Measured removal rates of chrysotile asbestos fibers from air and comparison with theoretical estimates based on gravitational settling and dilution ventilation

Our plain-English summary of Measured removal rates of chrysotile asbestos fibers from air and comparison with theoretical estimates based on gravitational settling and dilution ventilation, by J. Sahmel, H.J. Avens, P.K. Scott, et al (Inhalation Toxicology, 2015).

The study

• Authors: J. Sahmel, H.J. Avens, P.K. Scott, K. Unice, A. Burns, C.A. Barlow, A.K. Madl, J. Henshaw, D.J. Paustenbach
• Journal: Inhalation Toxicology
• Published: 2015;27(14):787-801
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Context: Industrial hygiene assessments often focus on activity-based airborne asbestos concentration measurements, but few empirical data exist regarding the fiber removal rate from air after activities cease.

Objective: Grade 7T chrysotile indoor fiber settling (FS) rates were characterized using air sampling (NIOSH Method 7402).

Materials And Methods: Six replicate events were conducted in a 58 m(3) study chamber (ventilation 3.5 ACH), in which chrysotile-contaminated work clothing was manipulated for 15 min followed by 30 min of no activity. The fiber concentration decay constant and removal rate were characterized using an exponential decay model based on the measurements.

Results: Breathing zone airborne chrysotile concentrations decreased by 86% within 15-30 min after fiber disturbance, compared to concentrations during active disturbance (p < 0.05). Estimated mean time required for 99% of the phase contrast microscopy-equivalent (PCME) fibers to be removed from air was approximately 30 min (95% CI: 22-57 min). The observed effective FS velocity was 0.0034 m/s. This settling velocity was between 4.5-fold and 180-fold faster than predicted by two different particulate gravitational settling models. Additionally, PCME concentrations decreased approximately 2.5-fold faster than predicted due to air exchange alone (32 versus 79 min to 99% decrease in concentration).

Discussion: Other measurement studies have reported similar airborne fiber removal rates, supporting the finding that factors other than gravitational settling and dilution ventilation contribute measurably to PCM fiber removal from air (e.g. impaction, agglomeration).

Conclusion: Overall, the scientific weight of evidence indicates that the time necessary for removal of 99% of fibers greater than 5 μm in length (with aspect ratios greater than 3:1) is approximately 20-80 min.

In plain English

The authors measured how quickly chrysotile asbestos fibers left chamber air after an activity stopped. In six replicate events in a 58 cubic-meter chamber (3.5 air changes per hour), a participant shook out clothing contaminated with Grade 7T chrysotile for 15 minutes, followed by 30 minutes without activity. Breathing-zone samples were analyzed by NIOSH Method 7402. From 15 to 30 minutes after shaking, concentrations were 86% lower on average (range 80 to 92%) than during shaking. For fibers in the size range this method counts, a decay curve fitted to all events gave a half-life of 4.8 minutes (95% CI 3.4 to 8.6) and 32 minutes (95% CI 22 to 57) to 99% removal. Ventilation alone was calculated to take 79 minutes. Two gravitational settling models gave settling speeds 4.5-fold to 180-fold slower than the measured effective speed. Drawing on this study and a reanalyzed 1972 insulation-spraying study, the authors called about 20 to 80 minutes a reasonable estimate for 99% removal of relevant fibers of this size. They noted their analysis assumed uniform concentration throughout the chamber.

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: Controlled chamber experiment (simulation study) with six replicate events, comparing measured fiber removal with theoretical estimates from ventilation-only and gravitational settling models
• Who was studied: No disease outcomes were studied. The study consisted of six clothes-handling events performed by a study participant in a chamber of about 58 cubic meters (4.7 by 4.8 by 2.5 m) at RJ Lee Group, Monroeville, Pennsylvania, USA. Air samples came from the participant's breathing zone. The article does not give study dates.
• Exposure: Work clothing contaminated with Grade 7T chrysotile from Thetford Mines, Quebec, Canada, was handled and shaken out for 15 minutes, followed by 30 minutes of no activity. The chamber ventilation was 3.5 air changes per hour. Fibers were counted as phase contrast microscopy-equivalent (PCME) chrysotile fibers by NIOSH Method 7402.
• Main results:
  • Breathing-zone concentrations from 15 to 30 minutes after shaking ended were reduced by 86% on average (range 80 to 92%) compared with concentrations during the shaking period.
  • The mean concentration in the last 15 minutes of the settling period was statistically significantly lower than during the clothes-handling period. The comparisons used Bonferroni-adjusted t-tests.
  • The decay curve was fitted to all six events combined, for chrysotile fibers in the size range counted by the PCME method. It gave a half-life of 4.8 minutes (95% CI 3.4 to 8.6 minutes) and a time to 99% removal of 32 minutes (95% CI 22 to 57 minutes).
  • When a curve was fitted to each event separately, the apparent half-lives ranged from 1.1 to 11.4 minutes.
  • Based on the measured ventilation of 3.5 air changes per hour, ventilation alone would have taken an estimated 79 minutes to remove 99% of fibers.
  • The effective settling velocity derived from the measurements, covering all removal other than ventilation, was 0.0034 m/s on average (95% CI 0.0009 to 0.0059 m/s). These non-ventilation effects alone corresponded to 54 minutes to 99% removal.
  • For fibers in the PCME size range, the two gravitational settling models (Sawyer and Spooner 1978; Hinds 1999) gave settling velocities of 0.000019 to 0.00076 m/s. This is 4.5-fold to 180-fold slower than the observed effective velocity.
  • With ventilation assumed to be zero, the gravitational settling models estimated 4 to over 14,000 hours to 99% removal. With ventilation of 3.5 air changes per hour included, the estimates were 59.4 to 79.2 minutes.
  • The authors fitted the same decay curve to published data from Reitze et al. (1972), a study of spraying asbestos insulation. That study did not specify the fiber type, but the authors judged it likely chrysotile. Their fit gave an estimated 61 minutes (95% CI 51 to 77 minutes) to 99% removal for that earlier study.
• The authors' conclusion: The authors concluded that measured removal of asbestos fibers from air was faster than predicted by commonly used gravitational settling models, especially models that assume little or no ventilation. Based on this study and previous research, they gave approximately 20 to 80 minutes as a reasonable estimate of the time for removal of 99% of relevant fibers (those in the size range counted by PCM or PCME methods). For the PCME-sized chrysotile fibers measured in this study, the mean was 32 minutes.
• Limitations the authors state:
  • The decay-curve analysis assumed a uniform concentration throughout the chamber, and a detailed analysis of how fibers spread through the chamber was beyond the study's scope. Samples came from the breathing zone near the release source.
  • The chamber ventilation rate of 3.5 air changes per hour is likely at the lower end of the range for many industrial settings.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Industrial Hygiene, Air Sampling, NIOSH, phase contrast microscopy (PCM), Aspect Ratio (Length-to-diameter ratio).

Related studies

• Mass transfer rates of polycyclic aromatic hydrocarbons between micron-size particles and their environment--theoretical estimates — P. Gerde, P. Scholander (1989)
• Comparison of direct and indirect methods of measuring airborne chrysotile fibre concentration — C. Eypert-Blaison, S. Veissiere, et al. (2010)
• Chrysotile asbestos exposure associated with removal of automobile exhaust systems (ca. 1945-1975) by mechanics: results of a simulation study — D.J. Paustenbach, A.K. Madl, et al. (2006)
• An exposure study of bystanders and workers during the installation and removal of asbestos gaskets and packing — C. Mangold, K. Clark, et al. (2006)
• Evaluation of take-home exposure and risk associated with the handling of clothing contaminated with chrysotile asbestos — J. Sahmel, C.A. Barlow, et al. (2014)
• Chrysotile asbestos in serpentinite quarries: a case study in Valmalenco, Central Alps, Northern Italy — A. Cavallo, B. Rimoldi (2013)

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