Heightened levels of CO2 are negligible in a mask. You aren't supposed to wear a mask 24/7. You wear a mask a strategic times like when you are inside, shopping or in a crowd. If you are outside away from people or crowds you don't need to wear a mask. Besides it's obvious that wearing a mask protects others. All you need to do is watch a video of a sneeze with and without a mask on to figure that out.