This book reveals the category and source of air contaminants in the air, compare the airworthiness regulation between other standards on cabin air quality, demonstrate the possible procedures for HEPA filter selection for aircraft in the initial design stage, summarize and analyze the major air purification methods and research of future air filter modification strategies to control the cabin contaminants. Retrospective studies on contaminates in aircraft cabin air emerge that the cabin might not be safe as manufacturers claim, also compulsory regulations on aircraft cabin air should be issued by airworthiness certification authority. Aircraft environmental control system on air quality purification showed that HEPA filter installed in recirculation air system was the dominant way to purify cabin recirculation air among current aircraft. Further research on the mechanism of HEPA filter gives a better understanding of air filtration.