Abstract A commonly applied “rule of thumb” for combustible dust is that a light bulb would be nearly obscured by a 6‐foot‐thick cloud of dust if the dust density was near that required to be explosible (i.e., the minimum explosible concentration MEC). This heuristic is based on Eckhoff's normative text on this topic: “a cloud of 40 g/m 3 of coal dust in air is so dense that a glowing 25‐W lightbulb can hardly be seen through a dust cloud 2 m thick.” The coal dust cloud described by Eckhoff would be close to the MEC. This heuristic is well known, and the author has used it repeatedly to illustrate how dense a dust cloud must be to be explosible. This paper demonstrates the heuristic is correct by evaluating light transmission fractions for combustible dust clouds over a range of densities and thicknesses. Large‐scale open‐air tests are presented to illustrate the appearance of an explosible dust cloud. It is also shown that horizontal surface dust deposition rates at cloud densities well below the MEC would create an intolerable housekeeping burden, and that the highest density tolerable from a worker respiratory protection standpoint is over 3 orders of magnitude less than the MEC.
J. Kelly Thomas (Wed,) studied this question.