high efficiency particulate air filter.
They are what we've used in the pharma and biotech industries for years for clean room air handling, till some bright spark decided to put them in a domestic vacuum cleaner. Yes, James Dyson, you again. Now everyone does it.

They are completely pointless in that role, save for almost certainly upping the rate of childhood asthma, and they cost a lot to replace, and block quite quickly. and have very small pores so restrict flow. Take them out, put a normal filter in, bosh, better cleaning.
( the mighty badger Aphrodite, on a bar stool, by your side, Mon 29 Jun 2009, 13:53, archived)