Appears to me there's no practical way to add air filters to the intakes on most Macs. Anyone ever try?
I've got some old high quality external HD cases that incorporate easy-to-clean filters. Always thought that a nice touch, though one did have to clean the filters once in a while (else they'd get clogged and cut off yer airflow.) Well, mine did, YMMV depending on whether your space is as dusty as is mine.