By the SightShield team · · 3 min read
The stuffy room effect
Everyone knows the feeling: an hour into a meeting in a closed, air-conditioned room, heads get heavy and attention drifts. In a classroom of 40 students with the door shut and the AC on, the same thing happens after lunch. A major reason is carbon dioxide. Every person breathes out CO2, and split ACs only recirculate room air — they do not bring in fresh air — so CO2 keeps rising until someone opens a door.
What the numbers mean
CO2 at these levels is not toxic, but it is a reliable sign that stale air, body odours and airborne germs are building up. Many ventilation guidelines aim to keep indoor CO2 around 1,000 ppm or lower.
| CO2 level (ppm) | What it usually indicates |
|---|---|
| About 420 | Outdoor air |
| Below 800 | Well-ventilated room |
| 800–1,000 | Acceptable; ventilation could be better |
| 1,000–1,500 | Stuffy; many people notice drowsiness |
| Above 1,500–2,000 | Poor ventilation; headaches and poor concentration are commonly reported |
Why it matters for work and learning
Research on office workers, including studies at Harvard on “green” office conditions, found better scores on decision-making tests in well-ventilated rooms than in rooms with higher CO2 and pollutants. Classroom studies have linked better ventilation with better attention and fewer absences. The exact effect varies, but the direction is consistent: fresh air helps people think.
How fast rooms fill up
A small meeting room of about 12 square metres with six people and the door closed can climb from 500 to well above 1,500 ppm within an hour. A classroom of 40 students can pass 2,000 ppm in a single period. The smaller the room and the more people in it, the faster it rises.
Measuring it properly
- Use a monitor with an NDIR CO2 sensor; cheap “eCO2” sensors estimate rather than measure.
- Place it at seated head height, away from doors, windows and people’s breath.
- Log readings through a full day or class schedule, not a single spot check.
- Compare rooms: the worst ones usually have the most people per square metre.
Fixes, from simple to complete
| Fix | Effect |
|---|---|
| Open doors or windows between meetings and classes | Quick reset, but lets heat, noise and dust in |
| Book fewer people into small rooms | Slows the rise |
| Exhaust fan in the room | Helps if replacement air can enter |
| Fresh air unit with heat or energy recovery (HRV/ERV) | Continuous filtered fresh air with less cooling loss |
| Demand-controlled ventilation | Fresh air rate rises automatically with CO2 levels |
Demand-controlled ventilation
The most efficient approach links a CO2 sensor in each room to the fresh air system. When the room is empty, ventilation runs at a low rate; when it fills up, the fan speeds up to hold CO2 near the target. This keeps air fresh without over-ventilating and wasting cooling energy, and suits meeting rooms and classrooms with changing occupancy.
An example
An office in Whitefield measured CO2 in its boardroom during a three-hour review: it rose from 600 ppm to over 2,200 ppm with ten people and the door closed. A compact ERV unit above the false ceiling, controlled by a CO2 sensor, now holds the room around 900 ppm during meetings. People noticed the difference within a week — fewer complaints about stuffiness and fewer breaks to “get some air”.
Get your rooms measured
We measure CO2 in meeting rooms and classrooms and design fresh air systems that keep levels down without losing cooling.
Fresh air ventilation service →Quick answers
What is a good CO2 level for a meeting room?+
Around 1,000 ppm or lower is a common target; below 800 ppm indicates good ventilation.
Does an air conditioner bring in fresh air?+
Split ACs do not; they recirculate room air. Fresh air needs a separate ventilation system or opening.
Does an air purifier reduce CO2?+
No. Purifiers remove particles; only ventilation reduces CO2.
Is high CO2 dangerous?+
At typical indoor levels it is not toxic, but it signals poor ventilation and is linked to drowsiness and poor concentration.
How do I measure CO2?+
Use a monitor with an NDIR CO2 sensor placed at seated head height and log readings over the day.