So, wearing masks forces us to breathe our own CO2.
What are the effects?
"Decrease of oxygen supply to the brain even though there is adequate blood flow caused by carbon monoxide poisoning, pollution in our cities, choking or suffocation can create conditions like tiredness, depression, irritability, poor judgment and health problems."
(random search for effects of oxygen deprivation https://www.oxygen-review.com/human-body.html )