CO2 sensors are useful for monitoring the level of carbon dioxide in the air of an enclosed room (classroom, canteen, teacher’s room, etc.). Given in ppm, the value indicated alerts the occupants to the need to open the windows to renew the air. Indeed, it is an excellent indicator of the level of containment in a room. The French High Council for Public Health (HCSP) recommends airing out as soon as this value reaches 800ppm. A CO2 monitor can also, by extrapolation, indicate an increase in the concentration of other pollutants in the air, such as suspended particles.