When you have a fire, you add water. Problem solved. Sometimes, though, adding water isn’t an option, which is why some fire systems involve adding materials that can decompose into poisons or smother everything in the building.

The Deadly Choice

Not all buildings use water to put out a fire. If you drench a museum, or a bank of computers, or collection of rare books, you might as well have let the place burn down. The solution is as bad as the problem, so some places have a system for depriving fires of oxygen. There’s a cheap, abundant substance that can do just that.


Instead of water, many fire suppressant systems carry carbon dioxide. This gave rise to legends about fire systems that “removed all the oxygen” from a building. These systems added instead of subtracting, flooding spaces with gas until between 35 and 75 percent of the atmosphere in a room was carbon dioxide.

The rumors weren’t entirely incorrect, though. The effect on humans is nearly the same as if the system did remove oxygen. To begin with, the carbon dioxide from a fire suppressant system displaces nearly all of the breathable, or combustible, oxygen. That’s what it’s supposed to do. But even if the level of oxygen in the room somehow stayed the same, high levels of carbon dioxide are extremely dangerous to people. Breathing in an atmosphere that’s two percent carbon dioxide can cause mild carbon dioxide poisoning to set in. In the early stages, carbon dioxide poisoning causes muscle twitches, slowed brain activity, and high blood pressure. As carbon dioxide levels increase, the victim’s heartbeat becomes fast and arrhythmic, and they become completely disoriented.


Sadly, one person has died due to a carbon dioxide fire suppression system. An employee at a financial company became trapped in a vault and pulled the fire alarm. She was unable to escape when the carbon dioxide flooded the area. Although deaths due to these systems are incredibly rare, they’re not freak accidents. Most buildings that house something valuable enough to install special fire suppression systems will most likely have locked doors.

The Cool (And Less Deadly) Choice

So fire safety companies came up with a cooler and less deadly option. Halons are combinations of carbon, fluorine, chlorine, and bromine. There are a lot of ways to combine these atoms, so chemists came up with an easy system to specify which one they’re talking about. Halon 2402 has two carbon atoms, four fluorine atoms, zero chlorine atoms, and two bromine atoms.


Halons can stop a fire when the gas is only 5% of the atmosphere of a room and a fifteen percent halon concentration isn’t fatal to humans. How can it leave enough oxygen for a human, but not enough for a flame? It can’t. Halons don’t smother flames, or douse them, or cool them, or deprive them of fuel. Instead, they stop the chemical reaction inside the flames.

A fire burns fuel, usually carbon (C), in the presence of oxygen, (O2). The end products of a flame are water and carbon dioxide, H2O and CO2. That’’s the beginning and the end of the process, but we don’t usually concentrate on the middle. First of all, the high temperature of a flame causes the fuel to fall apart before it “burns.” Pieces of fuel fall not only off the main body of, say, a book or a banknote, they fall off their own molecule. The first product of heat is a vapor of free radicals, molecule fragments with unpaired electrons. These free radicals react with oxygen to produce carbon dioxide and keep the reaction going. That’s where the halons come in. Armed with extremely reactive atoms, they pair with the radicals before anything else can. They simply stop the reaction, and they do it without producing enough gas to suffocate people.


Unfortunately, they can still poison people, or destroy the planet that people are living on. Numerous halons break down into toxic chemicals, especially when exposed to high heat. Others decay into products that deplete the ozone layer. Both of these factors are causing halons to be phased out. The gases being phased in still work on the same principle. They don’t remove any of the elements necessary for the fire to continue. They stop the inner reaction of the flames.

[Via Chemistry In Fire Fighting, Addressing the Need to Replace Halon, Why Fire Is Cool]

Top Image: Andrés Nieto Porras, Second Image: Defense Imagery , Third Image: Library of Congress.