The role of CO2 to control the indoor air quality in buildings is based on the fact that CO2 developed by people breathing may be used as a marker for the bio-effluents produced by people. The use of CO2 for the steering of ventilation systems is only appropriate in the case that no other pollutant is more dominant for the indoor environment. For instance when a person is taking a shower in a bathroom, moisture will be the more dominant pollutant. Nevertheless the use of CO2 as marker for the indoor air quality is widely used. CO2 sensors are often used in demand controlled ventilation to control bio-effluents from humans.