Significance of fibre length in the clearance of asbestos fibres from the lung.

Abstract

Rats were exposed by inhalation to radioactive anthophyllite asbestos. Animals were then killed serially over a period of 205 days and the fibre content of the lungs measured radiometrically. The lungs were subjected to selective bronchopulmonary lavage and the mean fibre content determined of free cells recovered from the alveolar spaces. The length distributions of fibres recovered from the lungs by lavage, and of those remaining in the lungs following lavage, were measured. Short (less than 5 ,um) fibres are cleared from the lung via the conducting airways more efficiently than longer fibres and fibres exceeding 50 ,m in length are not removed from the lungs by this route. Although fibres of about 200 jum in length were present in all the lungs examined, the longest which could be recovered by lavage, once fibre deposited in the airways had been cleared, was only about 100 ,tm decreasing to 60-70 ,um after 205 days. It is suggested that long fibres are more liable than short to penetrate the alveolar wall as they tend to bridge the alveolar ducts and alveoli.It has been shown by Morgan et al. (1977a) using autoradiographic techniques that when small amounts of asbestos are deposited in the rat lung the fibres tend, in the course of time, to accumulate not only in the tracheobronchial lymph nodes but also in subpleural foci. Similar peripheral accumulations of dust have been observed in the rodent lung following exposure to beryllium oxide (Sanders and Cannon, 1975) and in rodent, dog and human lungs following exposure to plutonium oxide (Clarke and Bair, 1964;Sanders and Dagle, 1974;Voelz et al., 1976). If asbestos fibres are transported in a similar manner to the periphery of the human lung, this could be an important factor in the aetiology both of asbestosis, which generally appears to affect subpleural areas first (Becklake, 1976) and of asbestos-induced mesothelial tumours. If, as seems likely, this movement occurs within or through the alveolar wall rather than over its surface, then it is important to know how the dimensions of asbestos fibres affect their penetration into the interstitium.To investigate the fate of asbestos fibres deposited in the rat lung, the selective bronchopulmonary lavage procedure of Brain and Frank (1973) applied by Morgan et al. (1977b) to the lungs of animals killed serially following exposure to radioactive asbestos by inhalation. In this technique the initial washings are with a balanced salt solution containing calcium and magnesium. Under these circumstances relatively few cells (only those which are loosely attached) are washed out of the lung. These include alveolar macrophages which are present on the mucous layer in the conducting airways and which are in the process of being removed from the lung. On subsequent washing with physiological saline, in the absence of divalent cations, alveolar macrophages detach themselves from the alveolar wall and are recovered in large numbers. To determine the length distribution of fibres present in the various washes, a ...

Concentrations and dimensions of coated and uncoated asbestos fibres in the human lung.

“…Most studies with experimental animals have indicated that fibrogenicity is dependent on fibre length. '-3 Fibres are probably only actively fibrogenic if they exceed a critical length, which is probably about [10][11][12][13][14][15][16][17][18][19][20] ,tm. Studies on the carcinogenic effects of fibres have also emphasised the significance of length.4-7 Long fibres are thought to be more carcinogenic than short and long thin fibres may be particularly active in this respect.…”

Health hazard evaluation and technical assistance report: HETA-94-0329-2574, Standard Steel, Burnham, Pennsylvania.

“…12 Animal studies demonstrate that fibers greater than 10-15 µm in length may not be as efficiently removed from the lung by cell-mediated mechanisms. 10,14 One study reported maximum RCF removal at fiber lengths of 11-15 µm, with decreased removal as fiber In vitro (literally, "in glass," as in "in a test tube") studies have added data to support conclusions reached by animal (in vivo) studies that certain fiber characteristics (surface chemistry, biopersistence, fiber structure) can affect the carcinogenic potential of a fiber. It is commonly reported that during in vitro tests, longer fiber structures are more toxic than short structures.…”

Health hazard evaluation report: HETA-86-038-1807, Morris Bean and Company, Yellow Springs, Ohio.

“…4 In rats, fibers with a mass median aerodynamic diameter between 3 and 6 uM were capable of alveolar deposition, although at much reduced levels (1-2%). 5 Animal studies 6,7 indicate that fibers longer than 10-15 uM may be less efficiently cleared from the lungs by macrophage mediated processes. In one study ceramic fiber clearance reached a maximum at fiber lengths of 11-15 uM and then decreased with increasing fiber length.…”