CO2 is used as a surrogate ventilation index for diagnosing ventilation inefficiency or distribution problems. As the number of people in the space increase or the level of activity increases, so will the CO2 concentration. Increased concentrations of CO2 in a space is also linked to discomfort and increased perception of odour. However, Indoor pollution sources and other indoor air quality (IAQ) problems might exist even when indoor CO2 levels are measured below 1000 ppm.