CO2 is a useful surrogate for what in indoor environmental assessments?

• A. Human occupants and their metabolic rate
• B. Outdoor air quality
• C. Building materials emissions
• D. Thermal comfort temperature

Answer: (A)

CO2 serves as a practical stand-in for how many people are in a space and how active they are, because humans exhale CO2 at a rate tied to metabolism. In indoor environments, the CO2 level reflects the balance between the people generating it and the ventilation removing it. If occupancy is high or activity is vigorous and ventilation isn’t sufficient, CO2 rises; if ventilation keeps up with generation, CO2 stays closer to outdoor levels. This makes CO2 a useful indicator of occupant load and the effectiveness of ventilation.

Outdoor air quality isn’t what CO2 in a room represents, and CO2 doesn’t measure emissions from building materials or directly indicate thermal comfort temperature.