Alice Hamilton Awards: Research Updates for 2012 - Exposure and Risk Assessment

Paul Middendorf, Ralph Zumwalde, Robert Castellan, Martin Harper, William Wallace, Leslie Stayner, Vincent Castranova, Frank Hearl, & Patricia Sullivan

Current intelligence bulletin 62: asbestos fibers and other elongate mineral particles: state of the science and roadmap for research.

The Asbestos Roadmap outlined a research agenda that will guide the development of specific research programs and projects that will lead to a broader and clearer understanding of the important determinants of toxicity for asbestos fibers and other elongate mineral particles (EMPs). Several fundamental issues must be addressed before addressing the larger toxicity issues: terminology, development of narrow size-range elongate mineral particle sample, provision of high-quality and well-characterized mineral samples, and improved analytical methods.

Terminology

Imprecise terminology and mineralogical complexity have affected progress in research. "Asbestos" and "asbestiform" are two commonly used terms that lack mineralogical precision. "Asbestos" is a term used for certain minerals that have crystallized in a particular macroscopic habit with certain commercially useful properties. These properties are less obvious on microscopic scales, so a different definition of asbestos may be necessary at the scale of the light microscope or electron microscope, involving characteristics such as chemical composition and crystallography. The lack of precision in these terms and the difficulty in translating macroscopic properties to microscopically identifiable characteristics contribute to miscommunication and uncertainty in identifying toxicity associated with various forms of minerals. NIOSH researchers have engaged with the Society of Mining Engineers and other federal agencies, as well as academic and industry stakeholders to explore the feasibility of a symposium to explore solutions to these issues.

Mineralogical repository

A high quality reference material of tremolite asbestos is anticipated to be available soon.

Develop techniques to separate elongate particles by length

Direct toxicological testing has been hampered by the inability to prepare significant quantities of length-classified asbestos samples. The Baron aerodynamic fiber classifier has been demonstrated to successfully separate fibers at the desired length scale. Research has been conducted to increase the throughput of this instrument in order to separate fibers in quantities sufficient for subsequent toxicological study. Although there are concentration limitations of the aerosol generation, the instrument performance is quite robust. Techniques have also been developed to monitor, in real time, the quality of the product from this instrument. In parallel with our work to improve the Baron aerodynamic fiber classifier, alternative techniques have been developed, in particular, a cross-flow liquid filtration technique (Bauer-McNett fiber classification), which has been successfully used in the paper industry to characterize pulp fiber length. We will report on our effort to extend the operation of the Bauer-McNett classifier from separation at the 10^-3-10^-4 m scale (appropriate for pulp characterization) to the 10^-5 m scale (appropriate for asbestos separation). Using these various techniques, samples have been prepared of long and short glass fibers. The separations are not perfect; however, contamination of each fraction is less than 20 %. These separated glass fiber samples are currently being used for an in vitro comparison of the response of murine alveolar macrophages to fibers of different lengths. The next phase of this work is to separate fibrous samples of amphibole asbestos.

Improve sampling and analytical methods

Phase contrast microscopy (PCM), the primary method specified by NIOSH, OSHA, and MSHA for analysis of air samples for asbestos fibers, has several limitations, including limited ability to resolve very thin fibers and to differentiate various types of EMPs. Occupational exposure limits for asbestos derive from lung cancer risk estimates from exposure of workers to airborne asbestos fibers in commercial processes. These risk assessments are based on fiber concentrations determined from a combination of PCM-based fiber counts on membrane filter samples and fiber counts estimated from impinger samples. The standard PCM method counts only fibers longer than 5 µm. Moreover, some fibers longer than 5 µm may be too thin to be detected by PCM. Thus, an undetermined number of fibers collected on each sample remain uncounted by PCM. More sensitive analytical methods are currently available, but standardization and validation of these methods will be required before they can be recommended for routine analysis.

A project to determine the effectiveness of the "cleavage fragment" discrimination procedure in American Society for Testing and Materials (ASTM) Standard D7200 has led to a successful recent ballot by ASTM International to make changes to the procedure in the Standard, and a publication:

• Harper, M, Lee, EG, Slaven, JE and Bartley, DL [2012]: An inter-laboratory study to determine the effectiveness of procedures for discriminating amphibole asbestos fibers from amphibole cleavage fragments in fiber counting by phase-contrast microscopy. Annals of Occupational Hygiene, 56: 645-649.