This is a mixture of three gases: approximately 52% nitrogen, 40% argon and 8% carbon dioxide. The basic concept with Inergen is to flood an enclosure with a mixture of these three gases to a 43% to 52% concentration. This reduces the available oxygen concentration below that necessary for combustion. It should be noted that any gas that displaces oxygen could also create an atmosphere detrimental to human life. The claim of Inergen is that the small amount of carbon dioxide in the mixture causes humans exposed to extinguishing concentrations to breathe faster and deeper and promote the uptake of oxygen by the blood.
Therefore, it depends on the modification of human physiology to claim safety for personnel exposed to extinguishing concentrations of the agent. The "envelope" of life safety with this concept is rather narrow. A little too much Inergen and there may not be enough available oxygen, despite the CO2. A little less Inergen and the extinguishing concentration may not be reached. Any modifications to the protected space – even the addition of equipment could change the volume of the space and thereby significantly alter the desired design criteria.
To ensure safety, it is required that the design concentration result in at least 10% oxygen. If the oxygen concentration falls below 10%, personnel must be evacuated within 30 seconds. This agent is approved for use in occupied areas under NFPA 2001 if it meets the stated criteria. Because Inergen is stored as a gas, it cannot be discharged to achieve rapid buildup of total flooding concentration. Discharge times in excess of one minute and as long as three minutes have been noted. Additionally, because Inergen is stored as a very high-pressure gas, as opposed to FM-200 that is stored as a liquid, its storage efficiency is low. Substantial numbers of containers are necessary to store enough Inergen for even the smallest hazards. The additional floor space required to accommodate the numerous Inergen cylinders along with added service costs to maintain, weigh and check all these cylinders must also be considered.