CO2 as a Ventilation Indicator
Learning Objectives
Students will be able to:
- Explain why CO2 is used as a ventilation indicator
- Describe the relationship between occupancy, ventilation, and CO2 levels
- Interpret CO2 readings and explain what they indicate
- Calculate rebreathed air fraction from CO2 data
Why CO2?
CO2 isn't the problem—it's the indicator.
At normal indoor levels (400-2000 ppm), CO2 itself is harmless. But CO2 tells us something important: how much of the air we're breathing has already been breathed by someone else.
The CO2 Balance
People breathing (exhale ~40,000 ppm)
CO2 builds up in room
Ventilation brings in fresh air (~420 ppm)
The CO2 level you measure is the result of this balance: More people or less ventilation = higher CO2. Fewer people or more ventilation = lower CO2.
CO2 Level Guidelines
| CO2 Level | Interpretation | Typical Scenario |
|---|---|---|
| ~420 ppm | Outdoor air | Baseline; no exhaled air |
| <600 ppm | Excellent ventilation | Windows open, few people, good HVAC |
| 600-800 ppm | Good ventilation | Typical well-ventilated classroom |
| 800-1000 ppm | Moderate ventilation | Acceptable but could be better |
| 1000-1500 ppm | Marginal ventilation | Crowded room, poor HVAC |
| >1500 ppm | Poor ventilation | Very stuffy; open windows! |
| >2000 ppm | Very poor ventilation | May cause drowsiness |
The Rebreathed Air Calculation
What fraction of the air you're breathing has already been exhaled by someone else?
Using typical values (outdoor = 420 ppm, exhaled = 40,000 ppm):
| Indoor CO2 | Calculation | Rebreathed % |
|---|---|---|
| 800 ppm | (800-420)/(40000-420) | 1.0% |
| 1000 ppm | (1000-420)/(40000-420) | 1.5% |
| 1500 ppm | (1500-420)/(40000-420) | 2.7% |
| 2000 ppm | (2000-420)/(40000-420) | 4.0% |
Why Rebreathed Air Matters
Disease Transmission
If someone is infected, the air they exhale may contain pathogens. The more rebreathed air, the higher your exposure to any respiratory pathogen in the room.
General Air Quality
Exhaled air contains not just CO2 but also water vapor, VOCs, and bioaerosols. High CO2 indicates these may be building up too.
Typical Daily CO2 Pattern
In a classroom, CO2 typically follows this pattern:
CO2 (ppm)
|
1200| ____
| / \ ____
1000| / \ / \
| / \/ \
800| / \
| / \
600| / \
|/ \____
400|____________________________________________
| 8am 10 12pm 2 4pm 6 8pm 10
Class Lunch Class After-school Empty
Notice: CO2 rises when room is occupied, drops during lunch/breaks (doors open, fewer people), and drops to near-outdoor levels when building is empty.
Practice Problems
- A classroom has 25 students and reads 1200 ppm. Is this good, moderate, or poor ventilation?
(Answer: Moderate-to-marginal—in the 1000-1500 ppm range) - If outdoor CO2 is 420 ppm and a room reads 1500 ppm, what percentage of the air is rebreathed?
(Answer: (1500-420)/(40000-420) = 1080/39580 = 2.7%) - A teacher opens windows and CO2 drops from 1400 ppm to 700 ppm in 20 minutes. What does this tell you about ventilation with windows open?
(Answer: Very good—the air is being replaced quickly with fresh outdoor air)
Key Takeaway
CO2 is a powerful indicator of ventilation because it comes almost entirely from human breathing. The difference between indoor and outdoor CO2 tells us how much of the air has been previously exhaled. Keeping CO2 below 800 ppm means less than 1% of the air you're breathing has been exhaled by someone else—good for both air quality and disease transmission risk.