An air purification apparatus comprising: a flow channel extending between an inlet and an outlet; a pollutant removal structure and an air displacement apparatus in the flow channel; a branched sensor channel including: a first branch extending between an ambient air port and a further outlet in the flow channel between the inlet and the air displacement apparatus; and a second branch extending between the ambient air port and a further inlet in the flow channel between the air displacement apparatus and the outlet, the first branch and second branch sharing a branch section; at least one sensor in the shared branch section, the at least one sensor comprising a pollutant sensor; a processor communicatively coupled to the pollutant sensor, the processor being adapted to derive a pollutant concentration from sensor data provided by the pollutant sensor; a valve arrangement (161, 163) in the branched sensor channel adapted to exclusively disconnect the first branch from the flow channel in a first configuration and exclusively disconnect the second branch from the flow channel in a second configuration; and a controller adapted to control the valve arrangement, the controller being adapted to: - periodically switch the valve arrangement (161, 163) between the first configuration and the second configuration; and wherein the processor is adapted to: - derive a first pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the first configuration; - derive a second pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the second configuration; and - determine a pollutant removal efficiency of the pollutant removal structure from the derived first pollutant concentration and second pollutant concentration. a flow channel extending between an inlet and an outlet; a pollutant removal structure and an air displacement apparatus in the flow channel; a branched sensor channel including: a first branch extending between an ambient air port and a further outlet in the flow channel between the inlet and the air displacement apparatus; and a second branch extending between the ambient air port and a further inlet in the flow channel between the air displacement apparatus and the outlet, the first branch and second branch sharing a branch section; a first branch extending between an ambient air port and a further outlet in the flow channel between the inlet and the air displacement apparatus; and a second branch extending between the ambient air port and a further inlet in the flow channel between the air displacement apparatus and the outlet, the first branch and second branch sharing a branch section; at least one sensor in the shared branch section, the at least one sensor comprising a pollutant sensor; a processor communicatively coupled to the pollutant sensor, the processor being adapted to derive a pollutant concentration from sensor data provided by the pollutant sensor; a valve arrangement (161, 163) in the branched sensor channel adapted to exclusively disconnect the first branch from the flow channel in a first configuration and exclusively disconnect the second branch from the flow channel in a second configuration; and a controller adapted to control the valve arrangement, the controller being adapted to: - periodically switch the valve arrangement (161, 163) between the first configuration and the second configuration; and wherein the processor is adapted to: - derive a first pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the first configuration; - derive a second pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the second configuration; and - determine a pollutant removal efficiency of the pollutant removal structure from the derived first pollutant concentration and second pollutant concentration. - periodically switch the valve arrangement (161, 163) between the first configuration and the second configuration; and - derive a first pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the first configuration; - derive a second pollutant concentration from sensor data provided by the pollutant sensor with the valve arrangement in the second configuration; and - determine a pollutant removal efficiency of the pollutant removal structure from the derived first pollutant concentration and second pollutant concentration.