> For instance, most commercial spaces have a MAX value on percentage of recycled air; a minimum amount of air fed into the return must be fresh air drawn from outside.

I live in a highrise in SF and this bit me during the wildfires this year. The air conditioning would pull in smoke from outside. I ended up turning it off altogether on the worst AQI days, and buying lots of replacement filters for both the furnace and some off-the-shelf air purifiers.