In a controlled environment where the issue has been scientifically studied, even people wearing an N95 mask while exercising experience no issues: http://www.rcjournal.com/contents/05.10/05.10.0569.pdf
The data is right there. You can choose to ignore it. You can choose to not believe it. But people have tried it, studied it, and demonstrated that even an N95 mask has no adverse impact on someone who doesn’t already have a respiratory problem.