They indicate when the air in a room is “used up”, i.e. it is no longer fresh. The devices remind you several times a day to open the windows. What ensured a good indoor climate in everyday life before Corona can now be decisive for health: Because frequent ventilation is one of the important measures to protect against the transmission of a corona infection within rooms.
CO2 detectors do not detect viruses in the air. They measure levels of carbon dioxide (CO2), a colorless and odorless gas that we breathe out. And with the air we breathe, aerosols from our lungs get into the room, which can transport viruses. The CO2 detectors are therefore an indicator of when the windows and doors should be opened at the latest, especially on cold winter days when frequent airing becomes uncomfortable.
In normal times, with a view purely to the good living climate, this is gradually the case with a value of over 1000 ppm (parts per million), at the latest at 1400 ppm ventilation should take place. During the pandemic, however, it is better to ensure air exchange from 800 ppm, according to the Stiftung Warentest with reference to the expert Konstantinos Stergiaropoulos, professor of heating and ventilation technology at the University of Stuttgart.
This is a shortcoming: The threshold values are not adjustable on all devices. This means that the well-rated devices only switch to the warning color yellow from the 1000 ppm range and even later to red. However, the majority of the tested devices also show the measured values on the display, which you therefore have to consciously pay attention to.
However, the testers were not always satisfied with the displays and warning colors: Not all CO2 traffic lights and measuring devices show red at a CO2 concentration of 1400 ppm in the room, but only react later, according to the magazine “test”.