeker (2007)
Panel B: Expanded vermiculite
Figure 2-2. Unexpanded and expanded vermiculite.
The primary product from the mine was vermiculite concentrate,
which was produced by
milling, screening, and grading the raw ore to enrich for the
vermiculite mineral. In general, mining practices sought to exclude
nonvermiculite material when harvesting the ore, and subsequent
processing steps were designed to eliminate nonvermiculite
materials from the
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finished product. Nevertheless, small amounts of other minerals
from the ore body tended to remain in the vermiculite (Zonolite)
product. This included a form of asbestos referred to as Libby
Amphibole asbestos (LAA).
This chapter provides a brief description of the mineralogical
characteristics of asbestos (see Section 2.2), an overview of
methods used to identify and measure asbestos in air and solid
materials (see Section 2.3), a review of the mineralogical
characteristics of LAA in particular (see Section 2.4), and an
overview of the potential for current human exposures to LAA (see
Section 2.5).
2.2. GEOLOGY AND MINERALOGY OF AMPHIBOLES 2.2.1. Overview
Asbestos is the generic name for a group of naturally-occurring
silicate minerals that crystallize in long thin fibers. The basic
chemical unit of asbestos and other silicate minerals is [SiO4]4−.
This basic unit consists of four oxygen atoms at the apices of a
regular tetrahedron surrounding and coordinated with one silicon
ion (Si4+) at the center (see Figure 2-3, Panel A). The silicate
tetrahedra can bond to one another through the oxygen atoms,
leading to a variety of crystal structures (see Figure 2-3, Panels
B, C, and D).
There are two main classes of asbestos: serpentine and
amphibole. The only member of the serpentine class is chrysotile,
which is the form of asbestos that was most commonly used in the
past in various man-made asbestos-containing materials (insulation,
brake linings, floor tiles, etc.). Chrysotile is a phyllosilicate
(see Figure 2-3, Panel D), occurring in sheets that curl into a
fibrous form.
There are many different types of amphibole asbestos. This
includes five types that were previously used in commerce
(actinolite, tremolite, amosite, crocidolite, and anthophyllite),
and these forms of asbestos are now regulated. Numerous other
asbestiform amphiboles exist, even though they were never used as
commercial products and are not currently named in regulations
(Gunter et al., 2007). All forms of amphibole asbestos are
inosilicates (see Figure 2-3, Panel C) in which the long axis of
the fiber (crystallographically called the c-axis) is parallel to
the direction of the chain of silicon tetrahedra.
2.2.2. Mineralogy of Amphibole Asbestos and Related Amphibole
Minerals
Different types of amphiboles differ from each other primarily
in the identity and amounts of monovalent and divalent cations that
bind to sites (referred to as A, B, or C sites) along the silicate
chains (see Figure 2-4). The specific cations between the two
double-chain plates define the elemental composition of the
mineral, while the ratio of these cations in each location is used
to classify amphiboles within a solid-solution series. The general
chemical formula for double-chain inosilicate amphiboles is shown
below:
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(A) Nesosilicates or single tetrahedron. The single tetrahedron
comprises four oxygen molecules covalently bound to the silicon, at
the center of the [SiO4]4−-tetrahedron. (B) Inosilicates [ino (gr.)
= thread]―Single-chain silicates. Chain silicates are realized by
linking [SiO4]4−-tetrahedrons in a way to form continuous chains.
They can be represented by a composition of [SiO3]2−. A typical
example is diopside CaMg[Si2O6], in which the “endless” chains are
also held together by Ca2+ and Mg2+ ions. (C)
Inosilicates―Double-chain silicates. Two silicate chains of the
inosilicates are linked at the corners, forming double-chains and
yielding [Si4O11]6− ions, as realized in the
tremolite-ferro-actinolite series Ca2(Mg,Fe)5Si8O22(OH,F,Cl)2.
Double-chain silicates are commonly grouped with the single-chain
inosilicates. (D) Phyllosilicates [phyllo (gr.) = sheet] or sheet
silicates. These are formed if the double-chain inosilicate
[Si4O11]6− chains are linked to form continuous sheets with the
chemical formula [Si2O5]2−. Examples of sheet silicates include
chrysotile Mg3Si2O5(OH)4 and vermiculite [(Mg,
Fe,Al)3(Al,Si)4O10(OH)2 ●4H2O].
Figure 2-3. Structure of the silicate minerals, illustrating
silicate subclasses by the linking of the basic silicon tetrahedron
(A) into more complex structures (B, C, or D).
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Figure 2-4. Cross section of amphibole fibers showing the
silicon tetrahedrons (triangles with open circles at apex) that
make up each double-chain plate (shown along the fiber axis).
Cations (shown as the darkened dots) occur between the plates
forming the basic fiber.
Source: Kroschwitz et al. (2007).
A0−1B2C5T8O22(OH, F, Cl)2 (2-1) where:
A = Na, K B = Na, Li, Ca, Mn, Fe2+, Mg C = Mg, Fe2+, Mn, Al,
Fe3+, Ti T = Si, Al
The mineral subgroup within amphiboles is determined by the
elemental composition.
• Calcic amphiboles (tremolite)
• Sodic-calcic amphiboles (richterite, winchite)
• Sodic amphiboles (riebeckite [also known as “crocidolite”],
arfvedsonite)
• Iron-magnesium-manganese-lithium amphiboles (anthophyllite,
cummingtonite-grunerite [also known as “amosite”])
Because the stoichiometry of the cations is not fixed, a
continuum of compositions may
occur. These are referred to as “solid solution series.” The
series are defined by their end
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members. For example, a solid solution series for the cation
Site A will have one end member with 100% sodium ions and one end
member with 100% potassium ions. This series would include all
intervening ratios.
Because each cation site has multiple possibilities, the
elemental composition of the amphibole silicates can be quite
complex. It is the complexity of the amphiboles that has
historically given rise to a proliferation of mineral names with
little systematic basis (Hawthorne, 1981). Currently, amphiboles
are identified by a clear classification scheme based on crystal
chemistry that uses well-established names based on the basic
mineralogy, with prefixes and adjective modifiers indicating the
presence of substantial substitutions that are not essential
constituents of the end members (Leake et al., 1997). As
implemented, this mineral classification system does not designate
certain amphibole minerals as asbestos. However, some mineral
designations have traditionally been considered asbestos (in the
asbestiform habit; e.g., tremolite, actinolite). Other commercial
forms of asbestos were known by trade names (e.g., Amosite) rather
than mineralogical terminology (cummingtonite-grunerite).
2.2.3. Morphology of Amphibole Minerals
Most amphibole minerals occur in a variety of growth habits,
depending on the temperature, pressure, local stress field, and
solution chemistry conditions during crystallization. The
nomenclature used to describe the crystal forms varies between
disciplines [field geologist, microscopist; e.g., see Lowers and
Meeker (2002)]. Text Box 2-1 provides definitions for common terms
used to describe the morphology of asbestos and other related
minerals.
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Text Box 2-1: Nomenclature
Acicular: The shape showed by and extremely slender crystal with
small cross-sectional dimensions (a special case of prismatic
form). Acicular crystals may be blunt-ended or pointed. The term
“needlelike” refers to an acicular crystal with pointed termination
at one of both ends.
Amphibole: A group of silicate minerals that may occur either in
massive or fibrous (asbestiform) habits.
Asbestiform (mineralogical): A specific type of mineral
fibrosity in which the fibers and fibrils are long, thin, and
possess high tensile strength and flexibility.
Asbestiform (regulatory): A specific type of fibrosity in which
the fibers and fibrils possess high tensile strength and
flexibility.
Asbestos: A group of highly fibrous silicate minerals that
readily separate into long, thin, strong fibers that have
sufficient flexibility to be woven, are heat resistant and
chemically inert, are electrical insulators, and therefore are
suitable for uses where incombustible, nonconducting, or chemically
resistant materials are required.
Bundle: A group of fibers occurring side by side with parallel
orientations.
Cleavage fragment: A fragment produced by breakage of crystal in
directions that are related to the crystal structure and are always
parallel to possible crystal faces.
Cluster: A group of overlapping fibers oriented at random.
Fiber (regulatory): A particle that has an aspect ratio (length
of the particle divided by its width), and depending on the
analytical methods used, a particle is considered a fiber if it has
a length greater than or equal to 5 µm and aspect ratio greater
than or equal to 3:1 (by PCM) or 5:1 (by transmission electron
microscopy [TEM]).
Fiber (mineralogical): The smallest, elongate crystalline unit
that can be separated from a bundle or appears to have grown
individually in that shape, and that exhibits a resemblance to
organic fibers.
Fibril: A single fiber which cannot be separated into smaller
components without losing its fibrous properties or appearance. A
substructure of a fiber.
Fibrous: The occurrence of a mineral in bundles of fibers,
resembling organic fibers in texture, from which the fibers can
usually be separated. Crystallized in elongated, thin, needlelike
grains or fibers.
Massive: A mineral form that does not contain fibrous
crystals.
Matrix: A particle of nonasbestos material that has one or more
fibers associated with it.
Prismatic: Having blocky, pencil-like elongated crystals that
are thicker than needles.
Structure: A term used mainly in microscopy, usually including
asbestos fibers, bundles, clusters, and matrix particles that
contain asbestos.
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Asbestiform morphology is present where the conditions of
formation allow crystals to form very long individual flexible
fibers which are parallel and easily separable and may become
visible to the naked eye when crushed (see Figure 2-5). Under the
microscope, individual amphibole structures may be described as
asbestiform, acicular, prismatic, or fibrous. Typically, a fiber is
defined as a highly elongated crystal with parallel sides. The
definitions for acicular crystals are “needlelike” in appearance
while prismatic crystals may have several parallel faces with a low
aspect ratio (ratio of length to width,
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2.3. METHODS FOR ANALYSIS OF ASBESTOS Because asbestos is a
solid that does not dissolve in water or other solvents, methods
for
the analysis of asbestos are somewhat different than for most
other chemical substances. This section provides a brief overview
of the most common methods for the analysis of asbestos.
2.3.1. Methods for Air Samples
The exposure pathway of primary health concern for humans is
inhalation of asbestos. Air is evaluated for the presence of
asbestos by drawing a known volume of air through a filter that
traps the solid particles in the air on the filter surface, and
the