Thoracic fraction of inhaled fiber aerosol

Our plain-English summary of Thoracic fraction of inhaled fiber aerosol, by Y.S. Cheng, W.C. Su (Journal of Occupational and Environmental Hygiene, 2013).

The study

• Authors: Y.S. Cheng, W.C. Su
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2013;10(4):194-202
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Size-selective sampling is a health-related method to collect airborne particles based on penetration of inhaled particles into different regions of the human respiratory tract; thus, it is the most relevant sampling method to correlate health risks with occupational exposure. The current practice of sampling asbestos and other fibers is not a size-selective method. The thoracic size fraction, defined as the portion of inhaled particles that can penetrate through the larynx, has been suggested as the most relevant size-selective sampling method for fiber aerosol. The thoracic fraction is based on 1-deposition of inhaled spherical particles in the human extrathoracic airways for mouth breathing and corrected for the particle inhalability. There is no comparable information for fiber aerosols; therefore, there is no technical basis to ascertain whether the current thoracic fraction definition is suitable for fiber aerosols. No human data are available from controlled experiments of inhaled fiber aerosols for the obvious reason that most fiber materials are potentially hazardous when inhaled. Our approach was to measure penetration of fiber aerosol in realistic human oropharyngeal airway replicas and to compare that with data from spherical particles. We showed that realistic human oral airway replicas (including the oral cavity, pharynx, and larynx regions) provided useful spherical and fiber particle deposition in the human head airway. These data could be used to test the thoracic fraction curves. The spherical penetration is in agreement with human in vivo data used to establish the thoracic fraction curve. Fiber penetrations through the larynx of two human oral airway replicas were higher than those for spherical particles for the same aerodynamic diameter using the same replicas. The thoracic curve as defined for spherical particles, therefore, may not include some fibers that could penetrate to the thoracic region.

In plain English

This laboratory study tested whether the 'thoracic fraction', a workplace air-sampling standard based on how round particles get past the mouth and throat, also fits fibers. No people were exposed. The authors used two silicone-rubber replicas of the human mouth, throat, voice box (larynx) and upper airways. They blew three man-made fibers through them: carbon, titanium dioxide and fiberglass. They compared the results with round plastic test particles. Asbestos itself was not tested. Round particles passed through the replicas in the same range as in the human volunteer data behind the standard. Fibers generally deposited less in the mouth-throat region than round particles of the same aerodynamic diameter, a standard measure of particle size, so more fibers got past the larynx. Some fell outside the current thoracic curve. Longer fibers deposited more than shorter ones. The authors concluded that the thoracic curve defined for round particles may miss some fibers that could reach the chest airways. They said data from two replicas are not enough and more data are needed.

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 deposition and penetration of fiber aerosols and spherical particles in human airway replicas (casts)
• Who was studied: Two conductive silicone-rubber human airway casts (Casts A and B). Each included the oral cavity, pharynx, larynx and the tracheobronchial tree to the third to fourth generation. No human or animal subjects were used.
• Exposure: Three man-made fibers, tested at constant inspiratory flow rates of 15, 43.5 and 60 L/min, compared with fluorescent polymer microspheres of aerodynamic diameter 0.5 to 30.0 μm. The fibers were carbon (count median diameter 3.66 μm, count median length 14.83 μm), titanium dioxide (0.59 μm, 3.20 μm) and fiberglass JM475/100 (0.62 μm, 7.67 μm). No asbestos was tested.
• Main results:
  • Spherical particle penetration through both replicas was in the same range as the in vivo human volunteer data used to establish the thoracic fraction.
  • In both oral replicas, round (compact) particles generally deposited more than fibers. The authors took this to imply that more fibers than spheres of the same aerodynamic diameter would penetrate the larynx.
  • Fiber penetration through the larynx was generally higher than that of spherical particles, and some fiber data fell outside the thoracic curve.
  • Titanium dioxide fibers and fiberglass had low deposition in the oral airway.
  • Long fibers deposited more than short fibers in every airway region of the replica.
  • Deposition of both fibers and spheres rose with the impaction parameter, which the authors took to show that impaction is the major deposition mechanism.
• The authors' conclusion: The authors concluded that the thoracic curve as defined for spherical particles may not include some fibers that could penetrate to the thoracic region. They suggested, as an example, that a curve with 50% penetration at 15 μm for fibers would be more conservative than the current curve.
• Limitations the authors state:
  • Data from only two airway replicas are not sufficient to establish a technical database for the thoracic fraction.
  • Only carbon, titanium dioxide and fiberglass were tested; data on other fiber materials are needed.
  • Because of the limits of visual measurement, only fibers above certain lengths were counted.

Topics

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Deposition.

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