Be aware of pulling in your furnace exhaust fumes, or gas hot water tanks exhaust fumes.
If your venting outside your good, but if your venting back into the room just to scrub smells say,
You could be putting carbon monoxide (not co2) back into your room.
I don't believe this is correct. If you create negative pressure near the inlet of a gas appliance and it stops drafting in the correct direction, or venting enough, the combustion becomes inefficient and it produces carbon monoxide fvrom the unspent oxygen which is then drawn back into the building.
If you take air in from that room and vent it back into the same room, the pressure differential is the same but your heat and small would get drawn into the cold air return or cracks around it.
If you take air from that room and vent it outside that room, depending on how well it is sealed, the room will be under negative pressure which could pull air and exhaust back in through the furnace causing carbon monoxide if the flu is the easiest path to pull air back into the room.
If you vent completely outside your house, you will put your entire house under negative pressure and depending on how air tight your home is, you will pull air in through door cracks, window cracks, exhaust vents, and potentially again gas appliance exhausts again causing carbon monixide, and environmental pollutants like pollen, dust, etc.
This is why balancing air flow in and out of houses such that it is generally positive rather than negative is important. It may never happen, and you might be venting so much air outside your home it takes any CO out with it. I can't really say or know this would happen, just that this is my understanding.
I'm happy to be corrected if I have this wrong, I never really questioned my HVAC/Gas buddy when he explained it to me a long time ago.
upon rereading
@Cookie Rider 's post, I think we are saying the same thing in different ways. Cookie is saying if you are causing negative pressure but venting outsdie, the Carbon Monoxide should be vented out too. All I was really saying I guess was that it is the negative pressure that is causing the Carbon Monoxide in the first place. Either way, just thought I'd add my noise to the post.