Above FL390 your lungs won't be able to suck the air,so the 100% won't help so much.You'll need oxygen under pressure,otherwise the oxygen from the mask will be of no use to you.In case of rapid depressurization,the TUC will be around 10 sec,so keep this in mind when flying above FL 390,and also to set your mask to 'emergency',in case of depressurization..
