Release of asbestos fibers during casting ring liner manipulation

Our plain-English summary of Release of asbestos fibers during casting ring liner manipulation, by D.R. Davis (The Journal of Prosthetic Dentistry, 1995).

The study

• Author: D.R. Davis
• Journal: The Journal of Prosthetic Dentistry
• Published: 1995;74(3):294-8
• Cited by: 4 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Civil lawsuits have been filed that allege wrongful deaths of at least one dentist and one dental technician from asbestos exposure. However, no known published scientific studies support such allegations. To assess what potential exposure could occur, air samples were collected while casting rings were lined with asbestos liner. Fibers were collected on filters and counted by use of phase-contrast microscopy and transmission electron microscopy. The concentrations of fibers 5 microns long or greater on all filters was less than allowed by federal guidelines. Based on this simulation the potential health risk from using asbestos ring liner seems low.

In plain English

In this laboratory simulation, the author measured airborne fibers released when dental casting rings were lined with dry asbestos liner. Whip Mix Corporation, whose liner was tested, supported the study. In a laboratory with sealed ducts and hoods, the author repeatedly tore liner strips, fitted them into ten rings and bagged them, over three 30-minute periods. Filters in the author's breathing zone and across the bench sampled air. By phase-contrast microscopy, the three breathing-zone filters showed 0.092, 0.061 and 0.065 fibers per milliliter. Their table variability was 0.065, 0.044 and 0.045, though the text gives 0.051 for the first. Area filters showed 0.005 fibers per milliliter before (plus or minus 0.005) and during (plus or minus 0.006) the work. Electron microscopy of breathing-zone filters found 0.349, 0.175 and 0.175 structures per milliliter, all chrysotile (white asbestos). The author reported that federal and state guidelines were not exceeded and that the potential health risk seemed low. The author noted that liner from only one source was tested, that filter placement strongly affected results, and that an epidemiologic study would assess risk more accurately.

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 simulation experiment measuring airborne fibers. The study was supported by the Whip Mix Corporation.
• Who was studied: No people were studied. Air was sampled in a laboratory of approximately 18.7 square meters with ducts and hoods sealed. The location and dates are not stated, and the author's affiliation is the University of California, San Francisco. The author personally lined the casting rings while an industrial hygienist monitored the pumps. The samples were three 30-minute breathing-zone samples, a 90-minute background area sample, and an area sample taken during the whole experiment.
• Exposure: Dry asbestos casting ring liner (Whip Mix Corporation). Pieces approximately 90 mm long, 38 mm wide and 0.6 mm thick were torn from rolls, fitted inside 10 metal casting rings, then removed and placed in a plastic waste bag. This was repeated for 30 minutes, three times. Fibers were counted by phase-contrast microscopy (fibers at least 5 micrometers long and at least 0.25 micrometers in diameter). Breathing-zone filters were also analyzed by transmission electron microscopy, with fiber type identified by energy dispersive x-ray analysis.
• Main results:
  • Area-zone filters (phase-contrast microscopy) showed 0.005 plus or minus 0.005 fibers/ml in the 90 minutes before the experiment, and 0.005 plus or minus 0.006 fibers/ml during the entire experiment.
  • Breathing-zone filter BZ-1, placed nearer the waste bag, showed the highest phase-contrast count: 0.092 fibers/ml from 80 ring liner manipulations, or approximately 0.001 fibers/ml per ring. Its variability is given as plus or minus 0.051 in the text but 0.065 in Table I (based on 95% confidence intervals).
  • Breathing-zone filters BZ-2 and BZ-3 showed 0.061 (plus or minus 0.044) and 0.065 (plus or minus 0.045) fibers/ml by phase-contrast microscopy. That averages approximately 8 x 10 to the minus 4 fibers/ml per ring.
  • Expressed as 8-hour time-weighted averages, the breathing-zone values were 0.006, 0.004 and 0.004 fibers/ml. The limits listed were 0.1 fiber/ml (NIOSH) and 0.2 fibers/ml (California OSHA). The author reported that federal and state guidelines were not exceeded.
  • By transmission electron microscopy, BZ-1 showed 0.349 total structures/ml, of which 0.200 structures/ml were at least 5 micrometers long. BZ-2 and BZ-3 each showed 0.175 total structures/ml, with 0.025 and less than 0.025 structures/ml at least 5 micrometers long. 'Structures' include free fibers, bundles, clusters, and fibers embedded in matrices.
  • All structures counted by transmission electron microscopy were identified as white asbestos (chrysotile).
  • Of 28 structures counted on all breathing-zone filters, only one was a true free fiber (3.6%), approximately 2.4 micrometers long.
• The authors' conclusion: The author concluded that, in this simulation, airborne concentrations of white asbestos fibers were below federal and California state guidelines, and that the potential health risk from using asbestos ring liner seemed low.
• Limitations the authors state:
  • Liner from only one source was tested, and the type and purity of ring liners used over the past 50 years cannot be known. The author states users therefore cannot be absolutely certain they are free of health risk from past activities.
  • Filter placement had a profound effect on the results.
  • Disposing of the dry liners differed from actual laboratory use, where the liner would have been saturated with water.
  • A more accurate risk assessment would require an epidemiologic study of dentists and dental technicians who worked with asbestos ring liner for many years.

Topics

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

Medical terms in this study, explained in our Lexicon: Wrongful Death, Asbestos Exposure, Transmission Electron Microscopy (TEM).

Related studies

• The effect of casting ring liners on the potential expansion of a gypsum-bonded investment — R. Earnshaw (1988)
• The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners — E.F. Morey, R. Earnshaw (1992)
• Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners — E.F. Morey (1992)
• An investigation of the potential biohazards associated with asbestos-substitute casting ring liners — G.J. Tulachka, W.A. Brantley, et al. (1987)
• Full-crown castings made with water-saturated and dry casting ring liners — F. Keyf, G. Erman (2000)
• Potential health hazards of ceramic ring lining material — D.R. Davis (1987)

Browse more research in The Journal of Prosthetic Dentistry, or return to medical research.