Outdoor air quantity and CO2 level

Stop Guessing CFM: How Metabolic Rate Dictates Indoor CO2 ?

Typical ventilation rules often treat all occupants the same. They are not.

The chart shows steady-state indoor CO2 levels against fresh outdoor air (CFM/person) across different metabolic (MET) rates, assuming an outdoor baseline of 400 ppm.

What the chart tell us:

• Activity Drives Contamination: At 15 CFM/person, a resting occupant (1 MET) keeps the room under 1,000 ppm. A person doing heavy work (4 MET) at the same airflow spikes the room to nearly 2,600 ppm.
• The Curve Flattens: Feeding more than 20 CFM/person yields massive diminishing returns. Throwing more fan energy at a space is an inefficient way to lower CO2 once you pass the knee of the curve.
• Codes are Minimums, Not Solutions: Designing a gym or high-activity zone with office-level fresh air rates guarantees a stuffy, high-CO2 environment.