Mammals exhale and a portion of that is C02, this is true. However, theirs also a lot of non c02-age in what we exhale; Exhaling many times into a bag and trying to seal it repeatedly after each breath is way more effort & time in the long run for a much much much less efficient (& totally impure) result. You would probably be better off breathing in your grow space for an hour a day but this is just as much a waste of time.