A "non-ignitable" atmosphere is usually defined as containing less than what percentage of oxygen?

A "non-ignitable" atmosphere is generally defined as one where there is insufficient oxygen to support combustion. In many industrial and safety contexts, this threshold is typically recognized as being less than 5% oxygen. At this level, the atmosphere lacks the necessary conditions for a fire to ignite and sustain itself, as combustion requires a minimum concentration of oxygen to occur.

While oxygen concentrations below this level significantly reduce the likelihood of ignition, higher percentages can still present risks depending on the context and the materials present. For instance, at 10% oxygen, while still lower than typical atmospheric levels (around 21%), there is enough oxygen present to potentially support combustion under the right conditions. Therefore, recognizing the importance of keeping oxygen levels below 5% is crucial for ensuring safety in various industrial settings where flammable materials may be present.