14 CFR § 25.1197

Fire extinguishing agents must—

Be capable of extinguishing flames emanating from any burning of fluids or other combustible materials in the area protected by the fire extinguishing system; and

Have thermal stability over the temperature range likely to be experienced in the compartment in which they are stored.

If any toxic extinguishing agent is used, provisions must be made to prevent harmful concentrations of fluid or fluid vapors (from leakage during normal operation of the airplane or as a result of discharging the fire extinguisher on the ground or in flight) from entering any personnel compartment, even though a defect may exist in the extinguishing system. This must be shown by test except for built-in carbon dioxide fuselage compartment fire extinguishing systems for which—

Five pounds or less of carbon dioxide will be discharged, under established fire control procedures, into any fuselage compartment; or

There is protective breathing equipment for each flight crewmember on flight deck duty.