Review: The Fiber Length of Coalinga Chrysotile: Enhanced Clearance due to Its Short Nature in Aqueous Solution with a Brief Critique on ``Short Fiber Toxicity''

Abstract

``Coalinga'' chrysotile is a short fiber chrysotile that historically has been noted to be very different from other forms of asbestos. Under the microscope the ``extreme fiber length'', typical of most forms of asbestos, was said to be neither ``visible'' nor ``even measurable''. This morphology has been recognized for decades. ``Short'' fiber also refers to the predominant length distribution of a fiber population in a given sample. The US Government used Coalinga chrysotile as its ``standard'' short fiber sample preparation for animal testing by ingestion and inhalation more than 30 years ago. The ``standard'' preparations were largely composed of fibers less than 5 μm long. This was only demonstrable in aqueous solution because Coalinga chrysotiles are composed of loosely bound short fibers and fibrils that fall apart into their constituent components in an aqueous milieu. The short length is thus not an artifact of sample preparation but a reflection of the nature of the starting materials. However, currently used regulatory counting methods do not discriminate between Coalinga type and typical long chrysotile fibers even though their biological potencies differ radically. Consequently, these methods grossly overestimate the attendant health risks. Biologically relevant fiber length discrimination is determined by macrophage mediated clearance and, more specifically, macrophage diameter. The diameter of a human macrophage approximates 10—15 μm providing a logical biological foundation for differentiating ``short'' vs. ``long'' fibers. The findings of this report support the importance of fiber length cut-offs at this level.