There are certification and airworthiness requirements related to the provision of clean crew and passenger compartment breathing air utilizing the aircraft bleed air system. There have been continuing reports and studies over the years regarding oil fumes in aircraft, including impaired crew performance. Oil fumes are viewed in varying ways ranging from rare seal bearing failures, to low-level leakage in normal flight. MSc research was undertaken to assess whether there is any gap between the certification requirements for the provision of clean air in crew and passenger compartments and the theoretical and practical implementation of the requirements using the bleed air system. A comprehensive literature search reviewed applicable certification standards and the documented and theoretical understanding of oil leakage. 2 types of Interviews were undertaken to address the research questions. Key aviation regulators were questioned about the process by which they certify and ensure compliance with the clean air requirements. Aerospace engineers and sealing professionals were interviewed about their understanding of how oil may leak past compressor oil bearing seals, and into the air supply under various flight conditions. The outcome of the research showed that there is a gap between the clean air certification requirements and the theoretical and practical implementation of the requirements using the bleed air system. Low-level oil leakage in normal flight operations is a function of the design of the pressurized oil and bleed air systems. The use of the bleed air system to supply the regulatory required air quality standards is not being met or being enforced as required.