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At what percent oxygen does air become spontaneously combustible (25%? 30 %?) Or does it ever without an ignition source?
Answer 1:

Oxygen alone won't combust without a spark. Think about it -- ambulances and hospitals often carry around canisters of 100% oxygen, and they are usually just fine. But they do have to be careful about keeping sparks away -- the "no smoking" signs in hospitals aren't just for preventing lung cancer.Like many highly exothermic reactions, the combustion of oxygen has an activation energy -- there needs to be an initial bit of energy introduced to the system to get the reaction going.Happy breathing,

Answer 2:

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 otherwise.

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 higher.

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 combust.

Thanks for the excellent question!


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