Differentiating Non-Asbestiform Amphibole and Amphibole Asbestos by Size Characteristics

Abstract

Mining or processing asbestos minerals can liberate isolated fibers or fiber bundles regulated as airborne asbestos fibers. Coarsely crystalline amphibole minerals are more common than asbestos in many geologic environments, and disturbance can result in the release of prismatic or acicular single crystals or cleavage fragments resembling asbestos fibers or fiber bundles but that are not currently regulated as asbestos. Bulk samples of six coarsely crystalline amphiboles and their five asbestos analogs were processed to maximize the number of particles meeting the criterion for counting under the current U.S. National Institute for Occupational Safety and Health Method 7400 “A” counting rules (> 5 μm long with an aspect ratio ≥ 3:1) and also within the respirable width range, i.e. < 3 μm width. The length distributions of the particles produced showed substantial overlap between cleavage fragments and asbestos fibers. Available data sets generally confirmed the relevance of the size distributions of particles generated from reference materials to airborne particles. The length criterion in the current ASTM International standard D7200-06 causes a large proportion (e.g., 40% grunerite and 39% tremolite) of the non-asbestiform particles to be considered potential asbestos. An alternative procedure may be to use a distinction based on width alone as some, but not the majority of, cleavage fragments were thinner than 1 μm (e.g., 9% of actinolite and 20% of grunerite particles), and not many amphibole asbestos particles were wider (e.g., 5% of crocidolite and 18% of amosite particles). This proposal would need further testing. This research should not be considered as addressing any controversy with regard to the toxicity of non-asbestiform amphibole particles of similar dimensions to asbestos particles.

Keywords:

• asbestos
• cleavage fragments
• mineral fibers
• mining

ACKNOWLEDGMENTS

The authors thank Michael Beard and Bruce Harvey (RTI International) for contract oversight and helpful discussions. This material was first presented at the 2008 ASTM International Johnson Conference on Critical Issues in Monitoring Asbestos, July 13–18, Burlington, Vermont.

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention or the views of RTI International

Notes

^A Percentage meeting the NIOSH 7400 method “A” counting rules definition of a fiber out of 300 particles observed under PCM.

^B For greater accuracy, measurements were made under TEM. 300 total particles were measured and the number of PCM-equivalent (PCMe) fibers counted. Percentage is the number meeting the stated classification out of the 300 PCMe fibers measured under TEM.

^C Anthophyllite contains talc, probably fibrous.

^A Percentage meeting the NIOSH 7400 method “A” counting rules definition of a fiber out of 300 particles observed under PCM.

^B For greater accuracy, measurements were made under TEM. Three hundred total particles were measured and the number of PCM-equivalent (PCMe) fibers counted. Percentage is the number meeting the stated classification out of the 300 PCMe fibers measured under TEM. (Procedure not carried out for HSE material.)

^A Cummingtonite cleavage fragments.

^B Grunerite cleavage fragments.

^C Actinolite cleavage fragments.

^A Percentage meeting the NIOSH 7400 method “A” counting rules definition of a fiber out of 300 particles observed under PCM. From Table I.

^B For greater accuracy, measurements were made under TEM. 300 total particles were measured and the number of PCM-equivalent (PCMe) fibers counted. Percentage is the number meeting the stated classification out of the 300 PCMe fibers measured under TEM.

^C Anthophyllite contains talc, probably fibrous.

^A Percentage meeting the NIOSH 7400 method “A” counting rules definition of a fiber out of 300 particles observed under PCM. From Table II.

^B For greater accuracy, measurements were made under TEM. Percentage is the number meeting the stated classification out of the 300 PCMe fibers measured under TEM.