Mich. Comp. Laws § 29.21b

Sec. 21b. Notwithstanding section 21a, a liquefied compressed gas extinguishing agent that meets both of the following conditions of toxicity and use may be used in properly engineered fire extinguishing or fire control systems: (a) The agent in its normal state is not a prohibited agent or propellant under section 21a(1). (b) The agent is used only under conditions approved by the national fire protection association and a nationally recognized independent testing laboratory that has considered the hazard of the thermal decomposition products and use approved by the bureau.