[nsporillo]

The box fan and high-merv filter hack is a good idea, but proper ventilation of filtered outdoor air is the best solution. Not always practical, but historically buildings breath via air infiltration due to pressure differentials, stack effects, wind, etc - might as well bring in the air on your own terms rather than it leak through your walls and insulation.

[macNchz]

On my leafy Brooklyn street the particulate pollution outside is well within the recommended levels nearly all the time, so thankfully there's not really a pressing need to retrofit modern ventilation into a 120 year old house. That is, assuming the presence of wildfire smoke from 3000 miles away remains a rare event, which...who knows.

[Tenoke]

I'm not actually sure it's always the best. My rooms face the street and I can't tell if the air I get on that side is actually better than just having air from the other side +purifier.

[nsporillo]

Quality Energy/Heat Recovery Ventilators (which are basically mandated by code [in some jurisdictions] in modern construction due to air tightness requirements) should allow you to select the filter with the MERV value that works best for your situation.

But to be fair, you may have a good point. Vehicle emissions and tire wear generates particulates consider to be ultrafine (0.1 max nanometer), which is even difficult for HEPA filters to deal with.