The present invention relates to a device, system and method for CO2 monitoring. To enable continuous monitoring at low cost and in a simple manner, the device comprises a signal input (10) for obtaining one or more monitoring signals (20) of a monitored area, a breathing monitor (11) for determining one or more breathing parameters (21) of one or more subjects present in the monitored area from the obtained one or more monitoring signals, and a CO2 estimation unit (12) for estimating the CO2 level (22) in the monitored area based on the determined one or more breathing parameters.