Any dead space in a breathing pathway can and will cause a certain degree of CO2 build up. You are rebreathing more of the gas you just exhaled from your lungs. There is always a small amount from your own airway. Put a mask on it is not even debatable you will rebreath more of the co2 you exhaled than with out. Long term exposure can have a negative impact on your blood chemistry, actually decreases the O2 getting to the brain. How much depends on the individual, the mask, their activities and the persons health. Ironic there has been a push in recent years to improve the air quality of work spaces by circulating more air to lower the levels of CO2 and now levels that are high enough to cause headaches and anxiety are of no concern. Agree with mandatory mask or not it’s pretty disingenuous to pretend the they have no affect on CO2 levels in the individuals using them. How come we allow TB carriers to mingle unmolested? People would be surprised how many carry that germ silently for years passing it on. Personally I would rather not get TB but that’s just me I guess. Just under 600000 have died worldwide from covid19. In 2017 1.6 million died from TB worldwide. TB kills an average of 4400 people everyday, year after year after year. What’s the point? Perspective, COVID is real it claims lives but it is being used to achieve political ends. They are doing a good job of it too.