fluctuating oxygen saturation

my husband has severe emphysema and is very concerned about oxygen sats. his are between 94-96% while
sitting and drop to 90 or less when active. he uses oxygen while walking on the treadmill and sleeping. is there a valid reason why it fluctuates and is this normal?

[National Jewish Health]

The fall in oxygen saturation with exertion is common in people with severe emphysema.  That your husband experiences this fall in saturation supports the use of supplemental oxygen with treadmill and other exercise.  The supplemental oxygen enables one to maintain a normal saturation with exertion and the benefit of that is that it enables the person to achieve a higher level of physical conditioning.  He should continue to use it for both, exertion and sleep, as many people who desaturate with exertion also desaturate with sleep, despite a normal daytime saturation, at rest.

Good luck.