I want to say that this warning is just a liability thing and it’s really probably fine, I probably am getting the right amount of oxygen even if it is a little uncomfortable — but my workplace just gave me some new tasks that involve cutting and lifting decently heavy objects, so I want to make sure that it isn’t like genuinely dangerous to wear a mask while I’m doing these things.
fun fact, here at least firefighters need to pass a fitness test that includes a treadmill run with full gear and a mask. to train for it they run while wearing two N-95s
so it does add some restriction but probably not enough to worry about