Claim: Wearing a mask will increase the amount of carbon dioxide I breathe and will make me sick.

Alabama state health officer Dr. Karen Landers: ““That is absolutely a myth,” said Dr. Karen Landers. “While it may be a bit hot, physicians, nurses, and other healthcare providers have worked in high-level PPE and have not had this problem.” (1)

Carbon dioxide easily escapes cloth masks.

Myth: Masks can cause carbon dioxide (CO²) build-up.

Fact: Some people have suggested that carbon dioxide from exhaling gets trapped under the cloth and can make you sick. This isn’t true. Properly fitted masks offer adequate airflow while still covering your nose and mouth. This makes the accumulation of carbon dioxide impossible. (2)

(1) https://www.wbrc.com/2020/07/15/mask-facts-vs-myths-do-masks-cause-you-breathe-carbon-dioxide/
(2) https://www.umms.org/coronavirus/what-to-know/symptoms-prevention/masks/wearing-mask