People inhale O2 and exhale CO₂, this is part of the body's metabolism. Thus, people in a confined space without proper ventilation, increase the indoor CO₂ concentration significantly. These CO₂ concentrations can range from 400 ppm, the atmospheric background concentration, to a value above 2000 ppm. Starting at a moderate concentration of 1000 ppm, the first symptoms appear, such as fatigue, loss of concentration or lower productivity. Above a concentration of 2000 ppm, symptoms like headaches can emerge. In addition, exhaled CO₂ correlates with human-borne exhaled aerosols, which may contain viruses and bacteria. When ventilation is poor, not only CO2 but also aerosols with potentially infectious viruses and bacteria accumulate. Thus, measuring CO₂ is a good proxy for indoor air quality and can indicate the risk for viral and bacterial infections.