The Importance of Width in Asbestos Fiber Carcinogenicity and Its Implications for Public Policy

Abstract

Evidence from human epidemiology, experimental animal implantation and inoculation studies, and lung burden studies show that fibers with widths greater than 1 micron are not implicated in the occurrence of lung cancer or mesothelioma. Furthermore, it is generally believed that certain fibers thinner than a few tenths of a micrometer must be abundant in a fiber population in order for them to be a causative agent for mesothelioma. These conclusions are fully consistent with the mineralogical characteristics of asbestos fibers, which, as fibrils, have widths of less than 1 micron and, as bundles, easily dissagregate into fibrils. Furthermore, the biological behavior of various habits of tremolite shows a clear dose-response relationship and provides evidence for a threshold between fiber width and tumor experience in animals. Public policy in regulating mineral fibers should incorporate this knowledge by altering the existing federal asbestos fiber definitions to reflect it.

A Meta-Analysis of Asbestos-Related Cancer Risk That Addresses Fiber Size and Mineral Type

“…Among environments in which fibers are predominantly asbestiform, the majority of fibers thicker than 0.5 µm are expected to be bundles (Veblen and Wylie, 1993;Virta et al, 1983;Wylie et al, 1982Wylie et al, , 1993 and, as described in greater detail in the discussion of crystalline habit later, bundles may be substantially more potent toward the induction of cancer than single-crystal fibers. Therefore, it may not be appropriate to apply the metrics proposed here to environments in which the majority of thicker fibers may be (nonasbestiform) single crystals.…”

“…Bundles may pose a special problem in distinguishing between risk posed by asbestiform fibers and nonasbestiform particles, since for structures thicker than about 0.5 µm, the majority of asbestiform fibers are comprised of bundles while virtually all nonasbestiform particles are single crystals ( Veblen and Wylie, 1993;Virta et al, 1983;Wylie et al, 1982Wylie et al, , 1993). This appears to be true even of amosite, as the mean thickness of single crystals in amosite may only be about 0.35 µm and, given that such distributions are skewed, the median thicknesses are likely even narrower (Wylie et al, 1982).…”

“…Given the characteristic distribution of widths anticipated among populations of asbestiform fibers and nonasbestiform particles (Veblen and Wylie, 1993;Wylie et al, 1982Wylie et al, , 1993, a much larger fraction of thick fibers is expected in these environments. Whether this is due to an actual lack of nonasbestiform particles or to an artifact in the manner in which the size distributions were analyzed in these environments cannot currently be determined.…”

Quantitative Analysis of Asbestos Fibres in Ophiolitic Rocks Used as Aggregates and Hazard Risk Assessment for Human Health

“…They have been used since ancient times as raw materials for the production of a large variety of materials and objects due to their exceptional attributes (high tensile strength, flexibility, chemical and heat re-sistance). The inherent properties of asbestos fibres appear to contribute to the toxicity of these mineral particles when lodged inside the human respiratory system (Stanton et al, 1981;Wylie et al, 1993). Occupational exposures to asbestos have been linked to asbestosis, lung cancer, malignant mesothelioma and various cancers of digestive tract (Mossman et al, 1990;Nolan et al, 2001;Constantopoulos, 2008;Murray and Nelson, 2008).…”

Introduction: INSERM-IARC-CNRS workshop on biopersistence of respirable synthetic fibers and minerals.

“…Fibers may also be subjected to either transversal breakage (for synthetic fibers) or longitudinal splitting (for natural fibers) and dissolution, which may result in dramatic changes in shape and dimensions, and in chemical composition resulting from selective leaching of component elements. The importance of diameter of elongated particles as a determinant for carcinogenesis is illustrated by tremolite asbestos, which, in its defibrilated thin form is highly carcinogenic (10), while thick cleavage fragments of the same mineral, often encountered in ores such as talc, are not carcinogenic (11). Thus, these slow disaggregation processes may produce changes in fiber number, size, crystalline structure, chemical composition and surface chemistry.…”