No activity shall emit any fly ash, dust, fumes, vapors, mists or gases in such quantities as to cause soiling or danger to the health of persons, animals, vegetation or property. No activity shall emit any liquid or solid particles in concentrations exceeding 0.3 grains per cubic foot of the conveying gas, nor any color visible smoke equal to or darker than No. 2 on the Ringelmann Chart described in the United States Bureau of Mine's Information Circular 7718 in the industrial districts.