amphibole hypothesis

The amphibole hypothesis is the idea that the amphibole forms of asbestos, chiefly crocidolite, amosite and tremolite, are far more potent causes of mesothelioma than chrysotile, and that mesotheliomas seen in chrysotile workers may come from small amounts of tremolite mixed into chrysotile ore^{1, 2}. A 1996 review in the American Journal of Public Health defined it in two parts: that amphibole contamination may explain mesotheliomas among chrysotile-exposed workers, and that chrysotile may be a weaker cause of cancer than the amphiboles¹. The question matters in lawsuits because most asbestos used in the United States was chrysotile^{3, 4}. The library holds the industry papers in which the idea was used, the medical articles on both sides of the debate, and the government rulemakings that weighed it.

Two families of asbestos

Chrysotile is the fibrous form of serpentine⁵. The other asbestos minerals are amphiboles: amosite and crocidolite, the two used most in industry, and anthophyllite, tremolite and actinolite⁶. Asbestos magazine told its readers in 1928 that South Africa produced chrysotile and "the two types of amphibole asbestos," crocidolite and amosite⁷. Johns-Manville's 1966 position paper on health said about 95 percent of the asbestos used in the United States was chrysotile, mainly from Canada³. In 1979 the magazine reported that the United States took some 98 percent of its chrysotile from Canada and almost all of its amosite and crocidolite from South Africa⁴.

Early observations of differences by fiber type

Dr. Philip Enterline's 1978 history of the medical literature, written for the industry's trade association, credits K. W. Smith with a 1955 review that blamed the higher lung cancer reports in Britain and Europe on greater use of amosite and crocidolite, "a harsher fiber" than the Canadian chrysotile used in North America⁸. Enterline called him one of the first writers to suggest that fiber types might differ in their power to cause cancer⁸.

The 1960 paper by J. C. Wagner, C. A. Sleggs and Paul Marchand described 33 cases of pleural mesothelioma in South Africa's North Western Cape; all but one had a probable exposure to crocidolite, many of them from living near the mines and mills⁹. Writing in 1997, Wagner recalled that in 1963 his group confirmed the tumors were occurring in the crocidolite mining area but not in the amosite or chrysotile areas of southern Africa¹⁰. A 1974 review of the asbestos health literature noted the "relative absence" of mesotheliomas among amosite and crocidolite miners in the Transvaal compared with the large number in the Cape crocidolite region¹¹.

Animal work pointed the other way. A 1967 report written for Union Carbide's British operation described Wagner's experiments in which chrysotile, amosite and crocidolite injected into rats all produced mesothelial tumors in a high proportion of animals¹². OSHA's 1983 emergency standard cited injection results from Wagner and colleagues, with mesotheliomas after crocidolite in 61 percent of animals and after Canadian chrysotile in 30 percent¹³.

The industry's use of the idea

An undated booklet of the Asbestos Information Association/North America said chrysotile made up 97 percent of U.S. use, in part because of "industry awareness of medical studies that show amosite and crocidolite to be more potentially hazardous to health than chrysotile."¹⁴ When the ACGIH said in 1980 it would recommend separate exposure limits by fiber type (2 fibers per milliliter for chrysotile, 0.5 for amosite, 0.2 for crocidolite), the Asbestos International Association wrote to protest that the standard would virtually ban amosite and crocidolite¹⁵.

Union Carbide, which by its own account mined and sold a chrysotile it called Calidria from 1963¹⁶, kept papers on the question. A 1983 packet of material from Dr. Hilton Lewinsohn, its assistant corporate medical director, included an article by Craighead and Mossman stating that, although the results were debated, the bulk of the evidence showed chrysotile was not an important cause of mesothelioma in Quebec and Italian miners, while adding that data on textile workers and insulators were "not as definitive."¹⁷ In February 1985 Malcolm Ross, a U.S. Geological Survey mineralogist, wrote to an EPA official that Dr. Churg's lung studies showed amphibole fibers far outnumbering chrysotile in mesothelioma victims and that mesothelioma was rare among Quebec chrysotile workers; the copy in Union Carbide's files is stamped as received by Lewinsohn¹⁸. Comments by Dr. Brian Commins, circulated by the Asbestos International Association in 1985, said mesothelioma from inhaled asbestos was "mainly associated with heavy exposure to amosite and crocidolite."¹⁹

The scientific debate

In a 1993 editorial, B. T. Mossman described a model in which crocidolite's durability and iron content explain its greater power to cause mesothelioma, while chrysotile dissolves in the lung²⁰. The same year, J. C. McDonald and colleagues reported 33 mesothelioma deaths in their cohort of about 11,000 Quebec chrysotile miners and millers, five of them in a factory that also used commercial amphiboles, and a higher risk at Thetford Mines than at Asbestos²¹.

In 1996 L. T. Stayner, D. A. Dankovic and R. A. Lemen reviewed the evidence and concluded that lung burden and mechanism studies did not convincingly support the hypothesis, that chrysotile was strongly linked to lung cancer and mesothelioma, and that it was prudent to treat chrysotile with "virtually the same level of concern" as the amphiboles¹. An accompanying editorial by M. R. Cullen traced the idea to two 1970s observations, faster clearance of chrysotile from the lung and lower disease rates in some chrysotile cohorts, and said that continued reference to it only distorted the debate²².

The reply letters of 1997 show the split. A. M. Langer and R. P. Nolan argued that the hypothesis had become a "tremolite hypothesis" and was not gone²³. Mossman and J. B. Gee wrote that its thrust was mesothelioma rather than lung cancer and that it remained viable²⁴. Wagner wrote that no mesotheliomas had been shown in chrysotile workers unless exposure was intense, lasted more than 20 years, and involved chrysotile contaminated with tremolite¹⁰. In 1998 A. L. Frank, R. F. Dodson and M. G. Williams examined more than 20,000 fibers of the UICC chrysotile reference sample used in animal studies, found no tremolite, and concluded that chrysotile itself can cause mesothelioma²⁵. A 2005 review by M. Greenberg described the defense of Canadian chrysotile as a public relations effort that blamed other agents for its harm²⁶.

What regulators decided

OSHA heard the argument in its 1986 rulemaking. Dr. Kenny Crump testified that the mesothelioma risk from chrysotile was lower than from amphiboles and that risk to modern workers should be estimated from chrysotile studies²⁷. OSHA answered that most future exposure would come from the demolition, renovation and removal of older asbestos products such as insulation, which generally contained amphiboles²⁸, and its final rule set one limit, 0.2 fiber per cubic centimeter, for asbestos, tremolite, anthophyllite and actinolite alike²⁹. In 1990 the EPA disagreed with a commenter who wanted chrysotile regulated less strictly than the amphiboles, calling the evidence for that view, as to lung cancer, "inconclusive and inconsistent."³⁰ A 2011 NIOSH research roadmap described the hypothesis and cited studies of workers exposed to chrysotile with very little tremolite who still showed strong links to lung cancer². The International Agency for Research on Cancer classes all forms of asbestos, including chrysotile, as causes of cancer in humans.

In asbestos cases

Defendants who sold chrysotile products, such as brakes, joint compound, gaskets and floor tile, use the hypothesis to argue that their fiber could not have caused a mesothelioma. Cullen wrote in 1996 that, in his experience, compensation claims involving "only" chrysotile had been contested or denied²². Even supporters of the hypothesis, such as Mossman and Gee, said its main concern was mesothelioma rather than lung cancer²⁴.

Related Terms

Chrysotile, Crocidolite, Amosite, Tremolite, Amphibole Asbestos, Serpentine Group, Amphibole Hypothesis Strong Form, Amphibole Hypothesis Weak Form, Chrysotile Defense, Tremolite-Not-Chrysotile Defense, Quebec Chrysotile Contamination Pathway, Stayner 1996, Wagner 1960 Study, Biopersistence, Fiber Burden Analysis, Helsinki Criteria, Mesothelioma

References

1. L. T. Stayner, D. A. Dankovic and R. A. Lemen, Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis (1996)
2. Asbestos Fibers and Other Elongate Mineral Particles: State of the Science and Roadmap for Research (2011), p. 55
3. Johns-Manville "Position Paper on the Health Aspects of Asbestos" (1966), p. 6
4. Asbestos Magazine, December 1979, p. 18
5. Asbestos Magazine, July 1953, p. 12
6. Asbestos Geology, Mineralogy, Mining, and Uses (2002), p. 4
7. Asbestos Magazine, May 1928, p. 5
8. Philip E. Enterline, Asbestos and Cancer: The First Thirty Years (1978), p. 30
9. J. C. Wagner, C. A. Sleggs and Paul Marchand, Diffuse Pleural Mesothelioma and Asbestos Exposure in the North Western Cape Province (1960)
10. J. C. Wagner, Asbestos-related cancer and the amphibole hypothesis: the first documentation of the association (1997)
11. Review of Asbestos-Health Literature (1974), p. 6
12. UC-1378: I. C. Sayers, Asbestos as a Health Hazard in the United Kingdom (1967), p. 12
13. Occupational Exposure to Asbestos; Emergency Temporary Standard (1983), p. 32
14. AIA-117: Asbestos Information Association/North America, What Asbestos Is: How and Where It Is Used (undated), p. 2
15. UC-3937: Asbestos International Association Director-General's Notes for Associations (1980), p. 17
16. UC-5123: Union Carbide Corporation's Responses to Plaintiffs' Standard Interrogatories, In re Asbestos Cases, Boulder County, Colorado (2002), p. 15
17. UC-2636: V. H. Johnkoski Transmittal of H. C. Lewinsohn's Asbestos Disease Abstracts and Articles (1983), p. 27
18. UC-3832: Malcolm Ross to Dr. Bernard D. Goldstein on Churg's Paper on Lung Fiber Content in Chrysotile Workers with Mesothelioma (1985), pp. 1–2
19. UC-4086: Asbestos International Association, Dr. Brian Commins' Comments on Nine Statements about Asbestos Health Risks (1985), p. 2
20. B. T. Mossman, Mechanisms of asbestos carcinogenesis and toxicity: the amphibole hypothesis revisited (1993)
21. J. C. McDonald and others, The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 (1993)
22. M. R. Cullen, The amphibole hypothesis of asbestos-related cancer: gone but not forgotten (1996)
23. A. M. Langer and R. P. Nolan, Asbestos-related cancer and the amphibole hypothesis: neither gone nor forgotten (1997)
24. B. T. Mossman and J. B. Gee, Asbestos-related cancer and the amphibole hypothesis: the hypothesis is still supported (1997)
25. A. L. Frank, R. F. Dodson and M. G. Williams, Carcinogenic implications of the lack of tremolite in UICC reference chrysotile (1998)
26. M. Greenberg, The art of perpetuating a public health hazard (2005)
27. Federal Register: Occupational Exposure to Asbestos, Tremolite, Anthophyllite, and Actinolite (1986), pp. 54–55
28. Federal Register: Occupational Exposure to Asbestos, Tremolite, Anthophyllite, and Actinolite (1986), p. 56
29. Federal Register: Occupational Exposure to Asbestos, Tremolite, Anthophyllite, and Actinolite (1986), pp. 1–2
30. UC-5195: EPA, National Emission Standards for Asbestos: Background Information for Promulgated Asbestos NESHAP Revisions (1990), p. 32