"what's the point if loose fitting cloth masks" - already answered.
Masks are worn by my nursing, surgical and anaesthetic colleagues every day for up to 12 hours for over the past 100 years? No evidence of altered lung physiology.
You said - wearing a mask leads to dangerous low levels of brain oxygen.
I dare say you made it up because you made it up. Your experience is irrelevant. You haven't experienced dangerous low levels of oxygen from wearing a mask. Nor have the people you know.
It's ironic that the establishment public health position is called project fear by people trying to spread "dangerous mask" nonsense.