I'm a 70 year old man with copd. I have severe copd which I understand is one step before end stage copd. My oxygen levels don't fall below 93 and with breathing exercise I can raise it to 96. I've considered purchasing a portable oxygen machine so I have it if I need it. Plus they are not cheap and they seem to keep creeping up in price. My question is a two part, at what level did you need to supplement your oxygen and which portable machine seems to be the easiest to take with you . Thanks in advance. 