The maximum amount of pollution allowed from fly ash or dust shall not exceed 83 pounds per 1,000 pounds of gas, of which not to exceed five pounds per 1,000 gas shall be of the size as to be retained in a 325 mesh, as provided in the United States Bureau of Standards regulations, provided that these conditions are to apply only when the excess air in the stack does not exceed 50% of the full load.
('73 Code, § 16-12)