Simulation tests to assess occupational exposure to airborne asbestos from artificially weathered asphalt-based roofing products

Our plain-English summary of Simulation tests to assess occupational exposure to airborne asbestos from artificially weathered asphalt-based roofing products, by P. Sheehan, F. Mowat, R. Weidling, et al (The Annals of Occupational Hygiene, 2010).

The study

• Authors: P. Sheehan, F. Mowat, R. Weidling, M. Floyd
• Journal: The Annals of Occupational Hygiene
• Published: 2010;54(8):880-92
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Historically, asbestos-containing roof cements and coatings were widely used for patching and repairing leaks. Although fiber releases from these materials when newly applied have been studied, there are virtually no useful data on airborne asbestos fiber concentrations associated with the repair or removal of weathered roof coatings and cements, as most studies involve complete tear-out of old roofs, rather than only limited removal of the roof coating or cement during a repair job. This study was undertaken to estimate potential chrysotile asbestos fiber exposures specific to these types of roofing products following artificially enhanced weathering. Roof panels coated with plastic roof cement and fibered roof coating were subjected to intense solar radiation and daily simulated precipitation events for 1 year and then scraped to remove the weathered materials to assess chrysotile fiber release and potential worker exposures. Analysis of measured fiber concentrations for hand scraping of the weathered products showed 8-h time-weighted average concentrations that were well below the current Occupational Safety and Health Administration permissible exposure limit for asbestos. There was, however, visibly more dust and a few more fibers collected during the hand scraping of weathered products compared to the cured products previously tested. There was a notable difference between fibers released from weathered and cured roofing products. In weathered samples, a large fraction of chrysotile fibers contained low concentrations of or essentially no magnesium and did not meet the spectral, mineralogical, or morphological definitions of chrysotile asbestos. The extent of magnesium leaching from chrysotile fibers is of interest because several researchers have reported that magnesium-depleted chrysotile fibers are less toxic and produce fewer mesothelial tumors in animal studies than normal chrysotile fibers.

In plain English

The authors ran a laboratory simulation with two recreated asphalt-based roofing products containing chrysotile asbestos: C-8 Plastic Roof Cement and B-10 Fibered Roof Coating. Coated roof panels were weathered outdoors for a year. A worker then hand scraped four panels for 30 minutes each, with air sampled at the worker's lapels. Total fibers counted by light microscopy ranged from 0.06 to 0.17 fibers per cubic centimeter for the cement and 0.07 to 0.09 for the coating. Under electron microscopy, 90% of asbestos fibers had low or essentially no magnesium and did not meet the definition of normal chrysotile. The authors assumed half an hour of scraping in an 8-hour day. On that basis, 8-hour averages counting all asbestos fibers were 0.005 to 0.011 fibers per cubic centimeter for the cement and 0.005 to 0.006 for the coating. Weathered products gave visibly more dust than cured products in the authors' earlier tests. The authors concluded that these exposures were well below the current OSHA limit of 0.1 fibers per cubic centimeter. They said magnesium loss would need to be quantified before any change in cancer-causing potency could be assessed.

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 exposure simulation. Roof panels coated with asbestos-containing products were put through one year of accelerated outdoor weathering, then scraped by hand while air was sampled.
• Who was studied: No human subjects were studied for health effects. Four weathered roof panels (two per product) were each scraped by a worker in an enclosed test chamber at Forensic Analytical Laboratories, Hayward, California, in September 2006. Personal air samples were taken from the worker's right and left lapels.
• Exposure: Chrysotile asbestos only. The products were C-8 Plastic Roof Cement (15.5% chrysotile) and B-10 Fibered Roof Coating (3.04% chrysotile), recreated in 2005 by Monsey and Henry. Each was removed by 30 minutes of hand scraping with a long-handled putty knife, sometimes with a hammer.
• Main results:
  • For C-8 Plastic Roof Cement, total fiber concentrations by phase contrast microscopy (all fiber types, including non-asbestos) ranged from 0.06 to 0.17 fibers per cubic centimeter over the 30-minute scraping.
  • For B-10 Fibered Roof Coating, total fiber concentrations by phase contrast microscopy ranged from 0.07 to 0.09 fibers per cubic centimeter over the 30-minute scraping.
  • Most asbestos fibers counted by electron microscopy (90%) fell into the authors' 'low-Mg' or 'no-Mg' categories. These fibers did not meet the definition of normal chrysotile.
  • For C-8 Plastic Roof Cement, concentrations of fibers meeting all definitions of normal chrysotile ranged from 0.005 to 0.05 fibers per cubic centimeter.
  • For B-10 Fibered Roof Coating, 0 to 1 normal chrysotile fibers were identified per sample. All but one sample were below analytical sensitivity.
  • The 8-hour time-weighted averages assumed half an hour of hand scraping in an 8-hour workday. A background level of 0.0008 fibers per cubic centimeter was used for the rest of the day.
  • For C-8 Plastic Roof Cement, 8-hour time-weighted averages counting all asbestos fibers (normal chrysotile plus low-Mg and no-Mg fibers) ranged from 0.005 to 0.011 fibers per cubic centimeter. Counting normal chrysotile only, they ranged from 0.001 to 0.002.
  • For B-10 Fibered Roof Coating, 8-hour time-weighted averages counting all asbestos fibers ranged from 0.005 to 0.006 fibers per cubic centimeter. Counting normal chrysotile only, the average was 0.001.
  • Scraping the weathered products produced visibly more dust and a few more fibers than the cured products the authors tested previously.
• The authors' conclusion: The authors concluded that limited hand scraping of weathered roof cements and coatings like those tested is likely to produce airborne asbestos concentrations well below the current OSHA permissible exposure limit of 0.1 fibers per cubic centimeter.
• Limitations the authors state:
  • The extent of magnesium leaching under the conditions of interest would need to be quantified before any change in cancer-causing potency could be assessed.
  • The animal studies of magnesium-depleted chrysotile that the authors cite (other researchers' work) likely do not represent realistic human exposures, for example injection versus inhalation.
  • The high air-sampling flow rate also collected large non-respirable particles on some samples.

Topics

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

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Fiber Release, Chrysotile, Time-Weighted Average (TWA), Permissible Exposure Limit (PEL).

Related studies

• Simulation tests to assess occupational exposure to airborne asbestos from asphalt-based roofing products — F. Mowat, R. Weidling, et al. (2007)
• Releasability of asbestos fibers from weathered roof cement — A.F. Oberta, L. Poye, et al. (2018)
• Exposure to airborne asbestos during abatement of ceiling material, window caulking, floor tile and roofing material — J.H. Lange, S.L. Sites, et al. (2008)
• Airborne concentrations of asbestos onboard maritime shipping vessels (1978-1992) — D.M. Murbach, A.K. Madl, et al. (2008)
• A review of changes in composition of hot mix asphalt in the United States — D.J. Mundt, K.M. Marano, et al. (2009)
• Exposure to airborne asbestos during abatement of ceiling material, window caulking, floor tile, and roofing material — J.H. Lange, S.L. Sites, et al. (2006)

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