Airthings sensors use a metal-oxide based sensor with a sensitive layer that reacts to chemicals by adsorption. This type of sensor will react to most volatile organic compounds, with some exceptions (like CO2), but is not able to differentiate between them.

Its sensitivity depends on the compound, which means it will give a larger signal for some chemicals than others, even though the concentration is the same. It is a relative sensor that adapts to its environment over time.

Like our nose, the sensor will eventually get used to the environment it is in. The sensor measures the amount of VOCs relative to the cleanest air it has seen the last week or so. This means that it must see clean, fresh air at regular intervals to be able to detect VOCs properly.