The recirc fans (cabin fans ) run independently of the packs, i.e., they run all the time the aircraft is powered unless turned off by us Engineers usually or crew abnormal ops. If you go and stand next to an outflow valve, which is normally the exhaust point for the old recirc air, you will feel a positive airflow being exhausted even with the packs off. That is the case on the aircraft I work on anyway.

I've changed the recirc filters numerous times. They are pretty dusty and we would normally wear a mask and paper all-over suit......certainly would at present if I had to change one.