A method of maintaining a ventilation treatment device positioned downstream of an exhaust hood, comprising: detecting an intensity of at least one ultraviolet light source continually by a sensor ; at successive points in time during operation thereof in the ventilation treatment device , polling the sensor with a controller to apply signals representing the intensity to a controller ; storing data responsive to the detected intensities in a data store ; characterised in that the method further comprises: the controller, at a first time, on a user output terminal , generating an indication that the at least one ultraviolet light source requires cleaning when the detected intensity is below a second threshold; the controller, at a second time subsequent the first time, determining whether a first command has been input by a user through the user output terminal as a first maintenance indication indicating the completion of a cleaning of the at least one ultraviolet light source ; the controller , at a third time subsequent the second time, on the user output terminal , generating an indication that the at least one ultraviolet light source requires replacement if the intensity of the at least one ultraviolet light source is not above a first threshold after the cleaning, the first threshold corresponding to a lower intensity than the second threshold; the controller , at a fourth time subsequent the third time, determining whether a second command has been input by the user through the user output terminal as a second maintenance indication indicating the completion of a replacement of the at least one ultraviolet light source has been received; and the controller preventing operation of the exhaust hood if no maintenance indication is received via an input by the user through the user output terminal within a predefined period of time or a predefined number of startups of the system. detecting an intensity of at least one ultraviolet light source continually by a sensor ; at successive points in time during operation thereof in the ventilation treatment device , polling the sensor with a controller to apply signals representing the intensity to a controller ; storing data responsive to the detected intensities in a data store ; the controller, at a first time, on a user output terminal , generating an indication that the at least one ultraviolet light source requires cleaning when the detected intensity is below a second threshold; the controller, at a second time subsequent the first time, determining whether a first command has been input by a user through the user output terminal as a first maintenance indication indicating the completion of a cleaning of the at least one ultraviolet light source ; the controller , at a third time subsequent the second time, on the user output terminal , generating an indication that the at least one ultraviolet light source requires replacement if the intensity of the at least one ultraviolet light source is not above a first threshold after the cleaning, the first threshold corresponding to a lower intensity than the second threshold; the controller , at a fourth time subsequent the third time, determining whether a second command has been input by the user through the user output terminal as a second maintenance indication indicating the completion of a replacement of the at least one ultraviolet light source has been received; and the controller preventing operation of the exhaust hood if no maintenance indication is received via an input by the user through the user output terminal within a predefined period of time or a predefined number of startups of the system. <EOS>