If re-circulated air is filtered, why isn bleed air off the engine filtered before it comes into the passenger cabin?
The ambient air outside the airplane at altitude cruise levels is very clean, cold (below -35 F/-37 C) and low in partial pressure of oxygen, too low to sustain life. Consequently, the air must be compressed to a density that is healthy for passengers and crew. Airplanes with a traditional bleed air system “bleed” or divert air from the airplanes’ engine compressors to accomplish the task of warming and pressurizing the air. The air taken from the engine compressors is upstream of the combustion chamber where fuel is added. The bleed air is essentially dry, sterile and dust free. It is cooled in air conditioning packs and is then mixed with approximately 50 percent filtered recirculated air. The mixed air is then supplied to the airplane cabin at the proper temperature. See the technical article on Commercial Airline Environmental Control System for more specific information.
