If we calculate the ventilation needed in a small room, we can start with the need for one human being. The normal calculated value is 10 litres/s. From that, it is possible to estimate the number of occupants multiplied with the minimum rate required in your country (e.g. 7 litres/s).
But how do you calculate the need for a courtroom, a store, or a movie theatre? If you calculate for the maximum amount of occupants, energy will be wasted when the room is empty.
If we apply a sensor that measures CO2, we can regulate the amount of air needed. With a pitot tube (the same tube that is used on aerplanes to measure speed by airflow), we can measure the amount of air provided.
A fan can produce a pressure in the air channels (i.e. 10 Pa) and the sensor regulates a damper.
It is not the sensor that is the brain of the system: our OEM customers apply logarithms and other means of control to make a system reliable and suitable for your application.
You might interject that air quality is not only about CO2 (the odourless gas that becomes dangerous at levels around 5,000ppm and above), and you are correct, but research* has shown that an adequate airflow measured with CO2 also reduces the risk of other problems.
* "Association of Ventilation Rates and CO2 Concentrations with Health and Other Responses in Commercial and Institutional Buildings", by Seppanen, O.A., Fisk, W.J., & Mendell, M.J. (1999).