Current statistics, such as those of the U.S. Environmental Protection Agency (EPA), show that people spend almost 90% of their time indoors, while the concentrations of some pollutants indoors are often 2× to 5× higher than typical outdoor concentrations.1 CO2 concentration is a key indicator of air quality. At this point, it is worth noting that about 140 years ago, Max von Pettenkofer laid the foundations for current regulations relating to air quality with his studies on CO2 levels. The higher the CO2 value in a building, the less comfortable it becomes for the people inside. In poorly ventilated rooms, the CO2 concentration increases rapidly. For example, in a space of about 4 m2 occupied by only one person, the CO2 value rises from 500 ppm (0.05%) to more than 1,000 ppm (0.1%) in just 45 minutes. At this level, the odorless and colorless gas can cause headaches, drowsiness, and poor concentration, often resulting in reduced productivity. From 2,000 ppm onward (0.2%), even the cognitive abilities of humans are influenced, and there is a significant risk to health at higher levels.

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