Deposition, clearance, and translocation of chrysotile asbestos from peripheral and central regions of the rat lung

Our plain-English summary of Deposition, clearance, and translocation of chrysotile asbestos from peripheral and central regions of the rat lung, by P.G. Coin, V.L. Roggli, A.R. Brody (Environmental Research, 1992).

The study

• Authors: P.G. Coin, V.L. Roggli, A.R. Brody
• Journal: Environmental Research
• Published: 1992;58(1):97-116
• Cited by: 49 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

We investigated the pulmonary deposition, clearance, and translocation of chrysotile asbestos in the context of our previously developed model of asbestosis in the rat. Adult male rats were exposed for 3 hr to an aerosol of chrysotile asbestos. Subgroups were sacrificed up to 29 days postexposure and the lungs of the animals fixed. Peripheral and central regions of the left lung were resected, digested, and analyzed for fiber content by scanning electron microscopy. Pulmonary deposition did not differ between peripheral and central regions. There was no evidence of translocation of fibers from central to peripheral regions. The average diameter of retained fibers decreased over time, consistent with longitudinal splitting. The average length of retained fibers increased over time, consistent with slower clearance of longer fibers. We employed a novel counting scheme to ensure accurate fiber number measurements, allowing the calculation of clearance rates for fibers 0.5 to greater than or equal to 16 microns in length. Fibers of length greater than or equal to 16 microns were cleared slowly, if at all. These findings could have important implications for the pathogenesis of asbestos-related pleural disease. Many fibers are deposited in the peripheral region, and the longest (greater than or equal to 16 microns) will persist there for extended periods.

In plain English

This was an animal experiment on chrysotile asbestos in rats. The authors exposed 23 adult male rats for 3 hours to an aerosol of chrysotile at 10 mg (respirable) per cubic meter. Groups of four or five rats were killed immediately after exposure and at 1, 8, 15 and 29 days afterwards. After adjusting for the amount of tissue in each region, fiber number and mass were similar in the peripheral (outer) and central regions of the left lung. The ratio of peripheral to central fiber mass showed no significant trend over time, and the authors found no evidence of pleural drift, meaning movement of fibers toward the lung surface. The average diameter of retained fibers decreased over time, which the authors said was consistent with fibers splitting lengthwise. Clearance slowed as fiber length increased. Half-lives were about 10 days for fibers 0.5 to 4 micrometers long and 114 days for fibers 16 micrometers or longer. For those longest fibers the clearance rate was not significantly different from zero. The authors noted that follow-up may have been too short to detect slow fiber movement.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal inhalation experiment with serial sacrifice; the left lung of each rat was divided into peripheral and central regions and fibers were counted and measured
• Who was studied: 23 adult male Sprague-Dawley rats aged 8-10 weeks, killed in subgroups of four or five at 0, 1, 8, 15 and 29 days after exposure; four unexposed rats were killed at 26 days as controls
• Exposure: A single 3-hour inhalation exposure to an aerosol of chrysotile asbestos at 10 mg (respirable) per cubic meter, which the authors put at approximately 5000 fibers 5 micrometers or longer per cubic centimeter
• Main results:
  • Fiber number and mass, normalized for the share of left-lung tissue in each region, were similar in the peripheral and central regions both immediately after exposure and at 1 day. The per-animal peripheral/central ratios for fiber number and mass, and the differences in average length and diameter, were not statistically different from equality.
  • The ratio of peripheral to central fiber mass from 1 to 29 days showed no significant trend over time (slope p = 0.59); no evidence of translocation toward the periphery was found
  • By sensitivity analysis, any translocation could be no greater than 20% of the overall clearance rate, corresponding to a process with a half-life of 73 days
  • Fiber numbers fell over time for all length classes under 8 micrometers. The 8 to 16 micrometer class did not change significantly up to 29 days, and the number of fibers 16 micrometers or longer increased significantly.
  • Average fiber diameter decreased over time within each length class, which the authors described as consistent with longitudinal splitting
  • Clearance half-lives based on fiber mass increased with fiber length, from about 10 days for fibers 0.5 to 4 micrometers to 114 days for fibers 16 micrometers or longer. The clearance rate for fibers longer than 16 micrometers was not significantly different from zero.
  • Total fiber mass in the left lung over time fit a two-compartment clearance model with half-lives of 3 days and 20 days
• The authors' conclusion: The authors concluded that, although they found no evidence of pleural drift, substantial numbers of inhaled chrysotile fibers were deposited within 1-2 mm of the visceral pleura of the rat. These included fibers longer than 16 micrometers that were cleared slowly, if at all. The authors suggested this could have implications for how asbestos-related pleural disease develops.
• Limitations the authors state:
  • Follow-up (29 days) may have been too short to detect slow translocation
  • The method could not distinguish translocation from central to peripheral regions from slower clearance out of the peripheral region
  • Translocation to the pleural regions might occur only at lung burdens much higher than those reached in this study
  • The dissection scheme was too crude to keep first alveolar duct bifurcations out of the peripheral tissue
  • Fiber splitting may alter the apparent clearance rate within a length class
  • The size-based calculations are approximations because all fibers in a size class are assumed to have the same dimensions

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Chrysotile.

Medical terms in this study, explained in our Lexicon: Deposition, Translocation, Chrysotile, Scanning Electron Microscopy (SEM), Asbestos-Related Pleural Disease.

Related studies

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• Clearance and dimensional changes of crocidolite asbestos fibers isolated from lungs of rats following short-term exposure — V.L. Roggli, M.H. George, et al. (1987)
• Deposition and clearance of chrysotile asbestos — A. Churg (1994)
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• Inferences on the kinetics of asbestos deposition and clearance among chrysotile miners and millers — M.M. Finkelstein, A. Dufresne (1999)

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