CO2 is a colorless gas with no taste or smell, heavier than air, and in low concentrations, it is a natural component of the surrounding air. Through the process of breathing, part of the oxygen contained in the inhaled air is converted into carbon dioxide. Other typical sources generally include combustion, for example, a gas stove is a significant source of CO2 in households.
The amount of CO2 in the air is expressed in units of “ppm”.
The unit “ppm” comes from the English “Parts Per Million”. 1 ppm is thus one part in a million = 1 millionth. For example, a concentration of 1,000 ppm CO2 in the air means that in every million air molecules, there are 1,000 molecules of CO2 and 999,000 molecules of other gases.
Outdoor air currently contains about 428 ppm CO2.
Historically, in the nineteenth century, Bavarian physician Max Josef von Pettenkofer, based on the study of the negative effects of exhaled air containing odors and scents released by breath and skin, proposed using the concentration of CO2 as a suitable indicator for assessing ventilation. He suggested a threshold concentration of 1,000 ppm as an indicator of insufficient ventilation. This criterion gradually began to be used more for assessing ventilation intensity and its control.
Subsequently, this criterion became increasingly enforced in practice, and the limit was raised to a range of 1,000 – 1,500 ppm. Principally controlled ventilation based on monitoring the current concentration of CO2 ensures controlled air exchange, thus maintaining both a low concentration of CO2 and automatically reducing the concentration of other pollutants.
These are various types of volatile organic compounds that are typically released due to the metabolic activity of the human body through breath, sweat, skin transpiration, etc. Furthermore, this controlled ventilation also contributes to energy savings, as it prevents uncontrolled over-ventilation.
Why monitor CO2?
CO2 is an excellent indicator of how much the air is currently exhaled. The main reason is that when a person inhales outdoor air, the exhaled air contains (besides various odors and scents) up to approximately 100 times higher concentration of CO2.
Thus, if people are in indoor spaces where adequate ventilation is not ensured, the concentration of carbon dioxide rises relatively quickly.
The concentration of carbon dioxide in buildings is closely related to how much fresh air is brought into the building. It therefore has a very good indicative ability of how exhaled the air is in a given space and how much it is currently ventilated.
Anatomy of exhaled air
Dispersion of CO2 in space
CO2 is heavier than air, the specific weight of CO2 is 1.839 kg/m3 at 20 °C and a pressure of 1,013 hPa, while the specific weight of air is 1.205 kg/m3. However, in normal space, due to thermal movement (descending and ascending air currents), the movement of people present, and ventilation, there is a constant mixing with the surrounding air, and the internal environment is largely relatively homogeneous.
Stratification, where heavier CO2 sinks to the ground, occurs mainly in basements or wells or caves, where there is minimal air movement.
