When you exhale, you exhale around 40.000ppm of CO2 (ppm = parts per million) and other gases. Also you might exhale some water drops small ones and very small ones. The smallest are called aerosols and if a person is infected with a virus like corona, the virus will also be in that aerosol. Now, if we would know how much contaminated air is in the room, we would be able to take counter measures and start ventilation.
But hey, we are lucky, since we can measure the amount CO2 in a room, which for sure comes form exhaling, we can say that at a certain point, the amount of CO2 (and so the viruses) are exceeding a point where it is not healthy anymore, a point where the risk of infecting others is much higher.
Studies show that this is when the CO2 level of a room exceeds 1000ppm. This is the point where you have to start ventilation or open the windows so to say. Below 1000ppm all is fine and the risk of spreading a virus is very low. If you reach or even pass 2000ppm you have to ventilate, no excuse.
SOC2 is capable of measuring 0 – 5000ppm which is far beyond what we really need. However, Sensors tend to be inaccurate when they come to their max or min values. So the sensor I used will be fine. There is enough headroom.
Fresh air outside btw. comes with around 400ppm so this is what we would call normal. If you have values between 400 and 1000ppm of CO2 you are fine.
The German Federal Environmental Agency issued a report on the influence of CO2 in rooms and what it means from a health perspective. The report is quite comprehensive but the part which is of interest is about the amount of CO2 in conjunction with the corona virus.
| CO2 ppm (Parts per million) in a room | Recommendation | SCO2 |
| 0 ppm – 400 ppm | normal (nature) | green |
| >400ppm < 1000 ppm | harmless | green |
| >1000ppm < 2000ppm | start ventilation | yellow |
| > 2000ppm | ventilation required | red |
SCO2 was specifically developed for schools to be build in the class. The parts for this very accurate sensor are around 50€. The case for the CO2 traffic light can be done basically with anything. I had some spare laminate laying around so I decided to use that.