That is an excellent question, and the answer is complicated. Air will never spontaneously combust, nor can it be made to burn non-spontaneously. Air is mostly nitrogen, which is not flammable.
Nitrogen is also non-reactive in general, so it
doesn't support the combustion of other materials,
either. After nitrogen, the most abundant gas in
our air is oxygen. Here's where it gets
complicated: Oxygen is also not flammable, but it is a high-energy gas that very readily oxidizes
other materials. For something to burn, the
reaction requires a fuel (the thing that burns) and an oxidizer like oxygen. Without the fuel, though, no combustion will take place no matter how high the concentration of oxygen is. Since air itself is not flammable, it is not a fuel and will not combust, spontaneously or
The danger we often hear about
with high oxygen levels is that other materials
that are not combustible or only very slightly
combustible under normal conditions, and therefore
not a danger, can become very combustible and
hazardous when oxygen levels are high. Also, many
things will be hot or will smolder when deprived
of air (and thus oxygen), and will suddenly burst
into flame when exposed to the oxygen that's in
our air. Examples of this include oily rags in a
trash can that ignite when someone lifts off the
can's lid, or toast in a toaster oven that is
black and smoky and that bursts into flame when
someone opens the oven door. Since oxygen is
required for the burning we see, the sudden
combustion in these examples would be more
dangerous if the oxygen concentration were
One final thing to note is the
difference between combustion and spontaneous
combustion. All burning is combustion, but it's only spontaneous combustion if the burning results from a heat-producing reaction instead of from a spark or some other ignition. So the toast example is of combustion, but the oily rags example is spontaneous combustion.
If you have a material that's not combustible in normal conditions but is extremely combustible in high-oxygen conditions, that doesn't necessarily mean that it will automatically explode when exposed to the oxygen. It varies depending on what substance we're talking about, but it might still take a little
spark or some other trigger to get it to
Thanks for the excellent question!
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