Indoor air

Indoor air quality is crucial to providing people with a healthy environment that has no effect on their productivity or well-being. Air quality is dependent on air temperature, humidity and pollution. The air temperature in rooms can be regulated with the help of heating and cooling systems, humidity with humidification equipment, while pollution can be resolved through room ventilation. However, all is not that simple. These are interrelated and interdependent factors, which transform apparently simple air quality control into a nuanced and refined system of regulation.

For example, air humidity should be considered in relative measurement units, because depending on air temperature, the saturation of air humidity varies. In this example, let’s assume a closed room contains 10 litres of water in the air, which at an air temperature of +20 °C makes up 30% of relative humidity. This degree of moisture is healthy for humans and is harmless to the eyes, skin and respiratory system. However, if the air temperature rises to + 24 °C, the relative humidity at the same absolute humidity will already be 22%, which will increasingly cause the skin, eyes and airways to dry out. Air ventilation is also a major cause of dehydration during the heating season – if the relative humidity of the air outdoors at 10 °C is 50%, heating this air to + 20 °C will reduce its relative humidity to just 8%. Therefore it is crucial, when thinking about air quality, to not only to regulate these criteria with the technical solutions available, but also to prevent overheating, over-cooling and over-ventilation of buildings.

In principle, when it comes to ascertaining the composition of air, two categories of sensors can be used. CO₂ gas sensors (the most common are NDIR – nondispersive infrared type sensors, while recently photoacoustic sensors have also become available) and VOC – volatile organic compound pollution sensors. The first type of sensor is very popular with the public for obvious reasons – CO₂ gas is the main component of our exhalation; its level in the exhaled air is about 38,000 PPM. Therefore, sensors of this type are adept at indicating the degree of exhalation within a room, which is the primary source of pollution in the living space. In turn, ventilating CO₂ also ventilates other types of pollution. However, CO2 sensors also have significant flaws, which is why we do not advise using them to automatically adjust ventilation criteria. Firstly, as an air quality indicator, such sensors will only be correct if the main pollutant is human exhalation. However, if this is not the case, these sensors will mislead users regarding the air quality in the room. Examples include freshly painted walls or new furniture generating significant chemical fumes, which often have odours that even people can detect, but which will not be recognized as pollution by CO₂ sensors. Likewise, organic pollution in pre-school education institutions could be caused by air pollution from kids’ diapers, while in the workplace, the source could be a worker who’s just come in after smoking outside. Of course, people can also detect these air pollutants, but automated ventilation with a CO₂ sensor cannot detect such pollution. While this is still considered a random drawback for such sensors, then the second flaw is even more important – one idiosyncrasy of the design of CO₂ sensors is their inability to ascertain the absolute CO₂ level with the help of fresh air without calibration. Therefore, these sensors will only show CO₂ levels if they are calibrated regularly. In the majority of cases, this calibration is integrated into sensors in this category automatically. By accumulating historical data over a period of time, the sensor equates it to the known condition that the CO₂ level of outdoor air is 400 PPM. Accordingly, at least once a week, the sensor must be allowed to measure the CO₂ level in an absolutely ventilated room or using air from outdoors. If the room has been ventilated for 15 minutes with the window fully open, this is can be done but if the CO₂ measurement is utilized as the basis for regulating ventilation, eventually the sensor will erroneously reduce the CO₂ level to a degree lower than its actual level, and in a significantly polluted room, the sensor will wrongly indicate that the air quality is that of fresh air.

The second type of sensors are VOC – volatile organic compound sensors, which are technologically much simpler, as well as a lot cheaper and easier to use. They can capture less widespread, but a much wider range of gases, including those found in human exhalation. Accordingly, if one assumes that the main source of indoor air pollution is human exhalation, knowing the proportions of the composition of the exhaled air, one can also calculate the theoretical indoor CO₂ level, which could be called the CO₂ equivalent, or eCO₂ level. Top quality sensors do this calculation very precisely, and one of their apparent disadvantages is actually their main advantage – they also take other types of indoor pollution in eCO₂ units into account. This is why we use high-quality Bosch Sensortec GmbH VOC sensors in our wall panels, which can not only precisely calculate the CO₂ level if the source of the room’s pollution is human exhalation, but will also show an increased level of CO₂ if there is other health-hazardous pollution in the room. Moreover, Bosch sensors calculate the total air quality index based on all perceived types of pollution. This index is considered to be the best criterion for determining air quality, contrary to the generally accepted public view that CO₂ is the only form of pollution. When choosing this type of air quality monitoring equipment, it is vital to check its certification according to ISO 16000-29: 2014. Otherwise, the selected equipment may not be accurate and may deceive users in regard to air quality readings.