For safety purposes at work, anything over 23.5% O2 is considered an elevated O2 atmosphere. It has been awhile, but I was thinking that when you get over 30% or so, all sorts of stuff that normally isn't that flammable becomes very flammable. Our company deals in pure industrial oxygen at say 600 psig. I have seen the results of a fire in an oxygen compressor. It isn't pretty. Some of the metal just burns, a lot of it melts and sprays all over the place once a hole is burned out. We put safety barriers around all those compressors for good reason. No one is allowed in while they are running.