Carbon dioxide is colorless and odorless and therefore not organoleptically detected by humans; it is 1.5 times heavier than air and usually accumulates in its lower layers, including in the zone of human respiration; in significant concentrations, it is found in basements, wells, abandoned mines, as well as in sealed rooms such as shelters or during prolonged immersion in submarines.
Special studies have found that a physiological reaction in the form of a slight expansion of the peripheral vessels is already detected when the concentration of carbon dioxide in the air is 0.1% (although for many people, especially healthy ones, concentrations of 2 and 3 are not noticeable %). The value of 0.1% – the norm established by K. Flygge and confirmed by domestic hygienists, is the maximum permissible hygienic norm for the content of carbon dioxide in the air of inhabited premises. It guarantees to a certain extent from the appearance of unpleasant odors; a carbon dioxide concentration of 0.07% is considered optimal for these rooms (the norm established by M. Pettenkoffer). However, one should not forget that due to the widespread use in the construction of polymeric and other synthetic materials, the indicated concentrations of carbon dioxide do not ensure complete cleanliness of the air in the premises.