Essentially, there are two categories of sensors used to determine air composition: CO2 gas sensors (the most common being NDIR – nondispersive infrared – type sensors, and more recently, photoacoustic sensors) and VOC (volatile organic compound) pollution sensors. For understandable reasons, the first type of sensors is particularly prevalent – CO2 gas is the main component of our exhaled breath, with its level in exhaled air being approximately 38,000 ppm. Therefore, such sensors can effectively indicate the air breathed in a room, which is the main pollution source in living spaces. However, ventilating CO2 also ventilates other types of pollution. Nevertheless, CO2 sensors also have significant drawbacks, which is why we do not recommend using them for automatic ventilation control criteria. Firstly, as air quality indicators, these sensors will only be accurate if the main pollution is human exhalation, but if it is not, these sensors will prove misleading about the air quality in the room. Examples include freshly painted walls or new furniture, which emit significant amounts of chemical compounds, often accompanied by odours perceptible to humans, but not detectable by CO2 sensors. They also do not detect all organic pollutants, such as air pollution in preschools from children's used diapers or in workplaces when an employee enters who has recently been smoking. While these air pollutants are perceptible to humans, ventilation with CO2 sensors will not respond to such pollution. Another significant drawback of CO2 sensors is their inability to determine the absolute CO2 level without calibration using fresh air. This means that these sensors will only show the CO2 level if regularly calibrated. Calibration in these sensor categories is mostly automatic, and over time, the sensor, by accumulating historical data, adjusts them to a known condition when the outdoor air CO2 level is 400 ppm. This means that at least once a week, the sensor must be allowed to measure the CO2 level in a fully ventilated room or outdoor air. If the room is ventilated for 15 minutes with fully open windows, this is possible, but if CO2 measurements are used as the basis for ventilation control, over time the sensor will erroneously display a lower CO2 level than the actual level, and in significantly polluted rooms, the level of fresh air displayed by the sensor will be deceptive.

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The second type of sensors is VOC or volatile organic compound sensors, which have a much simpler structure technologically, are significantly cheaper, and easier to use. They are capable of detecting a larger amount of less common gases, including those present in human exhalation. Thus, it is possible, assuming that the main indoor air pollution is human exhalation and knowing the proportions of exhaled air composition, to calculate the theoretical room CO2 level, commonly referred to as CO2 equivalent or eCO2 level. High-quality sensors perform this calculation very accurately, and the apparent disadvantage is, in fact, their main advantage – they calculate other types of indoor pollution in eCO2 units. That is why we use high-quality VOC sensors manufactured by Bosch Sensortec GmbH in our wall panels, which can accurately calculate the CO2 level in case the source of indoor pollution is human exhalation, as well as indicating an elevated CO2 level if there is another type of hazardous pollution in the room. In addition, Bosch sensors also calculate the total air quality index, based on all detectable types of pollution, and such an index is considered the best criterion for determining air quality, instead of the widely accepted public opinion that CO2 is the only pollution. When choosing such air quality monitoring equipment, it is essential to verify their certification according to ISO 16000-29:2014 requirements; otherwise, the selected equipment may not be accurate and could mislead the user about air quality indicators.