Sorry guys, you're right. Apparently my google source for that AC-air-comes-from-outside was wrong. XD
Tomorrow morning I've got a mold inspector coming to test the quality of both the inside & outside of my condo. When their lab gets results, they'll tell me what to do (if anything). All I know is, last night was the first night I spent an extended amount of time in my basement. I emerged from it hours later with a headache & kind of brain-fogged. It was weird. And while I was down there I realized I was having a bit harder of a time breathing than usual. And today I had unusual amounts of mucus.
That is what triggered my posting this thread, but I wrote it going on the outdoor pollution track instead of the basement mold one.
So if the test comes back with "Everything is fine," (given that they only test for mold) I'm going to assume it has to do with the air outside & buy some air purifiers for each floor. I'd kept the windows open for a long time after moving in, so... who knows.