The assessment of exposure in terms of fibres

Our plain-English summary of The assessment of exposure in terms of fibres, by G.W. Gibbs (The Annals of Occupational Hygiene, 1994).

The study

• Author: G.W. Gibbs
• Journal: The Annals of Occupational Hygiene
• Published: 1994;38(4):477-87, 409-10
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The membrane filter (MF) method for evaluating asbestos fibre concentrations was introduced in the 1960s. Before that time the midget impinger (MI) was used in North America, while the long running (LRTP) and regular thermal precipitator (TP) were used in the U.K. All studies from which estimates of long-term health risks can be derived (i.e. those with individual cumulative lifetime exposure estimates) were based on the now obsolete methods. The reliability of converting these indices of exposure to MF equivalent concentrations was reviewed. It was concluded that no overall single factor could be derived for the Quebec mining and milling industry. However, it has been possible to derive conversion factors at the individual mill and work area level. Applying these in one Quebec mortality study analysis based on all jobs held by persons in the cohort gave an overall MF/MI ratio of 3.6. An examination of the confidence intervals surrounding the Quebec data, ratios derived for other chrysotile mines by other investigators, and measurements of fibre concentrations in the 1970s suggest that this was probably not unreasonable. Side-by-side and other measurements were used to convert MI concentrations in the U.S. textile industry to MF fibre concentrations. While conversions involve considerable uncertainty, independent measurements of fibres in the lung tissues of workers from the U.S. textile plant and Quebec mills show that in lungs the ratios of the concentrations of chrysotile to those of tremolite are quite consistent with the ratio of assessed exposures to these fibres in the two industries. There is an apparently higher risk of mesothelioma in one Quebec mining area (Thetford Mines) than in another (Asbestos). A high concentration of fibrous tremolite has been found in the lungs of workers in Thetford. A method of evaluating the extent to which mesothelioma risk in the chrysotile mining industry might be explained by tremolite exposures was proposed. The slope of the lung cancer dose-response relationship for the textile industry is approximately 50 times that for the mining and milling industry. Available data on the length distributions of fibres from Quebec mines and mills (up to 5% > 5 microns) and the Charleston textile plant (up to 21% > 5 microns) and some marginal indication of longer fibres in tissues from Charleston workers suggest that further work specifically addressing differences in the size distributions of long fibres in these industries is needed.

In plain English

This 1993 workshop paper reviewed how exposure was estimated in studies of lung cancer and mesothelioma in chrysotile workers. None of those studies used today's membrane filter fibre counts directly. North American studies used midget impinger particle counts; U.K. studies used thermal precipitator particle counts before 1960, then a long running thermal precipitator that counted fibres in 1960 to 1966. The author reviewed attempts to convert these into fibre counts. For Quebec mines and mills, side-by-side samples agreed poorly; the author stated no single overall conversion factor applied. In one Quebec lung cancer case-control study, job-specific factors gave an overall factor of 3.64. At a U.S. textile plant, a cited study used a factor of about 3, or about 8 in preparation. Another cited study of that plant found a very steep slope relating lung cancer to estimated exposure; risk per unit of exposure appeared about 50 times that of Quebec mining workers. The author also discussed tremolite as a possible mesothelioma factor at Thetford Mines, and fibre length differences. The author concluded that all fibre estimates carry considerable uncertainty.

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: Review of exposure-assessment methods and data used in epidemiological studies of chrysotile workers (paper read at a workshop)
• Who was studied: Exposure data from cohort studies of chrysotile workers, mainly the Quebec chrysotile mines and mills, the Rochdale (U.K.) textile plant and the South Carolina (Charleston, U.S.) textile plant. Asbestos-cement and friction-material workers are also discussed.
• Exposure: Chrysotile dust measured by three kinds of method: midget impinger particle counts (in million particles per cubic foot), thermal precipitators (particle counts before 1960, and fibre counts with the long running thermal precipitator in 1960 to 1966), and membrane filter fibre counts. The U.S. textile plant also used some crocidolite and a small amount of amosite. The Rochdale plant bought crocidolite between 1932 and 1968. Tremolite is discussed for the Quebec mines.
• Main results:
  • In 87 side-by-side midget impinger and membrane filter samples from the Quebec mining industry, the overall correlation was 0.45 for the 56 samples with more than one fibre per field.
  • The author stated that no single overall conversion factor could be applied to the Quebec mine and mill data.
  • In a case-control study of lung cancer in Quebec miners and millers, job-specific conversion factors gave an overall conversion factor of 3.64, based on the jobs of cases and controls.
  • For the U.S. textile plant, the Dement et al. data suggested a conversion factor of about 3 for all textile operations except preparation, where the ratio appeared to be about 8.
  • The McDonald et al. (1983) study of the same U.S. textile plant found a very steep slope for the relationship between lung cancer and exposure in mppcf-years. In relation to exposure, the risk of lung cancer appeared to be about 50 times that of workers in the Quebec chrysotile mining industry.
  • From the lung tissue study by Sebastien et al. (1989), the author inferred that the fibre exposures of Quebec miners and millers, relative to Charleston textile workers, may have been even greater than estimated in the epidemiological studies.
  • A mesothelioma model fitted to the Quebec data (McDonald et al., 1992) gave a factor (b) of 0.1 times 10 to the minus 8 for miners and millers at Asbestos and 0.24 times 10 to the minus 8 at Thetford Mines. Tremolite exposures were considered greater at Thetford Mines.
  • Fibres longer than 5 micrometres made up as much as 21% of airborne fibres in textile carding and 24.5% in ring and gang spinning (unpublished data). This compared with 1.3% in mining and 4.1% in bagging fibre in a Quebec mill.
• The authors' conclusion: The author concluded that all fibre estimates carry considerable uncertainty. The uncertainties in the conversion ratios make risk per unit of fibre exposure less certain than risk based on unconverted data. The author added that combining data from tremolite-exposed vermiculite workers may help estimate mesothelioma risk from chrysotile containing tremolite.
• Limitations the authors state:
  • Uncertainty in the conversion ratios makes fibre-based risk estimates less certain than those based on unconverted exposure data.
  • The Dement et al. report did not say whether samples were personal samples or where the side-by-side samples were taken.
  • The lung tissue fibre-length measurements were based on small numbers.
  • Airborne tremolite concentrations in chrysotile workplaces are not known and may be difficult or impossible to measure.

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: Midget Impinger, Thermal Precipitator, confidence interval, Chrysotile, Tremolite, Mesothelioma, Thetford Mines, Lung Cancer.

Related studies

• A comparison of parallel dust and fibre measurements of airborne chrysotile asbestos in a large mine and processing factories in the Russian Federation — E. Feletto, S.J. Schonfeld, et al. (2017)
• An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers — L. Stayner, E. Kuempel, et al. (2008)
• Increased lung cancer mortality among chrysotile asbestos textile workers is more strongly associated with exposure to long thin fibres — D. Loomis, J.M. Dement, et al. (2012)
• Assessment of airborne asbestos exposure during the servicing and handling of automobile asbestos-containing gaskets — C.L. Blake, G.S. Dotson, et al. (2006)
• Lung dust analysis in the assessment of past exposure of man-made mineral fibre workers — J.C. McDonald, B.W. Case, et al. (1990)
• Lung cancer mortality and fibre exposures among North Carolina asbestos textile workers — D. Loomis, J.M. Dement, et al. (2009)

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