A halon system is a type of gaseous fire suppression system which operates with halon gas. In the 1980s, halon was recognized as an ozone-depleting compound, and production of additional halon was largely banned. Today, installation of new halon systems is only allowed when people can demonstrate that they absolutely must have a halon system. Older existing systems are perfectly legal, although many government encourage people to consider decommissioning their halon systems and replacing them with a safer system, if possible.
Halon was introduced in the 1960s as a form of gaseous fire suppression, and it also has a history of use as a refrigerant. In a halon system, pressurized tanks of the gas are connected to a series of pipes and nozzles. When a fire is detected, the gas is released. The halon gas interferes with the oxidation reaction between the fuel and the oxygen in the room which drives the fire, shutting the fire down. Other gaseous fire suppression systems work slightly differently, doing things like forcing the oxygen out of the room so that the fire cannot continue to be burn.
Halon systems were designed to be used in settings in which fire suppression might be necessary, but people wanted to avoid damage to expensive electronics and components. Such systems were widely used in the military, especially on aircraft, and some nations continue to allow halon systems on military aircraft because the halon system is the most suitable fire suppression system for that application. Halon systems were also used in server rooms and other facilities which contain expensive or delicate materials.
The primary problem with halon is that it depletes ozone. Every time a halon system is activated, halon is released into the environment, where it works its way into the atmosphere. Halon is also not very healthy for people to be around. While it will not suffocate people like some gaseous fire suppression systems, it can cause health problems, especially if people are exposed to it for prolonged periods. This is why alarms sound when halon systems activate, to alert people to the fact that they need to leave the area to avoid health risks.
Companies which specialize in maintenance of halon systems are still in business in several regions. They can recharge systems after a gas discharge and perform other maintenance tasks to keep existing systems working smoothly. Many such companies can also decommission a halon system and install an acceptable alternative. Since fire suppression systems are always evolving, it's worth consulting a fire suppression specialist every few years to see if a halon system is still necessary.