The levels of which airborne pollutant serve as a proxy for other indoor pollutants?

• A. Ozone
• B. Particulate Matter
• C. Carbon Dioxide
• D. Nitrogen Dioxide

Answer: (C)

Carbon dioxide levels are used as a proxy for other indoor pollutants because CO2 acts as a tracer of occupancy and how well a space is ventilated. People continuously emit CO2, so its concentration rises when a room is crowded or when outdoor air exchange is limited. When ventilation is insufficient, not only CO2 accumulates, but other contaminants—such as VOCs, odors, moisture-related compounds, and bioaerosols—also build up because they are not being diluted by fresh outdoor air. Seeing higher CO2 therefore signals reduced air dilution and a higher potential for exposure to a range of indoor pollutants. The other substances listed are specific pollutants with their own sources and dynamics, and their levels don’t reliably indicate overall ventilation adequacy the way CO2 does, which is why CO2 is considered a practical proxy for indoor air quality.