In the case of asbestos, visible airborne dust represents hazardous concentrations above established occupational exposure limits. Warren Cook stated the following in 1942: “In the case of asbestos dust condition, our evaluation of the exposure should be based on the knowledge that the present toxic limit for asbestos is five million particles of dust per cubic foot of air. This is a very small concentration, so small in fact that the condition may look good even to a critical eye and still present an exposure greater than this low limit.” He further noted, “In the case of asbestos dust, and this holds with even more certainty for dusts high in free silica content, the toxic limit is so low that the only safe procedure is to have recourse to actual dust determinations. This is especially important where the injury’s condition is not immediately evident but requires years to develop in the case of asbestosis and silicosis” (Cook, 1942).
A simple and valuable tool in the observation of dust processes is some form of Tyndall beam illumination. In dark areas, observations will be improved by almost any kind of illumination. Tyndall beam illumination consists of the following elements: (a) a light source of any kind or intensity (b) a shield interposed between the light source and the eyes of the observer, (c) arrangements to avoid or minimize reflection of light to the eyes of the observer from the elements of the equipment being studied, and (d) a relatively dark background for the observer’s eyes (Hemeon, 1955).
W. C. L. Hemeon (1955) suggested that dust and fume concentrations can be roughly described in terms of sight perception. He approximated dust concentration values in terms of the sense of light, which is provided in Table 1.
Table 1: Sight-Perception Dust Scale (Visible Dust Concentrations in General Air)
| Lighting | Perspective | Perspective |
| Short Distances (less than 10 to 15 feet) | Long Distances (50 to 200 feet) | |
| Concentration (mppcf) | Concentration (mppcf) | |
| Beam of sunlight, background relatively dark | Less than 2 (<12 f/cc)* (No Distance Effect) | Less than 2 (<12 f/cc)* (No Distance Effect) |
| Bright sunlight but no beam effect | 10 – 20 (60 – 120 f/cc)* | 2 – 5 (12 – 30 f/cc)* |
| Bright daylight “north” illumination (i.e., interior but no direct sun) | 10 – 20 (60 – 120 f/cc)* | 5 – 10 (30 – 60 f/cc)* |
| Low intensity daylight | 20 – 40 (120 – 240 f/cc)* | 10 – 20 (60 – 120 f/cc)* |
| Dim artificial light (night) | 100 – 200 (600 – 1200 f/cc)* | 75 – 100 (450 – 600 f/cc) |
* Added by author for comparison of million particles per cubic foot (mppcf) to fibers per cubic centimeter (f/cc) based on OSHA factor of 1 mppcf = 6 f/cc.
By comparing the above sight‑perception scale of values with the threshold concentration of 5 mppcf, Hemeon noted that hazardous concentrations are apparent visually only under the most favorable conditions of illumination (Hemeon, 1955).
Even if the asbestos portion of the dust is in the minority, significant asbestos concentrations exist, and exceedances of the contemporary OELs are likely. If the dust is substantially all asbestos, it is readily apparent that any visible dust generated from asbestos‑containing materials will exceed all OELs since the 1969 Walsh‑Healey OEL of 2 mppcf (12 f/cc) and will most likely exceed the original OEL of 5 mppcf.
Petty (2020) researched the literature on visible dust from asbestos-containing materials. The purpose of the research was to determine if exposure to visible dust created from asbestos‑containing materials (>1% asbestos) likely exceeds recommended occupational exposure limits for asbestos. The analysis of the literature found that 1) the 5 mppcf asbestos standard was based on total dust, not just asbestos dust; 2) the presence of visible dust from operations involving asbestos‑containing materials is likely greater than 5 mppcf; and 3) the presence of visible asbestos-containing dust likely results in levels above ACGIH and OSHA standards (Petty, 2020).
The National Research Council’s Committee on Indoor Air Pollutants (1981) reported that fiber settling velocity depends far more heavily on fiber diameter than on fiber length. The committee estimated that in still air in a room with a height of three meters, a fiber with dimensions of 5 µm long by 1 µm in diameter would remain airborne for approximately four hours. The committee further estimated that a fiber of the same length with a 0.1 µm diameter would remain airborne for closer to 20 hours (Donovan, Donovan, Sahmel, Scott, & Paustenbauch, 2010).
In 1978, the U.S. EPA also evaluated the speed with which asbestos fibers may settle out of the air. The EPA estimated that settling velocity was more strongly dependent on fiber diameter than on length, and they determined that settling velocities for fibers that were 5, 2, and 1 µm in length with a 5:1 aspect ratio would require 4, 20, and 80 hours, respectively, to settle out of still air from a height of 9 feet. They further estimated that air turbulence would prolong the settling and cause re-entrainment of fallen fibers (EPA, 1978).
References
Cook, W. (1942). The Occupational Disease Hazard: Evaluation in the Field. Industrial Medicine, 11(4), 193‑197.
Donovan, E. P., Donovan, B. L., Sahmel, J., Scott, P. K., & Paustenbauch, D. J. (2010). Evaluation of Bystander Exposures to Asbestos in Occupational Settings: A Review of the Literature and Application of a Simple Eddy Diffusion Model. Critical Reviews in Toxicology, 1‑23.
EPA. (1978). Sprayed ACM in Buildings: A Guidance Document. Triangle Park, NC: U.S. EPA, Office of Air and Waste Management, Office of Air Quality Planning and Standards.
Hemeon, W. C. (1955). Plant and Process Ventilation. New York: The Industrial Press.
Petty, S. (2020). Visible dust and asbestos: what does it suggest regarding asbestos exposure? Journal of Scientific Practice and Integrity, 2(1).: What does it suggest regarding asbestos exposure? Journal of Scientific Practice and Integrity, 2(1).