A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles (2009) / Chapter Skim
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3 Major Scientific Issues: State of the Science and Future Research Directions
3 Major Scientific Issues: State of the Science and Future Research Directions
Pages 33-76
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From page 33... ...
The proposed studies are founded upon a broad span of scientific literature on these topics that has been well summarized in the draft Roadmap. This chapter provides the committee's review of the major scientific issues discussed in the Roadmap.
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From page 34... ...
NIOSH has introduced the term elongated mineral particle to encompass the broad range of mineral particles that are the primary focus of the proposed research. The committee urges the use of the descriptive term elongate, rather than elongated so as to describe the physical appearance of the particles as opposed to implying that they have been actively lengthened (see also 1 Suggestions for terms that need well-referenced definitions include acicular, actinolite, amphibole, anthophyllite, asbestiform, asbestos, chrysotile, cleavage fragment, crocidolite, fiber, fibril, fibrous, solid solution series, and tremolite.
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From page 35... ...
As such, the term elongate mineral particle is a convenient, neutral, and uniform means for the disciplines of mineralogy, toxicology, and epidemiology to discuss broad categories of mineral particles with potentially widely varying potency for causing cancer and other health effects. In the NIOSH Roadmap and in this report, the focus is on minerals, which are naturally occurring substances; discussions of research on synthetic materials are included to provide examples of potential research direc
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From page 36... ...
. The potential health effects of some nonasbestiform mineral particles have not been studied, and the potency of respirable prismatic, acicular, and fibrous particles that do not meet the definitions of commercial asbestos is not known.
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From page 37... ...
. This dichotomy between the generally precise IMA definitions and the less detailed and less precise CAS definitions presents significant difficulties for those involved in work on asbestos and other elongate minerals.
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From page 38... ...
These criteria are well defined and can be quantified. The Roadmap would benefit from further emphasis on the mineralogical research needed and from discussion of the development of standardized reference mineral samples that could be used in toxicological studies to assess the variability in the toxicity of different types of elongate mineral particles.
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From page 39... ...
This form of minerals characterization is used routinely by the mineral industry to identify and quantify mineral samples in terms of their variability in composition, size, and shape. Whether establishing a new reference mineral sample for eventual use in health-related research or characterizing an unknown suite of elongate mineral particles from an air sample filter, a basic set of mineral
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From page 40... ...
The Roadmap recognizes that workers may be exposed to any number of crushed and ground and/or contaminant particles introduced during mining, milling, manufacturing, and demolition of the materials. In these cases, the size and shape criteria used to describe elongate mineral particles encompass many mineral groups in addition to asbestos and analogous minerals.
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From page 41... ...
graphic analysis of granular, acicular, thin sections and radiated growth hab hand specimen its)
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NOTES: AFM = atomic force microscopy; EBSD = electron backscattered diffraction; EPMA = electron probe microanalysis; EXAFS = extended X-ray absorption fine structure; PLM = polarized light microscopy; SEM = scanning electron microscopy; TEM = transmission electron microscopy; XRD = X-ray diffraction; XRF = X-ray fluorescence. When analyzing natural materials for basic mineralogical research and characterization, data from steps 1 and 2 are required, whether from historical sources or newly collected by the analyst.
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From page 43... ...
Standardized Reference Minerals Well-characterized reference mineral samples are important for research on the potential health effects of elongate mineral particles, and the need for a well-managed repository should be emphasized. The identification, classification, and characterization of unknown mineral particles from workplace or environmental exposures require comparison to rock-forming minerals that have been characterized mineralogically by conventional petrographic techniques.
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From page 44... ...
. The existing mineral samples vary in the extent to which they have been characterized, and all existing samples and new mineral samples would eventually have to be examined and represented by the same types of basic analytical data, generated using viable modern techniques (see Table 3-1)
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From page 45... ...
Given likely developments in instrumentation and classification, the standards should be assessed to allow for future enhancements in the data set. TOXICITY SCREENING AND TESTING State of Science and Future Directions As discussed in Chapter 2, the Roadmap needs to explicitly outline a standardized approach to screening for the potential hazards of elongate mineral particles whose biological effects have not yet been characterized.
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In vivo toxicity assays (animal studies) FIGURE 3-1 Overview of a possible testing strategy for elongate mineral particles.
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If the particle is respirable or of a thoracic size fraction, it should be considered potentially hazardous and war rants further evaluation. Concern is heightened if the in haled particles include dimensions that are consistent with other known harmful elongate mineral particles.
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From page 48... ...
helps place the results in context; this is particularly valuable when toxicity data may also be available for these other elongate mineral particles. Although solubility assays are reproducible using the same type of elongate mineral particles, the assay should include concurrent control particles with known solubility.
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From page 49... ...
The studies also can be used to assess the effects of dimension, chemical composition, and interaction of particles with other environmental toxicants such as chemical carcinogens, viruses, or radiation (radon) in modulating the biological behaviors of the elongate mineral particles.
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. The thoracic or respirable fraction of the elongate mineral particles would be tested using a range of doses reflecting those that may occur during human exposure, depending on the choice of the in vitro target cell, in parallel with control particles having extremes of responses in each assay.
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From page 51... ...
The description of genetic toxicology studies in the Roadmap needs to be improved. Although various types of asbestos have been shown to induce chromosomal aberrations and sister chromatid exchanges in human mesotheliomas and lung cancers and in cultured human and mammalian cells, mutagenic studies at most mammalian genetic loci have largely been negative (Jaurand, 1996; Hei et al., 2000; Schins and Hei, 2006, for review)
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In Vivo Toxicity Assays Elongate mineral particles ranked as likely hazardous based on their mineralogical characteristics or having characteristics that prevent close correspondence to particles of known toxicity would be tested in vivo. It should be acknowledged that the three general types of responses of greatest concern, based on experience with asbestos, are fibrosis, mesothelioma, and lung cancer.
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From page 53... ...
If the correlations were adequately reviewed, it might become clear which types of animal studies would or would not be useful to predict the relative toxicity of an unknown elongate mineral particle with a reasonable degree of certainty. The committee considers that existing data may be sufficient to provide guidance on the design of animal studies.
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From page 54... ...
• Animals should be exposed to multiple concentrations. Exposure levels should be selected based on several considerations, such as maximum tolerated dose, number of thoracic elongate mineral particles in the aerosol having the length range of concern (e.g., >20 μm long)
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, lung cancer, and meso thelioma. In the rat, pulmonary fibrosis has consistently preceded the development of lung cancer and mesothelioma from inhaled elongate mineral particles.
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Appendix C provides a more detailed overview of these potentially useful experimental design tools. EPIDEMIOLOGY Assessment of the State of the Science The Roadmap discusses epidemiological studies of workers with mining and/or milling exposures to mineral particles that have been reported to be nonasbestiform from three different regions: the talc mining region of upstate New York, the Homestake gold mine in South Dakota, and the taconite iron ore mines in northeastern Minnesota.
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From page 57... ...
This subsection concludes that the studies of the Homestake gold miners provide at best weak evidence of an excess risk of lung cancer, and there are inadequate data on worker exposures to nonasbestiform minerals. The subsection on studies of taconite iron ore miners and millers in northeastern Minnesota indicates that recent sampling and analysis of the ores from the taconite iron ore mines reported no asbestos, but did find ferroactinolite, ferrian sepiolite, grunerite-ferroactinolite, and actinolite, some of which was fibrous.
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The committee agrees with the Roadmap's assessment of the results of the epidemiological studies on elongate mineral particles as inconclusive.
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From page 59... ...
Multiple international studies of exposure to amphibole particles have not been included. While the committee understands that the Roadmap was not intended to be a comprehensive review of the asbestos epidemiological literature, some attention to these studies seems relevant for planning future research on the health effects of exposure to elongate mineral particles.
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From page 60... ...
The Roadmap also notes the need to determine whether elongate amphibole particles pose a risk to human health and recommends that an expert panel be assembled to evaluate whether the existing epidemiological evidence could support development of a likely maximum risk estimate associated with exposure to these elongate mineral particles. Based on the review of the epidemiological literature contained in this document, it does not appear that the epidemiological evidence is sufficiently robust for such an endeavor.
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From page 61... ...
. The committee notes that although the development of sensitive bioassays for intermediate end points on the pathways from exposure to asbestos and other elongate mineral particles to pulmonary fibrosis, lung cancer, and mesothelioma is a laudable research goal, the application of such bioassays to medical surveillance of exposed workers would require a major and lengthy validation effort given the long latency of these health outcomes.
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From page 62... ...
. Further, the Roadmap states that opportunities for epidemiological studies of workers exposed to asbestos and the other elongate mineral particles are present in other countries.
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2. Develop analytical methods with improved sensitivity to visual ize thinner elongate mineral particles to ensure a more complete evaluation of airborne exposures.
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From page 64... ...
The document presents a good case for why PCM is an inadequate tool for elongate mineral particle exposure assessment yet falls short of recommending that it be replaced with PLM and electron microscopy techniques. PCM does not provide the detailed exposure characterization needed to conduct the complex risk assessments described in the Roadmap.
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From page 65... ...
Regardless of method used, more discussion of the statistics of elongate mineral particle counting and counting quality control is needed. The advancement of science with respect to elongate mineral particle exposure assessment requires nationally recognized quality control programs.
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From page 66... ...
Statisticians who are also familiar with exposure data analysis should be actively involved in addressing some of the challenging data analysis issues in this area of research. Additional Statistical Issues There is considerable discussion in the NIOSH Roadmap on the effect of fiber counts and fiber dimensions on exposure risk.
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From page 67... ...
If the research is intended to address all elongate mineral particles, not just asbestos or its analogs, a different title might be appropriate. If the Roadmap is expanded at some point to include a larger range of elongate particles, more generally (whether minerals, man-made materials [e.g., ceramics]
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From page 68... ...
If necessary, a note could be added to clarify or update the terminology. Because elongate mineral particle is a broad descriptive term and not a rigorous mineralogical term, the preference when feasible is for providing the correct mineral names.
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In Silica, Silicosis, and Lung Cancer, edited by D Goldsmith, D
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1997. Malignant transformation of immortalized human bronchial epithelial cells by asbestos fibers.
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Presentation to the Committee for the Review of the NIOSH Roadmap on Asbestos and Other Elongate Mineral Particles. Wash ington, DC, March 30.
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1997. Nomenclature of amphiboles: Report of the subcommittee on amphi boles of the International Mineralogical Association, Commission on New Minerals and Mineral Names.
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1997. Relevance of particle-induced rat lung tumors for assessing lung carcinogenic hazard and human lung cancer risk.
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From page 74... ...
NIOSH current intelligence bulletin. Asbes tos fibers and other elongated mineral particles: State of the science and roadmap for research.
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From page 75... ...
2007. Vermiculite, respiratory disease and asbestos exposure in Libby, Montana: Update of a cohort mortality study.
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76 REVIEW OF THE NIOSH ROADMAP solution rate relevant to biopersistence at neutral pH: An interlabo ratory round robin. Inhalation Toxicology 9(6)
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