How does DCV maintain indoor air quality? Fresh or outdoor air has a low CO2 concentration which is indicative of good air quality. As people breath, they exhale CO2. When more people are gathered in a space, the CO2 levels rise. These heightened levels of CO2 indicates poor indoor air quality. When the CO2 concentration is measured, high or rising CO2 levels tell the HVAC system more fresh air is required. More fresh air is then added which lowers the CO2 levels, maintaining good Indoor Air Quality (IAQ).