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 microm long with an aspect ratio >or= 3:1) and also within the respirable width range, i.e. < 3 microm 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 microm (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.