Section 293.400 - Coal dust controls1. The operator of a coal mine shall not permit excessive accumulation of coal dust in any part of the mine. Where mining operations raise an excessive amount of dust, water or water with a wetting agent added to it or other effective methods shall be used to allay such dust at its source.2. All coal mines, except those mines or those locations in a mine in which dust is too wet or too high in incombustible content to propagate an explosion, shall be rock-dusted to within forty feet of all active working faces and all open crosscuts within the forty-foot zone shall be rock-dusted. If the dust in a mine or any part of it is wet but becomes dry, the mine or any portion of the mine so affected shall be rock-dusted as soon as it becomes dry.3. Where rock dust is applied it shall be distributed upon the top, floor and sides of all open places and maintained in such quantity that the incombustible content of the mine dust will not be less than sixty-five percent.