Mo. Rev. Stat. § 71.760
The emission or discharge into the open air of dense smoke within the corporate limits of any city of this state is hereby declared to be a public nuisance. The owners, lessees, occupants, managers or agents of any building, establishment or premises from which dense smoke is so emitted or discharged shall be deemed guilty of a misdemeanor, and upon conviction thereof, in any court of competent jurisdiction, shall pay a fine of not less than twenty-five dollars nor more than one hundred dollars. And each and every day whereon such smoke shall be emitted or discharged shall constitute a separate offense; provided, however, that in any suit or proceeding under this section, it shall be a good defense if the person charged with a violation thereof shall show to the satisfaction of the jury or court trying the facts that there is no known practicable device, appliance, means or method by application of which to his building, establishment or premises the emission or discharge of the dense smoke complained of in that proceeding could have been prevented.
§ 71.760, RSMo
RSMo 1939 § 7575