There are many types of CO2 sensors on the market. The two most common types are NDIR and a method that works from a calculation.

There is a compact NDIR sensor installed in Airthings sensors. It measures the amount of CO2 gas molecules in the chamber, which samples air from the room it is placed in. A light shines on a sensor at the other end of the chamber. As the light passes through the chamber, only CO2 is absorbed. The sensor then measures how much light has passed through the chamber, excluding the CO2 molecules. This gives an extremely accurate result of exactly how much CO2 is present in the air. It actively counts the molecules every five minutes to ensure the most up to date statistics are provided.