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
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
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.