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Halon and carbon dioxide were once the go-to extinguishing agents for protecting special hazards, but both fell from grace for this application. Halon has the problem of depleting stratospheric ozone. And carbon dioxide is not safe to use in enclosed spaces that have people in them.
Special hazard fire-extinguishing agent alternatives were developed to take their place. All told, about a dozen such agents exist. These agents put out fires, avoid collateral damage, and are safe for people and for the environment. In addition to protecting special hazards, these agents are used in spaces where dumping many gallons of water would cause too much collateral damage or would disrupt critical operations.
When considering which extinguishing agent to use in a fire suppression system, several variables have to be considered: What is the hazard? What kind of enclosure is it in? What are the life-cycle costs? What is the impact on the environment? What is its impact on building occupants? How will the agent be stored?
An alternative to the Halon alternatives is water mist. Water mist systems use a very fine water spray to extinguish a fire. Because water mist systems require less water than a standard sprinkler system to adequately control a fire, water mist is effectively being used to protect areas normally protected by fire sprinklers that have water supply limitations or where system weight is a factor. They are still gaining acceptance in commercial facilities and have so far been primarily used to protect hazards such as passenger ships, machinery spaces, museums and hotels.