Human exposure to asbestos: dust controls and standards. The inhalation of fibrous dusts

Our plain-English summary of Human exposure to asbestos: dust controls and standards. The inhalation of fibrous dusts, by V. Timbrell (Annals of the New York Academy of Sciences, 1965).

The study

• Author: V. Timbrell
• Journal: Annals of the New York Academy of Sciences
• Published: 1965;132(1):255-73
• Cited by: 105 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy.

In plain English

This 1965 single-author paper from the Medical Research Council Pneumoconiosis Research Unit in Great Britain was a laboratory and mathematical study of fibers in airways; it did not study people. Using an aerosol spectrometer, the author measured how fast glass fibers and amosite, chrysotile and crocidolite asbestos fell through air. Falling speed depended mainly on diameter; length mattered little for long fibers. The author estimated that long, regular asbestos fibers thinner than about 3.5 microns could reach the lung's deepest air spaces. In the spectrometer, fibers with length more than five times their width tended to align with the airflow, glass more than asbestos. Lacking direct evidence for the lung, the author assumed random orientation there in the calculations. In a model with nasal hairs 100 microns apart, only five per cent of 200-micron fibers were calculated to pass two stages of hairs. The author concluded that pathologists could expect respiratory bronchiole branch points to be preferred deposit sites for long fibers. The author noted that the asbestos results came from highly selected fibers and that the calculations used simplifying assumptions.

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: Laboratory experiment measuring the falling speeds of fibers in an aerosol spectrometer, plus theoretical (mathematical) calculations of fiber deposition in airways; no human or animal subjects
• Who was studied: Glass fibers from glass filter paper manufacture and three types of asbestos in the finest commercial grades, shortened in a hammer mill; work done at the MRC Pneumoconiosis Research Unit, Llandough Hospital, Great Britain
• Exposure: Airborne glass, amosite, chrysotile and crocidolite fibers of varying diameters and lengths; falling speed expressed as equivalent (aerodynamic) diameter
• Main results:
  • For all fibers studied, falling speed was determined mainly by fiber diameter and showed little sensitivity to length, especially for fibers with a high length-to-diameter ratio
  • For long fibers, equivalent diameter could be approximated as the actual diameter times 3.5 for amosite, 3 for crocidolite and 2.5 for chrysotile (3 if the type is unknown)
  • Taking 10 microns as the upper limit for unit-density spheres reaching the pulmonary air spaces, the expected upper limit for the diameter of long regular asbestos fibers reaching those spaces was about 3.5 microns
  • In the aerosol spectrometer, fibers with aspect ratios above 5 tended to align with the airstream, and this tendency was stronger for glass fibers than for the three asbestos types. For the deposition calculations the author assumed random fiber orientation in lung airways.
  • In calculations assuming nasal hairs spaced 100 microns apart, only five per cent of fibers 200 microns long penetrated two stages of nasal hairs
  • Calculated penetration of 200-micron fibers through an airway dividing into four branches rose from 2 per cent for a 200-micron respiratory bronchiole to 70 per cent for an 800-micron airway. With two branches it rose from 30 per cent to 86 per cent.
  • Long fine fibers would be expected to deposit at preferred sites in the respiratory bronchioles, especially where they branch
• The authors' conclusion: The author concluded that fiber diameter, not length, governs the main deposition mechanisms, so long thin asbestos fibers can reach deep into the lung. In the author's view, nasal hairs and the narrow respiratory bronchioles limit fiber length, and pathologists can expect respiratory bronchiole branch points to be preferred deposition sites for long fibers.
• Limitations the authors state:
  • The asbestos results relate to highly selected fibers, because most asbestos fibers were parts of aggregates
  • Particles attached to asbestos fibers were hard to detect, and some such fibers were suspected to have entered the results
  • There was no direct evidence on whether fibers align with air streamlines in the lung, and the calculations assumed random orientation
  • The deposition calculations assumed fibers of infinitely small diameter and considered interception only

Topics

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

Medical terms in this study, explained in our Lexicon: Dust Control.

Related studies

• A simple dispenser for generating dust clouds from standard reference samples of asbestos — V. Timbrell, A.W. Hyett, et al. (1968)
• Accumulation of mixed mineral dusts in the lungs of rats during chronic inhalation exposure — C.H. McMillan, A.D. Jones, et al. (1989)
• Quantitative assessment of inorganic fibrous particulates in dust samples with an analytical transmission electron microscope — F.D. Pooley, N.J. Clark (1979)
• Inhalation of rod-like carbon nanotubes causes unconventional allergic airway inflammation — E.M. Rydman, M. Ilves, et al. (2014)
• A review of inhalation toxicology studies with para-aramid fibrils — D.B. Warheit (1995)
• Economics of dust control — D.W. Hills (1965)

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