Emission Of Dirt, Dust, Fly Ash And Other Forms Of Particulate Matter. Emission of dirt, dust, fly ash and other forms of particulate matter shall not exceed eighty-five hundredths (0.85) pounds per one thousand (1,000) pounds of gases of which amount not to exceed five-tenths (0.5) pounds per one thousand (1,000) pounds of gases shall be of such size as to be retained on a 325-mesh U.S. Standard Sieve. In the case of emission of fly ash or dust from a stationary furnace or combustion device, these standards shall apply to a condition of fifty percent (50%) excess air on the stack at full load, which standards shall be varied in proportion to the deviation of the percentage of excess air from fifty percent (50%).