Most people believe oxygen is flammable, but it’s not that simple. Have you ever seen signs that say “No smoking, Oxygen Present” or a sign that says “Danger Oxygen Present, Extremely Flammable”? If oxygen is flammable, then why doesn’t the ambient air catch fire when you light a match?
Oxygen is not flammable, but it does intensify the combustion process and can cause fire to burn hotter and faster. It does not act as fuel for the fire to burn, it does however act as the oxidizing part of the fire triangle.
It might seem like I am just splitting hairs, but that’s not really true. Oxygen does not burn, let’s take a closer look at why, as well as the components necessary for fire.
Also read: What Makes Something Flammable?
How Oxygen Behaves in Fire
Any knowledgeable firefighter, scientist, or chemistry student must know the properties of many chemicals, and oxygen is one of them.
In fact, the language of oxygen being highly flammable is not at all true. That is not even a little bit true. (but that doesn’t mean oxygen is safe near a fire)
Oxygen supports combustion but is not what we would call flammable.
Oxygen cannot be ignited. Have you ever heard of an oxygen explosion while smoking a cigarette and simultaneously breathing oxygen from a supply tank?
The various warnings are not wrong but are very unlikely. In actuality, pure oxygen only makes fires burn hotter.
Folks who are addicted to tobacco, are the very folks who have to use oxygen to assist with their breathing. Their cigarette does not explode, nor ignite, and neither does the oxygen around the burning cigarette and supply tank.
Here is a crude test you can easily try:
- Obtain a cigarette.
- Take a mouthful of oxygen from your resuscitator supply line, but don’t inhale it.
- Now blow the raw Oxygen into the burning cigarette. If you observe the burning end of the cigarette, notice that the burning piece turns from a red/orange to a brilliant yellow.
You can see a similar experiment, here:
That is how oxygen supports combustion. It does not burn up the cigarette as you may imagine. It only makes the cigarette burn hotter.
Think of oxygen as a fertilizer for fire. So what is this about oxygen being flammable?
Elements of Fire
It is well known that in order for fire to occur, it takes three elements called the fire triangle.
- Oxygen (Actually any oxidizer, oxygen is just the most common)
Without those elements, fire cannot occur.
Firefighters know that in order to stifle (put out) a fire, you must remove one or more of the three elements.
- You can remove the heat by cooling the fire with water
- You can remove the fuel by digging a path between the fire and the unburned fuel
- You can remove the oxygen by smothering the fire with a layer of carbon dioxide (CO2), or by placing a lid over a burning pan of grease (Another common method is to place a fire-resistant blanket over the active fire)
If you were to smother the fire, hence removing the oxygen, it would eventually burn out. But if you remove the blanket too soon, the remaining heat would find more oxygen and reignite.
The presence of oxygen in an area that is sufficiently heated, will cause a fire to become supported by the oxygen, but it is not burning; otherwise, we would call it fuel.
To complicate matters slightly, firefighters and scientists add a fourth dimension to the fire triangle, called the fire tetrahedron. A tetrahedron is a four-sided triangle which in this case adds a fourth element to the fire triangle.
This addition includes the concept of “free radicals”, referred to as the chemical chain reaction. Free radicals are combinations of many flammable elements which also exacerbate the combustion process.
Among other techniques, they can be eliminated by a layer of dry chemical agents. The removal of free radicals can easily be accomplished in smaller fires.
Structure fires are a different challenge.
The production of smoke and its various elements call for large quantities of water, and effective application. Oxygen gases and other components of combustion make fire more difficult to extinguish in these large fires, but once the job is done, respiratory issues cease to be a factor.
It should be noted that, as long as a fire is still smoldering and is emitting products of combustion, you can still be breathing lots of poisonous gases, (free radicals) and breathing apparatus (SCBA) must still be worn.
What About Liquid Oxygen?
Liquid oxygen is simply regular oxygen that is compressed from a gas into a liquid. This high pressure causes that change in form.
It is abbreviated LOx in written form or sometimes LOX. (In this text, we’ll use the abbreviation LOx.)
Like all oxygen, LOx will accelerate combustion, but it is not a fuel and is not flammable. Because it is stored in such a concentrated form, LOx can cause a fire to violently increase in size and temperature.
While it is not technically flammable, it should be treated with extreme care and safety.
This concept of oxidizer, not fuel is shown well in this video:
Firefighters should know where liquid oxygen tanks are located within their jurisdiction.
LOx storage tanks are commonly found around hospital and healthcare properties. It is also a useful commodity in many other industrial areas that firefighters should be aware of in their district.
LOx tanks are pressurized in order to store and deliver the most available O2. The primary hazards of a LOx spill, are identified by what product it comes in contact with.
For example, LOx tanks are usually set into a concrete foundation, and perhaps even adjacent to an asphalt parking lot. A spill of LOx onto an asphalt parking lot could lead to a disaster. In a number of ways, it could be ignited very quickly and cause a large fire.
The good news is that the LOx is not initially a respiration problem, but the fire is. As we know, O2 is not flammable, but it makes fire burn more furiously. Obviously, we are not looking for new ways to lose control of a fire.
The secondary containment storage area is a welcome safety feature, but it can still spill over and come into contact with a variety of organic materials. Here is the shortlist of hazards to keep away from loose LOx:
- Oil, grease, tar
- Waste containers of trash
LOx cannot survive for long in a normal living atmosphere due to its rapid evaporation rate, but it can do a lot of damage in the process.
During a spill incident of LOx, cars can run through the area providing heat, sparks, oil, grease, and tires. Any number of occurrences can cause a fire in that case, and fire will subsequently seek to combine with a boost of available oxygen.
Also read: Is Argon Flammable? What You Should Know