No, Basic Physics says that CO2 will pass totally unhindered through even the finest-mesh masks. Any impairment you experience is due solely to the merely mechanical effect, which causes a slight pressure differential - i.e., it's harder to suck air through a filter.
A mask is, of course, an article of clothing, and thus provides a little warmth (i.e., provides insulation).
Regards,
Please see my reply to oincobx (post 118, above) to understand how and why this is true.
I even include a simple way to test whether the effect of this is serious for you or not. The severity is different for different people in different environments (such as altitude, health etc.)
For me, the effect is quite severe.