Fibers that have been released can remain suspended in the air for many hours. After the fibers settle, they can be resuspended in the air by disturbances created by occupant activities or custodial work such as dusting or sweeping. Resuspension of asbestos fibers in the air is called re-entrainment. Re-entrainment may cause repeated exposures after the fibers are released from the asbestos material (EPA, 1979).
Airborne fibers exhibit a settling velocity proportional to their dimensions and aspect ratios. Fibers released from an overhead surface may take many hours to complete settling under quiet conditions, and turbulence can interrupt and prolong fiber settling indefinitely. According to Sawyer (1979), settled fibers may enter repeated cycles of re-entrainment and be available for human exposure in a nearly continuous fashion. The asbestos fibers also possess sufficient inertia to result in relative containment within the confines of a structure, and do not exhibit completely free dispersal to the outside environment (Sawyer, Non-Occupational Indoor Asbestos Pollution, 1979).
The electrostatic forces between particles and other particles and between particles and surfaces are proportional to surface area, and the effect of the force is proportional to the mass of the particles. Smaller particles are influenced by these attractive forces more strongly than larger particles. Individual small particles can be strongly attached to a surface by electrostatic forces. According to Millette and Hays (1994), a single layer of very small particles, uniform in size, will be hard to dislodge from a surface, but mixtures of particles of different sizes may be more readily entrained (Millette & Hays, 1994).
In a survey of storehouses insulated with sprayed asbestos, Lumley et al. (1971) reported that disturbance of the insulation or the fallen asbestos debris is likely to cause airborne asbestos dust. The lowest mean asbestos dust concentration was obtained in the storehouses in which there was hardly any activity. A higher mean concentration was obtained where there was much activity (Lumley, Harries, & O’Kelly, 1971).
In the Yale University Art and Architecture Building where chrysotile asbestos-containing material had contributed asbestos fibers to the dust on top of books, the fiber concentration during quiet conditions was found to be 0.02 f/cc [n=15]. During book removal, airborne asbestos fiber levels in the library rose from the routine activity level of 0.3 [n=10] to 1.2 f/cc [n=8] due to re-entrainment of settled fibers (Sawyer,1977).
Kominsky and Freyberg (1991) conducted a study to evaluate the potential for asbestos fiber re-entrainment during cleaning of carpet contaminated with asbestos. Two types of carpet cleaning equipment were evaluated at two carpet contamination levels. Overall, airborne asbestos concentrations during carpet cleaning were two to four times greater than concentrations prior to cleaning. The level of asbestos contamination and the type of cleaning method used had no statistically significant effect on the relative increase of airborne asbestos concentrations during carpet cleaning (Kominsky & Freyberg, 1991).
References
EPA. (1979, March). Asbestos-Containing Materials in School Buildings.
Kominsky, J., & Freyberg, R. (1991, May). Asbestos Fiber Reentrainment During Dry Vacuuming and Wet Cleaning of Asbestos Contaminated Carpet. EPA, Risk Reduction Engineering Laboratory. Cincinnati, OH: EPA.
Lumley, K., Harries, P. G., & O’Kelly, J. (1971). Buildings Insulated with Sprayed Asbestos: A Potential Hazard. Annals of Occupational Hygiene, 14, 255-257.
Millette, J., & Hays, S. (1994). Resuspension of Settled Dust. In Chapter 8: Settled Asbestos Dust Sampling and Analysis (pp. 59-65). Lewis Publishers.
Sawyer, R. (1977). Asbestos Exposure in a Yale Building: Analysis and Resolution. Environmental Research, 13, 146-169.
Sawyer, R. (1979). Non-Occupational Indoor Asbestos Pollution. In R. Lemon, & J. Dement (Eds.), Dusts and Diseases. Park Forest South, Illinois: Pathotox Publishers, Inc.
