GeologyRocks

I think it had smth to do with the giant insects. In todays atmosphere isnt enough oxygen for these insect they could not survive. I think someone once calculated how much O2 would be needed for these insect to be able to live.

So 35% would be an absolute minimum to sustain these insects? The actual O² level, hypothetically speaking, could have been higher?

I don't know if these numbers are minimum, average or maximum. I would have to do some serious digging in some articles and the libary to find out. I see no reason why O2 couldnt be even higher, however, at some certain percentage the flora would incinerate with some one spark or one volcano eruption.

A quick google scholar search seems to indicate that the amount of oxygen is also determined by the amount of charcoal found in carboniferous sediments. Lot's of charcoal = high oxygen. This has been backed up (in one case I saw) by some experiments and numerical modelling. Not 100% sure where 35% comes from though or whether it's minimum, maximum or average as I couldn't view the papers (even though I was at Uni at the time *tsk*).

I know that it isn't currently in vogue, but the information relating to oxygen percentage came from Wiki: http://en.wikipedia.org/wiki/Oxygen

 I know this is probably a physiological question rather than geological, but does anybody know the sub-lethal effects of oxygen (i.e. 40-50% at atmospheric pressure) on the human body?

From own experience breathing pure or nearly pure oxygen will result very quickly in dry mouth and increaed thirst, the skin of the lips and inside the mouth because brittle like when spending a really long, icy day outside.

It essentially is a toxin that will damage cell membranes and oxydize any substance it can.

Another note...assuming the air would be like 50% oxigen I doubt we would be living in a world like we know it. Any kind of fire would be a desaster of national scale.