Since I mentioned fans first with a very oversimplified comment I’ll expand a little on where I was going.
Particles and gasses behave differently so while some methods will reduce both, others will be more targeted. So it is very possible to have little CO2 and lots of particles…
A simple way of thinking about it is that removal of particles from air requires either moving them from the room (air changes) or removing them from the air (settling on a surface or adsorbtion onto filter media). Particles tend to like to float around in a small area in stale air, but moving air can force them to enter your air filter or leave the room (for example an exhaust fan).
Anyone ever notice you’re supposed to put your free standing HEPA filter in the middle of the room? This is so it actually has the opportunity to not just clean a pocket of air in the corner… Similarly HVAC systems will place the air vents and returns to encourage flow through the room. So proper use of ventilation fans can encourage particles to settle and move to the air filter. I am of course oversimplifying a bit.
Anyone who really wants to dig into details might also enjoy this presentation from the CDC.
