Agree with Symphony. The MD22 just has a very coarse steel mesh (fine enough to keep a wasp out) at the air intake by design and is absolutely fine with that. Air filters on engines are to prevent abrasive particles from entering the cylinders, where they could cause wear. Fairly easy to pick those up on a car driving at high speed over various dirty surfaces (sand, asphalt, small stones, perhaps even metal dust from somewhere). Not a lot of that in our engine room, which doesn't travel at high speed and the air has to find a fairly convoluted path from intakes under the sprayhood down some fibreglass channels and to the engine - at worst we might get a bit of household dust from clothing or pollen, neither of which are very abrasive.