Evaluation of bystander exposures to asbestos in occupational settings: a review of the literature and application of a simple eddy diffusion model

Our plain-English summary of Evaluation of bystander exposures to asbestos in occupational settings: a review of the literature and application of a simple eddy diffusion model, by E.P. Donovan, B.L. Donovan, J. Sahmel, et al (Critical Reviews in Toxicology, 2011).

The study

• Authors: E.P. Donovan, B.L. Donovan, J. Sahmel, P.K. Scott, D.J. Paustenbach
• Journal: Critical Reviews in Toxicology
• Published: 2011;41(1):52-74
• Type of article: Review
• Cited by: 18 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This article presents a review of the publicly available information as it relates to airborne asbestos concentrations at varying distances from a source in an occupational environment. Personal and area samples collected 5-75 feet from the primary worker from workplace surveys conducted in the 1970s and area samples collected 5-50 feet from the primary worker during more recent simulation studies were identified, compiled, and analyzed. As expected, airborne asbestos concentrations generally decreased with distance from the worker who performed a given task. Based on this review, however, the authors found that no systematic research to quantitatively relate fiber concentration with distance from the source (including consideration of fiber length, dilution ventilation, and initial momentum of the particle) has been conducted to date. A simple mathematical model was therefore used, and the results were considered, along with available published data comparing exposure data for both workers and persons/areas near workers. From this analysis, the authors offer guidance for estimating airborne asbestos concentrations at distance from a source. Based on the available data and our modeling results, the authors propose the following approach as a rule of thumb: for persons 1-5 feet from the source, airborne asbestos concentrations can be roughly approximated at 50% of the source concentration; 35% at >5-10 feet, 10% for >10-30 feet, and less than 1% at distances greater than 30 feet. This approach should be helpful for bracketing the range of likely exposures to bystanders being evaluated in asbestos-related dose-reconstruction analyses.

In plain English

This review combined published asbestos air-sampling data at varying distances from a worker with a simple mathematical model. Of nearly 100 studies screened, 19 qualified: seven 1970s workplace surveys and twelve 2000s simulation studies. Bystander samples were nearly always lower than matching worker samples. Pooled average bystander-to-worker ratios showed no clear trend with distance: 0.08 at 1-5 feet, 0.61 at more than 5-10 feet, 0.11 at more than 10-30 feet and 0.10 beyond 30 feet. The authors noted that 44 of the 51 samples behind the 0.61 figure came from one study with many undetectable results. A model with deposition and wall reflection predicted 37% of the source concentration at 3 feet and 2% at 10 feet. Combining the model with professional judgment, the authors proposed a rough rule of thumb for bystanders under 50 feet away: 50% of the source concentration at 1-5 feet, 35% at more than 5-10, 10% at more than 10-30 and 1% beyond 30 feet. They meant it for historical exposure reconstruction and called it too uncertain for respiratory-protection decisions where air samples can be taken.

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: Literature review of published air-sampling studies, combined with an indoor-air eddy diffusion mathematical model (Drivas et al. 1996 and Keil 2000 models)
• Who was studied: 19 published studies: 7 workplace surveys conducted in the 1970s and 12 simulation studies conducted in the 2000s, selected from about 100 studies screened. Studies of asbestos abatement and of building occupants during routine maintenance were excluded.
• Exposure: Airborne asbestos fibers counted as OSHA fibers (longer than 5 micrometers, aspect ratio at least 3:1). Tasks covered were spray insulation, spackling/patching/taping compounds, brake repair, gasket work, clutch and brake handling, coatings/mastics/adhesives, and phenolic molding material (Bakelite). The model assumed a single 15-minute release equal to a peak worker concentration of 5 f/cc in a 100 by 100 by 20-foot room.
• Main results:
  • Across the studies, bystander samples were nearly always lower than the corresponding worker samples.
  • Concentrations in the 1970s workplace surveys were orders of magnitude higher than in the 2000s simulation studies.
  • In the pooled published data, bystander-to-worker ratios did not follow a linear pattern with distance. Average ratios were 0.08 at 1-5 feet, 0.61 at more than 5-10 feet, 0.11 at more than 10-30 feet, and 0.10 at more than 30 feet.
  • Of the 51 samples in the more than 5-10 feet group, 44 came from the Mowat et al. (2005) study, which had a very high proportion of samples below the limit of detection. The authors said the higher ratio from that study should be interpreted with caution.
  • The model with reflection and deposition (Drivas et al. 1996) predicted bystander concentrations of 37% of the source concentration at 3 feet, 2% at 10 feet and 0.3% at 15 feet.
  • The upper-bound model without reflection and deposition (Keil 2000) predicted 83% at 3 feet, 38% at 10 feet, 14% at 15 feet, 7% at 20 feet and 0.8% at 30 feet.
  • The proposed rule of thumb was 50% of the source concentration at 1-5 feet, 35% at more than 5-10 feet, 10% at more than 10-30 feet, and 1% at greater than 30 feet. It was intended for bystanders less than 50 feet from the source, and the model results were adjusted on professional judgment.
• The authors' conclusion: The authors proposed general guidelines for estimating bystander exposure when the worker's concentration is known and the distance is under 50 feet, intended for historical exposure reconstruction. They said the rule of thumb would be too uncertain for deciding whether respiratory protection is needed in an active workplace, where air samples can be collected. They concluded that much more research is needed.
• Limitations the authors state:
  • Few workplace studies gave air sampling data at known distances. The 1970s studies were not directly comparable, and none were designed to answer this question.
  • In some simulation studies, half or more of the samples were below detection limits.
  • Using one-half the detection limit for undetected bystander samples could have underestimated the reduction with distance.
  • The model was simplistic and cannot account for local exhaust ventilation, open windows, fans or high dilution ventilation.
  • There is moderate uncertainty about the precision of the rule of thumb.
  • The rule of thumb would be too uncertain for deciding on respiratory protection where workplace-specific air samples can be collected.

Topics

Medical terms in this study, explained in our Lexicon: Bystander Exposure, Fiber Length.

Related studies

• Evaluation of take home (para-occupational) exposure to asbestos and disease: a review of the literature — E.P. Donovan, B.L. Donovan, et al. (2012)
• History of knowledge and evolution of occupational health and regulatory aspects of asbestos exposure science: 1900-1975 — C.A. Barlow, J. Sahmel, et al. (2017)
• Revising the British Occupational Hygiene Society asbestos standard: 1968-1982 — M. Greenberg (2006)
• Commentary on 'evaluation of take home (para-occupational) exposure to asbestos and disease: a review of the literature', Donovan et al.1 — D.S. Egilman, N.M. Druar (2013)
• Evaluation of asbestos exposures during firewood-harvesting simulations in Libby, MT, USA--preliminary data — J.F. Hart, T.J. Ward, et al. (2007)
• Airborne concentrations of asbestos onboard maritime shipping vessels (1978-1992) — D.M. Murbach, A.K. Madl, et al. (2008)

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