Persistence of Natural Mineral Fibers in Human Lungs: An Overview

Abstract

Virtually all available data on persistence of naturally occurring mineral fibers in human lungs have been derived from studies of asbestos fiber loads. These studies indicate that, although both amphibole and chrysotile asbestos fibers are found in the lungs of the general population and exposed workers, amphibole fibers are universally present in disproportionately large and chrysotile fibers in disproportionately small amounts compared to their known abundance in the original inhaled dusts. Why this should be remains unclear. Most reports have shown that fiber accumulation is proportional to measured exposure for amphiboles, but this is not generally true for chrysotile. Very little information is available on actual fiber clearance rates from human lungs. For amosite and crocidolite, estimated clearance half-times are measured in years to decades, whereas for chrysotile the available, rather indirect, data suggest that the vast majority of fibers are cleared within months, although some fibers may be sequestered and very slowly cleared. Overall these studies suggest that the differences between amphibole and chrysotile fiber burdens in man reflect much faster clearance of chrysotile fibers, rather than a failure of chrysotile deposition. A variety of other naturally occurring fibers are commonly found in human lungs, but there are no data on their rates of accumulation or disappearance. -Environ Health Perspect 1 02(Suppl 5): 229-233 (1994)

Mechanisms in the Pathogenesis of Asbestosis and Silicosis

“…Both animal and human studies show that continuing exposure to amphiboles results in continuously increasing amphibole fiber levels recoverable from the lung, whereas continuing exposure to chrysotile is associated with a negligible increase in chrysotile fiber burden over time (reviewed in reference 38). The estimated half-life for amphibole fibers, while difficult to estimate accurately because of the type of laboratory-to-laboratory variations mentioned above, appears to be on the order of decades, whereas that of chrysotile appears to be around a few months (38,52). For this reason, virtually all studies of human fiber burdens show a predominance of amphiboles, even with nominal exposure only to chrysotile (38) (see below).…”

Only One Lesson to be Learned

“…This hypothesis ignores that asbestos fibers are actually detected in human lung tissue and bronchoalveolar lavage fluid many years after exposure cessation [7 -11]. This is true also for chrysotile fibers, which show compartimentalization [7,9]. These findings are not new: "… studies indicate that, although both amphibole and chrysotile asbestos fibers are found in the lungs of the general population and exposed workers, amphibole fibers are universally present in disproportionately large and chrysotile fibers in disproportionately small amounts compared to their known abundance in the original inhaled dusts" [7].…”

Collegium Ramazzini: Comments on the 2014 Helsinki consensus report on asbestos

“…">Over‐reliance on asbestos fiber counts in lung tissue as an indicator of past exposure to asbestos.Asbestos fiber counts obtained from human lung tissue are now recognized to be a highly insensitive measure of past exposure to chrysotile asbestos. Chrysotile asbestos fibers are now well documented to have only a short residence time in lung tissue and therefore, their measurement in the lung cannot be used as a measure of cumulative past exposure [Wagner et al, ; Baker, ; Kohyama and Suzuki, ; Churg and Wright, ; Finkelstein and Dufresne, ; Roggli et al, ; Suzuki and Yuen, ; Dodson et al, ; Egilman, ]. As with asbestos bodies, the Collegium Ramazzini has no concern about using a finding of asbestos fibers in lung tissue as an indicator of past exposure to asbestos.…”

“…">There is no recognition that chrysotile is the predominant type of asbestos fiber found in pleural mesothelioma tissue.Multiple studies have demonstrated that chrysotile fibers are the predominant type of asbestos fiber found in pleural mesothelioma tissue. The relative abundance of chrysotile fibers in mesothelioma tissue contrasts with their relative scarcity in lung tissue [Wagner et al, ; Kohyama and Suzuki, ; Churg and Wright, ; Finkelstein and Dufresne, ; Suzuki and Yuen, ; Dodson et al, ; Egilman, ]. 5.Threshold for the development of an asbestos‐related lung cancer.…”