An air treatment device (100; 300; 400) intended to be arranged at a ventilation air inlet of a room or cabin, at an air inlet or an air outlet of an air conditioning device, the air treatment device comprising: a casing (104; 304; 404) having an air inlet side (120; 302; 402) and an air outlet side (110; 308; 408), said casing comprising an air inlet (121; 306; 406) arranged on the air inlet side and an air outlet (131; 310; 410) arranged on the air outlet side of the casing, such that air from the ventilation air inlet of the room or cabin, or air to the air inlet of the air conditioning device, or air from the air outlet of the air conditioning device is lead through the casing; an air treatment section arranged in the casing, said air treatment section comprising: a fan for generating a flow of air from the air inlet to the air outlet; and filtering means arranged such that the flow of air passes through the filtering means; a first pressure sensor adapted to measure a first pressure in the flow or air into, or out from, the air treatment device; a second pressure sensor adapted to measure a second pressure in the room or cabin surrounding the air treatment device outside the flow or air into, or out from, the air treatment device; and a control unit; wherein the control unit is configured to adapt the speed of the fan, based on the detected first pressure and the detected second pressure, to generate a flow of air through the air treatment device corresponding to the flow of air from the ventilation air inlet to the room or cabin, or the flow of air to or from the air conditioning device and wherein said casing comprising a protruding portion (140; 320) extending from the air inlet side around the air inlet in the casing, said protruding portion is adapted to enclose the ventilation air inlet of the room, or the air outlet of the air conditioning device, to form an at least partly enclosed space such that air from the ventilation air inlet of the room, or air from the air outlet of the air conditioning device, enters the enclosed space before it continues into the air treatment device, said first pressure sensor is arranged to measure the pressure within the at least partly enclosed space. a casing (104; 304; 404) having an air inlet side (120; 302; 402) and an air outlet side (110; 308; 408), said casing comprising an air inlet (121; 306; 406) arranged on the air inlet side and an air outlet (131; 310; 410) arranged on the air outlet side of the casing, such that air from the ventilation air inlet of the room or cabin, or air to the air inlet of the air conditioning device, or air from the air outlet of the air conditioning device is lead through the casing; an air treatment section arranged in the casing, said air treatment section comprising: a fan for generating a flow of air from the air inlet to the air outlet; and filtering means arranged such that the flow of air passes through the filtering means; a first pressure sensor adapted to measure a first pressure in the flow or air into, or out from, the air treatment device; a second pressure sensor adapted to measure a second pressure in the room or cabin surrounding the air treatment device outside the flow or air into, or out from, the air treatment device; and a control unit; a fan for generating a flow of air from the air inlet to the air outlet; and filtering means arranged such that the flow of air passes through the filtering means; a first pressure sensor adapted to measure a first pressure in the flow or air into, or out from, the air treatment device; a second pressure sensor adapted to measure a second pressure in the room or cabin surrounding the air treatment device outside the flow or air into, or out from, the air treatment device; and a control unit; wherein the control unit is configured to adapt the speed of the fan, based on the detected first pressure and the detected second pressure, to generate a flow of air through the air treatment device corresponding to the flow of air from the ventilation air inlet to the room or cabin, or the flow of air to or from the air conditioning device and wherein said casing comprising a protruding portion (140; 320) extending from the air inlet side around the air inlet in the casing, said protruding portion is adapted to enclose the ventilation air inlet of the room, or the air outlet of the air conditioning device, to form an at least partly enclosed space such that air from the ventilation air inlet of the room, or air from the air outlet of the air conditioning device, enters the enclosed space before it continues into the air treatment device, said first pressure sensor is arranged to measure the pressure within the at least partly enclosed space.