Air, or rather the oxygen it contains, is essential to life as we know it. An adult breathes in and out approximately 15 times per minute; children 20 to 30 times; babies 40 times. The air we breathe in contains 21 % oxygen, whereas when we breathe it out, it contains 17 % oxygen. A person at rest needs approximately 20-30 m³ of fresh air per hour. The minimum air change rate for good hygiene is 0.5-1, which means the air in the room should be completely changed at least every two hours. If the air is replaced solely via a ventilation system, this system must have a certain air change rate to ensure the moisture, CO₂ and air pollutants that occur are extracted. The rate varies depending on the size of the room, its purpose, the indoor air quality requirements and the number of occupants.