According to the Centers for Disease Control and Prevention (CDC), wearing a mask does not raise the carbon dioxide (CO2) level in the air you breathe. Because a face mask is not airtight, carbon dioxide completely escapes into the air through and around the sides of the cloth mask when you breathe out or talk. Carbon dioxide is small enough to easily pass through any cloth mask material. In contrast, SARS-CoV-2 which is the virus that causes the COVID-19 disease is much larger than CO2, so it cannot pass through a properly designed and properly worn cloth mask. 

See also:

• Are masks really effective in preventing the spread of COVID-19?
• What is the best type of face mask to use?

Source: CDC