Besides installing a cabin filter, cleaning out the intake helps big time. First vacuum any crud in there. Next make up a solution of bleach and water, soak a rag in it clean the entire area and swipe down as far as you can go, let dry repeat. Let it totally dry out and spray with “Fridgee Fresh” or the equivilent, to rid the area of mold and mildew.
Then replace or install a cabin filter.