As one poster pointed out, the filter traps dust and lint. I have a dog
that sheds a LOT. Amazing how much dog hair goes in the filter.
I have used the disposable filters, but really like the washable kind.
I also spritz them with a soluable oil for air filters, which picks up
more stuff. I simply either hose them down outside, or in the shower.
I clean the filters once a month, though it a;ppears that once every
two months would suffice. If you can't buy a ready-made washable
filter, any good HVAC place can fabricate you one to the specific size.
Mine cost $10, which is quite cheap enough.
I have also see electrostatic filters (had one of those, but a PITA to
clean) that have a water system for cleaning. Mine didn't have that, so
can't speak to that issue.
The alternative is to buy the disposable ones. Get them in quantity and
get a better price. When I had that kind in a house, I changed them
about every 3 months. You could hear the fan labor when the filter got
clogged, and the chief domestic engineer complained about the house
getting dusty. Same with the electrstatic filter, which had a mesh
screen backup filter.
Your milage will vary, depending on how dusty your environment is, how
many pets you have, etc., etc.
I think people make much to big a thing about it. Guess what... they
are selling expensive air "purifiers," and filter systems... Just my 2