An air treatment system (100, 100') for managing a condition of air in an enclosed environment, said system comprising a supply (10, 10') of a gas comprising oxygen; a discharge for a flow of indoor air from said enclosed environment; a flow path adapted to circulate a flow of air through said enclosed environment, said flow of air comprising at least one of a first portion comprising said gas comprising oxygen and a second portion comprising indoor air from said enclosed environment, said flow path further comprising means (41, 42) for selectively providing said first portion comprising said gas comprising oxygen and said second portion comprising indoor air from said enclosed environment; an air processing assembly adapted to receive said flow of air, comprising at least a first air processing element adapted to remove a first pollutant from said flow of air; a second air processing element adapted to remove a second pollutant from said flow of air; and means (51a-55a) for selectively coupling said first and second processing elements to said first flow path, comprising a first valve element adapted to direct said flow of air through said first air processing element; and a second valve element adapted to direct said flow of air through said second air processing element, wherein said first and second valve elements, respectively, may be arranged in at least a first position wherein said flow is directed through the respective air processing element and a second position wherein said flow of air is allowed to bypass the respective air processing element, a first sensor adapted to sense a first parameter indicative of a condition of said indoor air, a second sensor adapted to sense a second parameter indicative of a condition of said gas comprising oxygen; and a control system adapted to control said air treatment system; wherein said control system is adapted to receive a value of a parameter indicative of a desired condition for said airflow; receive said first and second actual parameters from said first and second sensor; and based on the received parameters, determine said first portion of gas comprising oxygen and said second portion of indoor air from said enclosed environment by means of said means for selectively providing said first portion comprising said gas comprising oxygen and said second portion comprising indoor air from said enclosed environment, and control said means for selectively coupling said first and second air processing elements to said flow path. means (41, 42) for selectively providing said first portion comprising said gas comprising oxygen and said second portion comprising indoor air from said enclosed environment; an air processing assembly adapted to receive said flow of air, comprising at least a first air processing element adapted to remove a first pollutant from said flow of air; a second air processing element adapted to remove a second pollutant from said flow of air; and means (51a-55a) for selectively coupling said first and second processing elements to said first flow path, comprising a first valve element adapted to direct said flow of air through said first air processing element; and a second valve element adapted to direct said flow of air through said second air processing element, wherein said first and second valve elements, respectively, may be arranged in at least a first position wherein said flow is directed through the respective air processing element and a second position wherein said flow of air is allowed to bypass the respective air processing element, a first sensor adapted to sense a first parameter indicative of a condition of said indoor air, a second sensor adapted to sense a second parameter indicative of a condition of said gas comprising oxygen; and a control system adapted to control said air treatment system; wherein said control system is adapted to receive a value of a parameter indicative of a desired condition for said airflow; receive said first and second actual parameters from said first and second sensor; and based on the received parameters, determine said first portion of gas comprising oxygen and said second portion of indoor air from said enclosed environment by means of said means for selectively providing said first portion comprising said gas comprising oxygen and said second portion comprising indoor air from said enclosed environment, and control said means for selectively coupling said first and second air processing elements to said flow path. a first air processing element adapted to remove a first pollutant from said flow of air; a second air processing element adapted to remove a second pollutant from said flow of air; and means (51a-55a) for selectively coupling said first and second processing elements to said first flow path, comprising a first valve element adapted to direct said flow of air through said first air processing element; and a second valve element adapted to direct said flow of air through said second air processing element,