Section R. 325.2440 - Recirculation of air from exhaust systemsRule 40.
(1) The recirculation of air containing a contaminant whose MAC is at least 1000 ppm, 15 mg/m3, or 50 mppcf, shall be permitted if the exhaust ventilation system is equipped with an air cleaning device capable of reducing the contaminant concentrations to 10% or less of their MAC in the returned air.(2) The director may allow the recirculation of air containing a contaminant whose MAC is less than 1000 ppm, 15 mg/m3 or 50 mppcf, if the toxicity of the contaminant and the degree of air cleaning to be achieved create an environment which will not impair the health of the workers, and if the contaminant concentrations in the returned air do not exceed 10% of its MAC.(3) A recirculation system shall include an alternate air duct connection to discharge the returned air outside of the building if necessary to protect the workers' health.Mich. Admin. Code R. 325.2440