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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 m3 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.
Acknowledgements
The authors would like to thank and acknowledge Tony Havics, Shannon Gaffney, James Keenan, Joshua Maskrey, Lauren Spicer, Gretchen DeRose, Rich Lee, Drew Van Orden, Matthew Sanchez, and Matthew Zock for their support and assistance in completing this study.
Declaration of interest
The authors’ affiliations are as shown on the cover page. All of the authors are employed by Cardno ChemRisk, a consulting firm that performs scientific research and support for the government, corporations, law firms, and various scientific/professional organizations. The underlying data used in this analysis were generated from a study that was partially funded by John Crane, Inc., a manufacturer of sealing devices which historically manufactured or supplied asbestos-containing gaskets and packing. Cardno ChemRisk has been engaged by John Crane, Inc., as well as other corporations, to provide general consulting, expert advice, and litigation support on scientific matters involving asbestos. The analysis reported in this article and the work associated with preparing this manuscript for publication was conceived and funded entirely by Cardno ChemRisk. This paper was prepared and written exclusively by the authors without any review or input by John Crane, Inc. employees or legal counsel, or any other outside source. Four of the authors (D.J.P., J.L.H., J.S., A.K.M.) have served as expert witnesses regarding historical exposures of various tradesmen to asbestos.