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In regards to a material being flammable or combustible (i.e. if a higher than normal concentration of oxygen is present, something that is considered a combustible could now be considered flammable). What is the assumed normal oxygen concentration (expressed as a percentage) in the atmosphere?
Question Date: 2017-09-25
Answer 1:

Currently it is 21% or partial pressure of 0.21 bars of O2 .

In the distant geologic past it was quite low… but it has been this value for millions of years now.

Answer 2:

Earth's atmosphere is roughly 20% oxygen. The question of whether a substance is flammable is generally assumed to be in the context of the Earth's real atmosphere.

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