I live in an apartment over a busy road, and my building doesn’t have forced-air ventilation to the units, so I need to pull in fresh air somehow. Just leaving the windows open results in black soot accumulating in the carpets (and is probably why both my SO and my cat developed asthma while living here.) Regular air purifiers (we have two) only do so much.

What ended up making a difference for us, is buying a portable industrial air scrubber, of the kind construction/remediation sites use to clean the smoke and dust (and mold, and asbestos, etc) they’re generating out of the air before it gets vented outside (see e.g. https://a.co/d/01lTtyuZ); and plumbing this machine into our apartment in reverse: putting it out on our (covered) balcony, with it exhausting the “scrubbed” air into the apartment. (I made a little sliding-door shim of the type people use to pipe portable AC units outside, just with the hose on the other side.)

Generally works great. We run it on a very low speed, so it’s surprisingly quiet (no louder than other fans we have for air circulation, and a more pleasant frequency because of the large diameter of the fan it has inside.) The pre-filters get absolutely caked with exhaust and road dust after ~2wks, and start to let through an odd stench at that point, so we’re actually replacing those faster than even is recommended for the industrial use-case. But the actual main HEPA filter inside the unit seems to last longer than in industrial usage.