For ventilation purposes, clean air is defined as air that is free of hazardous concentrations of flammable gases and vapors.
Source: API RP 14F, Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class 1, Division 1 and Division 2 Locations, Fifth Edition, July 2008. Global Standards
Clean Air (gas properties)
Air that is free of flammable gases and interfering or contaminating substances.
Source: IEC 60079-29-1, Explosive atmospheres – Part 29-1: Gas detectors – Performance requirements of detectors for flammable gases. Global Standards