§ 3367.24. Smoke.

The emission of smoke by any use permitted in the M-2 manufacturing district shall be controlled so as to be less dark in shade than that designated as No. 2 on the Ringlemann Chart, published and used by the United States Bureau of Mines; provided, however, that smoke of a density equal to that designated as No. 2 on the Ringlemann Chart may be permitted for not more than eight minutes during any 30 minute period and smoke of a density not exceeding that designated as No. 3 on the Ringlemann Chart may be permitted for not more than three minutes during any 30 minute period while starting or cleaning a fire.