It can be mounted inside or outside the room...it just depends on your situation.
Not sure what co2 thing yer talking about...but IMO, unless it's a *well built* home made carbon/charcoal filter or a charcoal filter you purchased at a grow store you are fooling yourself to think that a "masking" agent (pinesol, ONA, fartin in the bucket...etc) will eliimnate the odor of marijuana from your exhaust.
Whether a human can smell it or not matters little.
But I guarantee the drug dogs can smell it...and that is all that need to be said.
good luck
bt dt