The present specification relates to a system for managing the condition of air in an enclosed environment comprising a flow path adapted to circulate a flow of air through the enclosed environment, the flow of air comprising at least one of a first portion comprising a gas comprising oxygen and a second portion comprising indoor air from the enclosed environment. The flow path selectively provides the portions. An air processing assembly is adapted to receive the flow of air. A first and a second air processing element is provided. A control system is adapted to determine the first portion of gas comprising oxygen and the second portion of indoor air from the enclosed environment and control the selectively coupling of the air processing elements to the flow path.