Studies on the deposition of inhaled fibrous material in the respiratory tract of the rat and its subsequent clearance using radioactive tracer techniques. 1. UICC crocidolite asbestos

Our plain-English summary of Studies on the deposition of inhaled fibrous material in the respiratory tract of the rat and its subsequent clearance using radioactive tracer techniques. 1. UICC crocidolite asbestos, by J.C. Evans, R.J. Evans, A. Holmes, et al (Environmental Research, 1973).

The study

• Authors: J.C. Evans, R.J. Evans, A. Holmes, R.F. Hounam, D.M. Jones, A. Morgan, M. Walsh
• Journal: Environmental Research
• Published: 1973;6(2):180-201
• Cited by: 47 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Radioactive tracer methods were used to study the distribution of crocidolite between the upper and lower respiratory tracts after single short inhalation exposures of the "nose only" type. With the UICC standard reference sample of crocidolite, it was found that 50 ± 5% of the deposited asbestos was in the lower respiratory tract. After 30 days the lung content of crocidolite had been reduced to 73% of the initial value. Autoradiography was used to study the distribution of asbestos within the lung immediately after exposure and after 30 days. The clearance of crocidolite from the respiratory tract was also followed by the measurement of radioactivity in faeces.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This was an animal experiment. Eight albino rats breathed, through the nose only, a cloud of UICC standard reference crocidolite asbestos that had been made radioactive so it could be traced. Four rats were exposed for 42 minutes and four for 90 minutes. In each group, two rats were killed right away and two after 30 days. On average, 35% of the inhaled asbestos was deposited, and about half of the deposited asbestos was in the lower respiratory tract. Upper-airway deposits were cleared almost completely within about an hour. After 30 days, lung content was 73% of the initial value. Crocidolite in faeces showed a fast phase (half-time 0.43 days) and a slow phase (half-time 29 days), which the authors said probably reflected clearance from the lower tract. In rats killed right after exposure, the largest number of fibers seen in the conducting airways was at branch points of the smaller bronchioles. The authors noted that with few rats, differences between animals could hide differences in exposure. They concluded that tracer methods can follow inhaled fibers using only a few milligrams.

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 using radioactive tracer techniques (neutron-irradiated crocidolite) with brief nose-only exposures, radiometric measurement of organs and faeces, and autoradiography of lung sections
• Who was studied: Eight albino rats free of chronic respiratory disease, at the Atomic Energy Research Establishment, Harwell, U.K. In each of two experiments, two rats were killed immediately after exposure and two were kept for 30 days before being killed.
• Exposure: UICC standard reference sample of crocidolite asbestos, made radioactive by neutron irradiation. In experiment 1, four rats (Nos. 1 to 4) were exposed to an elutriated cloud for 42 minutes. In experiment 2, four rats (Nos. 5 to 8) were exposed to an unelutriated cloud for 90 minutes. Mean airborne concentrations were 13.0 and 8.1 micrograms per liter. Airborne fibers had a median length between 3 and 4 micrometers by optical microscopy (fibers longer than 2 micrometers) and between 1 and 2 micrometers by electron microscopy. The diameter distribution peaked between 0.2 and 0.3 micrometers.
• Main results:
  • In the rats killed immediately after exposure, about 50% of the deposited asbestos was in the lower respiratory tract (printed as 50 plus or minus 5%).
  • On average, 35% of the inhaled asbestos was deposited (range 24 to 47%).
  • In rats killed shortly after exposure, clearance from the upper respiratory tract was virtually complete after 1 hour. The authors stated that this material was removed very rapidly via the oesophagus.
  • After 30 days, the lung content of crocidolite had been reduced to 73% of the initial value, a 27% reduction in the initial lung burden.
  • Excretion of crocidolite in faeces followed two components, with mean half-times of 0.43 days and 29 days. The authors stated that the long-term component probably represents slower clearance of crocidolite initially deposited in the lower respiratory tract.
  • In rats killed immediately after exposure, autoradiographs of the conducting airways showed the largest number of fibers at bifurcations of the smaller bronchioles, just downstream from the carina.
  • Fibers deposited in the bronchioles were generally longer and thicker than those found in the alveolar region. The longest fiber seen was at least 20 micrometers long.
  • After 30 days, all fibers in the alveolar region appeared to be inside cells. Some comparatively long fibers (more than 10 micrometers) could be seen in the interalveolar septa.
  • Only a few percent of the radioactive iron had leached from the crocidolite over 30 days, so lung radioactivity still measured the crocidolite remaining.
• The authors' conclusion: The authors concluded that radioactive tracer techniques provide a method for studying the fate of inhaled fibrous particles. They stated that the method can quantify deposition between the upper and lower respiratory tract and follow clearance. They added that it needs only a few milligrams of fiber, so comparative studies could be made on dust samples from factory or mining operations.
• Limitations the authors state:
  • With a small number of animals, differences in deposition between rats (from variations in tidal volume and respiratory rate) were enough to obscure differences due to exposure conditions.
  • Contamination of the fur occurred (about 18% of the respiratory tract deposit in the first experiment). Ingestion of this material during preening probably inflated total deposition estimates in the 30-day rats.
  • Fibers shorter than 2 micrometers could not be detected by autoradiography under the exposure conditions used.
  • Absolute fiber lengths could not be measured in lung sections, because fibers may be cut during sectioning and lie in random orientations.
  • There was probably a selective loss of longer fibers inside the exposure chamber before the cloud was inhaled.

Topics

Medical terms in this study, explained in our Lexicon: Deposition, UICC, Crocidolite.

Related studies

• Studies on the deposition of inhaled fibrous material in the respiratory tract of the rat and its subsequent clearance using radioactive tracer techniques — A. Morgan, J.C. Evans, et al. (1975)
• Deposition of inhaled asbestos and man-made mineral fibres in the respiratory tract — A. Morgan (1995)
• The effects of chrysotile and crocidolite asbestos on the lower respiratory tract: analysis of bronchoalveolar lavage constituents — E. Kagan, Y. Oghiso, et al. (1983)
• Deposition pattern and clearance pathways of inhaled chrysotile asbestos — A.R. Brody, L.H. Hill (1981)
• An overload hypothesis for pulmonary clearance of UICC amosite fibres inhaled by rats — R.E. Bolton, J.H. Vincent, et al. (1983)
• Kinetics of deposition and clearance of inhaled mineral dusts during chronic exposure — J.H. Vincent, A.M. Johnston, et al. (1985)

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