Optical phase contrast microscopy (PCM)

The exposure index available for this and previous studies of Libby workers is based on asbestos fiber count data obtained using optical phase contrast microscopy (PCM). PCM measures fibers longer than 5 µm and wider than 0.25 µm [i.e., the fiber size regulated under the OSHA standard]. Fiber count data obtained using PCM do not distinguish between the various amphiboles in the Libby vermiculite.

Thus, fiber count estimates used in the exposure assessment include not only the regulated tremolite asbestos fibers but also the asbestiform amphibole fibers not mentioned in the regulations (winchite and richterite). Recall that Meeker et al. (2003) characterized the Libby amphibole as 6% tremolite.

If the observed health effects are explained by tremolite asbestos alone, then exposure has been considerably overestimated, and the effect of each fiber per cubic centimeter-year increment in exposure has been substantially underestimated.