Which factors can affect CO2 concentration in a space?

• A. Temperature fluctuations and humidity.
• B. Ventilation system fan speed only.
• C. Air leakage through building envelope.
• D. Occupant count, air transfers between spaces, and variability of metabolic CO2 production.

Answer: (D)

CO2 concentration in a space is governed by a balance between what is produced inside the space and what is removed or redistributed by the air that moves in and out. The main sources and pathways include how many people are present and how active they are (these determine how much CO2 is generated), how air moves between spaces or is exchanged with outdoor air (interspace transfers and ventilation), and how those generation rates can vary with activity over time.

This is why the best answer includes occupant count, air transfers between spaces, and variability of metabolic CO2 production. More people or higher activity raises CO2 generation; air moving between spaces or with the outside dilutes or concentrates CO2 depending on the differing levels in connected spaces; and CO2 production itself can change as activity changes, even with the same space, adding to the dynamics of concentration.

The other options don’t capture the full picture. Temperature and humidity can influence air properties and sensor readings but aren’t the primary drivers of CO2 levels. Ventilation speed matters, but focusing only on fan speed ignores generation from occupants and interzone air movement. Air leakage through the building envelope is part of air exchange, but by itself it doesn’t account for occupancy-driven generation or metabolic variability.