This is also something i wondered why porsche did not have on the 997's, interior recirculation filter. Bmws, mercedes, toyotas, the whole lot of cars had it since as far as 1996 if i recall right. Purpose of course is easier maintenace and it protects evap from getting dirty from recirculated air. My 09 mitsubishi did not have the filter but there was a retrofit item available if owner wants to install one. Anyone knows if its possible for our 997.2 ac's. The 991's already have it.