https://www.usatoday.com/story/news/factcheck/2020/05/30/fact-check-wearing-face-mask-not-cause-hypoxia-hypercapnia/5260106002/
"It is common for surgeons and other scientists or health care workers to wear face masks, particularly N95 respirators, for prolonged periods of time.
Neither the CDC nor the World Health Organization has issued warnings suggesting the use of surgical face masks would result in dangerous oxygen level depletion within the general public.
The CDC has requested the general public reserve N95 respirator masks for health care workers, but members of the public and other industries do still have access to them."
"There is no evidence to support that the general public — which doesn't typically wear masks for prolonged periods of time — will experience significant reductions in oxygen intake level, resulting in hypoxemia. While CO2 can build up in face masks, it is unlikely that wearing a mask will cause hypercapnia, according to the CDC
It is true that those who are most at risk of negative effects from face masks have been advised by the CDC to avoid the face coverings and reach out to their health care providers for additional guidance."