Does Low Exposure to Chrysotile Pose a Health Risk?
Dr. Tom Hesterberg Principal Toxicologist Center for Toxicology and Environmental Health
American Conference Institute’s Asbestos Claims and Litigation January 30-31, 2014 San Francisco, California
5120 North Shore Drive | North Little Rock, AR 72118 | Main Line: 501.801.8500 Overview of Fiber Toxicology Studies
• Basics of Fiber Toxicology • Chromosomal Effects • Fiber Biopersistence • Rodent Inhalation Studies • Thresholds for Fiber Toxicity
2 Basics of Fiber Toxicology
3 Dose The Cornerstone of Toxicology
“All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.”
Paracelsus (1493-1541).
4 Definition of Threshold
The dose below which a given effect is not observed
A threshold is a result of our bodies’ protective mechanisms
5 Doses of Common Substances
Substance Normal Lethal
Water 1.5 qts 15 qts
Aspirin 2 tablets 90 tablets
Table Salt 3 tsp. 60 tsp.
Cyanide in Lima Beans 0.5 cups 11 cups
Toxicity is the adverse effect caused when a chemical reaches a sufficient dose—threshold.
6 Asbestos Types
Chrysotile Asbestos Crocidolite Asbestos The Three Ds of Fiber Toxicology
Dose - Amount reaching the deep lung
Dimension - Thin fibers deposit in the deep lung; long fibers are more toxic
Durability - Dissolution and breakage; more durable fibers are more toxic
Hesterberg and Hart., Inhal. Tox., 2001
8 Biopersistence Determines Toxic Potential of Fibers
Long Fiber (> 20 µm)
Incongruent Dissolution Congruent Dissolution
Transverse Breakage
Complete Altered Biological Translocation Dissolution Reactivity Macrophage Uptake
Mucociliary Clearance Epithelial Cell Uptake Intracellular Degradation Translocation to Interstitium
Hesterberg and Hart., Inhal. Tox., 2001 9 Important to Link Exposure and Dose and to the Lung’s Response to Asbestos Fibers
Exposure
Dose
At High Doses: Response Inflammation Fibrosis Mesothelioma Lung cancer
10 What happens to fibers in the lung?
Movement Up the Mucociliary Escalator
Macrophage with fiber Asbestos in the Lung
Macrophage Exposure Early Clearance of Short Fibers Soluble Long Fibers Rapid Recovery of Normal Durable Long Fibers Chronic Inflammation Chromosomal Effects of Fibers in Cells Grown in Culture Dishes
17 Asbestos Fibers are Taken Up by Cells Grown in Culture and Accumulate in the Perinuclear Region of the Cell
Hesterberg, Oshimura, Brody and Barrett, 1986 Asbestos Induces Anaphase Abnormalities in Syrian Hamster Embryo (SHE) Cells in Culture Dishes
Hesterberg and Barrett, 1985 Asbestos and Fiber Glass Induce Cytogenetic Effects in SHE Cells in Culture Dishes
Hesterberg et al., 1985 But Asbestos Does Not Induce Cytogenetic Effects in Primary Cultures of Human Bronchial Epithelial Cells Grown in Culture Dishes
Kodama et al., 1993 Toxicity of Asbestos to Primary Cultures of Mesothelial Cells
Untreated Mesothelial Cells
Apoptosis in Asbestos-Treated Mesothelial Cells
Broaddus et al., 1996
22 Toxicity of Asbestos to Primary Cultures of Human Mesothelial Cells • Induced apoptosis, not chromosome effects Broaddus et al., 1996: • All cells die within one week Yang et al., 2006 • Programmed necrosis Yang et al., 2010 How can asbestos cause mesothelioma if human mesothelial cells exposed to asbestos die? Yang et al., 2006
23 Fibers With the Same Length Have the Same Toxicity to CHO Cells in Culture
Hart et al., 1994 No Correlation Between Cell Culture Chromosomal Effects and Inhalation Toxicity of Fibers
In Vitro Toxicity Rat Inhalation Toxicity Fiber Chromosome Effects Fibrosis Tumors
Crocidolite Yes Yes Yes Chrysotile Yes Yes Yes RCF1 Yes Yes Yes 475 Fiber Glass Ye s Yes No MMVF 22 (Slag Wool) Yes No No X607 Fiber Yes No No MMVF10 (Fiber glass) Yes No No MMVF22 (Mineral wool) Yes No No
Hesterberg and Hart, 2001 Hart et al., 1994
25 Fiber Dissolution and Biopersistence Studies
26 Measurement of In Vitro Dissolution of Fibers Simulated Lung Fluid
Fibers
Analyze Solutes: Outflow Na +,Ca +2 , etc. Leaching of the Outer Layer of a Fiber Glass Fiber Leaching of Metals From Fibers Leaving Weakened Silicon Dioxide Matrix
Na+ Ca+2
Mg+2
K+ B+3
SiO2 Matrix Vitreous Fibers Dissolve and Break
Glass Fibers Lung Macrophage After 3 Days in Lung Fluid with Fiber
30 Asbestos Physical/Chemical Structure Determines Biopersistence
Chrysotile Asbestos Crocidolite Asbestos
31 Inhalation Biopersistence Study Protocol
32 Biopersistence of Fibers in the Lung
120
100
80
60 % of Day 1 Day of % Amosite
40
Fibers/Lung >20 µm >20 Fibers/Lung E Glass 20 475 Glass Cert. Glass 901 Glass 0 0 90 180 270 360 Recovery Days Biopersistence of Amosite and Chrysotile
Bernstein and Hoskins, 2006
34 Rodent Studies of Lung Disease
35 Advantages of Rodent Inhalation Studies
• Respirable fibers used • Exposure determined • Dose = Lung burdens • Fibrosis, lung cancer, and mesothelioma • Biopersistence • Determine threshold • Relationship between fibrosis and tumors
36
Lungs 14 Days After Similar Exposures to Tremolite or Chrysotile
Tremolite Chrysotile
Bernstein, 2003 Chronic Inhalation Studies of Fiber Toxicity and Tumorigenicity
At Risk Animals
Inhalation Exposure Recovery
3 Mo 6 Mo 12 Mo 18 Mo 24 Mo 20%
Recovery
Lung Histopathology and Burden Rodent Chronic Fiber Inhalation Studies
Fiber Type Species Fibrosis Lung Cancer Meso
Air Control R - 1-3.7 0 Asbestos Amosite H + 0 26* Asbestos Amosite R + 38* 5 Asbestos Crocidolite R + 13.2* 0.9 Asbestos Chrysotile R + 18.5* 1.4 Asbestos Chrysotile H + 0 0 FG-SA MMVF32 R + 23* 4.7 Ceramic RCF1 R + 12.4* 1.6 Ceramic RCF1 H + 0 41* FG-SA MMVF33 H + 0 1.2 Rock MMVF21 R + 4.4 0 FG-BI MMVF10 R - 5.8 0 Hybrid X607 R - 0 0 FG-BI MMVF11 R - 2.6 0 Slag MMVF22 R - 2.6 0 HT MMVF34 R - 0 0
Hesterberg and Hart, CRT, 2001 39 If Chrysotile is Biosoluble in the Lung, then Why did it Produce Lung Disease in the Animal Studies?
40 Lung Dose = Deposited Dose – Amount Cleared Lung Burdens of Chrysotile and Other Fibers During a Chronic Inhalation Study
Two Year 6 Month Lung Burden Recovery Percent Retained Produced Fiber Exposure Fibers/Lung Fibers/Lung After 6 Mo Lung
Type Fibers/cc (X 106) (X 106) Recovery Disease X607 174 58 15 26% No RCF 187 143 61 58% Yes Chrysotile 10,600 1,600 216 14% Yes
Hesterberg et al., 1998 Lung Burdens of Chrysotile and Other Fibers During a Chronic Inhalation Study
Two Year 6 Month Lung Burden Recovery Percent Retained Produced Fiber Exposure Fibers/Lung Fibers/Lung After 6 Mo Lung
Type Fibers/cc (X 106) (X 106) Recovery Disease X607 174 58 15 26% No RCF 187 143 61 58% Yes Chrysotile 10,600 1,600 216 14% Yes
Hesterberg et al., 1998 Chrysotile Does Not Cause Mesotheloimas in Hamsters
Num ber Lun g Mesoth elial Group Animals Fibrosis Hyperplasia Meso thelioma Control 83 No No No MMVF 10a 81 No No No Chrysotile 49 Yes No No MMVF 33 83 Yes Yes Yes RCF 1 102 Yes Yes Yes Amosite 83 Yes Yes Yes
McConnell et al., 1995
Hesterberg et al., 1999
McConnell et al., 1999 Chronic Bioassays of Chrysotile Asbestos
• Chronic exposure resulted in lung fibrosis, cancer and a single mesothelioma in rats • Chrysotile exposure levels were very high — 10,600 f/cc • This is 100,000 times the OSHA PEL of 0.1 f/cc • Massive dose to the lung target tissues • Lung clearance likely impaired due to “overload”
Hesterberg et al., 1993
45 Threshold for Chrysotile-Induced Lung Disease is Above 500 f/cc
• Three month rat inhalation study of chrysotile • Exposure to 536 WHO fiber/cc • No lung fibrosis was observed at this level • 200-fold lower than tumorigenic exposure of 10,500 f/cc • Lack of fibrosis suggests no lung cancer or mesothelioma • This study indicates a threshold of > 500 f/cc
Bernstein et al., 2006.
46 Fiber Biopersistence Correlates with Inhalation Toxicity of Fibers
Biopersistence Rat Inhalation Toxicity Fiber Half-Life (Days) Fibrosis Tumors Crocidolite 817 + + Amosite 418 + + MMVF32 79 + + RCF1 55 + + MMVF33 49 + - MMVF21 67 + - MMVF 10 (Fiber glass) 14.5 - - X607 9.8 - - MMVF11 (Fiber glass) 9 - - MMVF22 (Mineral wool) 9 - - MMVF34 (Mineral wool) 6 - -
Hesterberg and Hart, 2001
47 Fiber Biopersistence Correlates with Inhalation Toxicity of Fibers
Biopersistence Rat Inhalation Toxicity Fiber Half-Life (Days) Fibrosis Tumors Crocidolite 817 + + Amosite 418 + + MMVF32 79 + + RCF1 55 + + MMVF33 49 + - MMVF21 67 + - MMVF 10 (Fiber glass) 14.5 - - Chrysotile (Canada) 11.3 X607 9.8 - - MMVF11 (Fiber glass) 9 - - MMVF22 (Mineral wool) 9 - - MMVF34 (Mineral wool) 6 - - Chrysotile (Brazil and CA) 1.3, 0.3 Hesterberg and Hart, 2001
48 Lung Fibrosis Occurs Before Lung Cancer and Mesothelioma in Rodents
Hesterberg and Hart, 2001 Airborne TWA Fiber Concentrations from Drywall Workers (fibers/cc ≥5 µm) Study TWA
Contemporary Studies
Brorby et al. (2013) (respirable PCM 0.26 (median) fibers, enclosed space)
Brorby et al. (2013) (respirable PCM 0.078 (median) fibers, nonenclosed space)
Contemporary Estimates of Historical Exposures
Brorby et al. (2013) 0.11 (median)
Historic Studies
Rhodes and Ingalls (1976) 0.1 to 0.9
Rhodes and Spencer (1977) 0.005 to 0.3 (personal TWAs)
Verma and Middleton (1980) 2.1 to 4.5
Verma and Middleton (1981) 4.2
National Loss Control Service None Corporation, 1972
Soule (1973) None
Rohl et al. (1975)/Fischbein et al. (1979) None
PCM=phase contrast microscopy 50 Low Exposures to Chrysotile Do Not Result in Lung Disease
12000
10000
8000
6000
4000
Exposure Levels in Fibers/cc in Levels Exposure 2000
0 Lung Disease in No Lung Disease in Drywall Installers Animals Animals
51 Summary and Conclusions
• In vitro studies in non-lung rodent cells and cell lines have shown chromosomal effects, but – These effects are not seen in primary cultures of human lung cells – In vitro chromosome studies were invalidated by chronic animal inhalation studies • Chrysotile is very biosoluble in the lung • Threshold for chrysotile disease > 500 f/cc • Low chrysotile exposures do not cause disease
52 Does Low Exposure to Chrysotile Pose a Health Risk?
Dr. Tom Hesterberg Principal Toxicologist Center for Toxicology and Environmental Health
American Conference Institute’s Asbestos Claims and Litigation January 30-31, 2014 San Francisco, California
5120 North Shore Drive | North Little Rock, AR 72118 | Main Line: 501.801.8500