Hi Athene, I've only kept the machine at 2 litres per minute. At that level, an hour didn't seem to be enough to keep me from being short of breath, so I went up to 2 hours. 3 hours may be too much. I don't know too much about the side effects, so I'm hoping not to over do it. I haven't been taking any anti oxidants but that's a good idea. Do you think Co-q10 would be helpful? Roma, swimming may be easier than other exercises because cold or warm water can have a good effect on orthostatic intolerance, by causing vaso-constriction (I think). I got mine on craigslist for $300, and luckily it only has a few hundred hours on it. A portable one would be ideal, but all of those were upwards of $1000. Bear in mind if you get a used one it may need filters or cleaning, especially if it was used by a smoker. Mine had very little signs of use. I'll try to update again soon. I'm hoping to be able to do a certain amount of exercise this week, above the limit that I was able to do before the oxygen. If I can do that level, I'll know it is helping with PEM, otherwise I'll have to back off and rest more again. Take care all!