Thank you, @merpreb That really is an interesting article. It's good to know that CO2 is not a problem.
For those of you who do not want to bother going to the link, here is what the article says:
"There is a myth that if u wear a mask, u breathe your own exhaled CO2. Firstly, the space between your face and the mask is 40-50mm. You breathe and exhale about 500ml of air with each breath. Frankly, the 8-10% air that u may RE-breathe with a mask without an exhale valve actually makes your lungs stronger - like a training mask, as seen in the movie Pink. Doctors actually recommend those masks that RE-circulate CO2 to high performing athletes for improved lung function."
The author is Richa Upadhyay, here are her credentials:
Studied at Makhanlal Chaturvedi National University of Journalism and Communication
Lives in Bengaluru, Karnataka, India
So frankly I am not sure what qualifies her to answer the question, but the answer does make sense. I would prefer to see something by someone who had better qualifications though.
JK