Information from CDC on a topic we get questions about:
Wearing a mask does not raise the carbon dioxide (CO2) level in the air you breathe
A cloth mask does not provide an airtight fit across the face. The CO2 completely escapes into the air through and around the sides of the cloth mask when you breathe out or talk. CO2 is small enough to easily pass through any cloth mask material. In contrast, the virus that causes COVID-19 is much larger than CO2, so it cannot pass as easily through a properly designed and properly worn cloth mask.
See CDC Considerations for Wearing Masks
Mask Use and Carbon Dioxide