I thought something was up when I couldn't keep up my stamina despite trying really hard to get fit so I got a sats monitor and tested it myself, to find they always go below 90 when exercising. My FEV1 is 66, resting sats is 91-97 (it varies) and exercising made it drop to 84-89, I even once had a reading of 78 but don't know if that is actually possible! I did feel like I was going to pass out, tho, my limbs went numb and stuff. My doc didn't want to hear about supplemental oxygen for exercise, he told me to cut down the intensity and thought a course of antibiotics would fix it, I doubted this but took Cipro...and it made no difference. BUT recently I also did a real thorough IV visit with all the trimmings and that actually made a remarkable improvement, and for quite a while they stayed above 90 when exercising. So my guess is being run down/tired, having underlying infection (even if it is your 'normal') and old clogged mucus decreases sats. Don't just accept it, try and find the source of the problem. Does anyone know what "hyperinflation" means? I've read somewhere CFs battle to exercise due to this...correct me but I think it is when you have an air cushion of 'old air' in your lungs?