I see a lot of pictures on social media platforms where people measure Carbon Dioxide (CO2) concentrations in indoor environments incorrectly.
CO2 sensors are very sensitive instruments, and when you keep them close to your face they measure the CO2 concentrations of your breath and not of the room. The CO2 level in exhaled air is about 38,000ppm. When CO2 is exhaled it is quickly mixed with the ambient air and, if the ventilation is good, the concentration is quickly reduced to safe levels.
Keep the monitor away from your face, at least 3m/10ft. Preferable in a fixed point on the wall.
Composition of the air during inhaling & during exhaling
The typical composition of the inhaled air is:
- 78% Nitrogen
- 20.95% Oxygen
- 0.93% Argon
- 0.04% Carbon Dioxide
- 0.0005% Helium
- 0.00005% Hydrogen
- and small amounts of other gases
The gas exhaled is 3.8% to 5% by volume of CO2, about 100 times increase over the inhaled amount. The volume of oxygen is reduced by a small amount, 4% to 5%, compared to the oxygen inhaled.
The typical composition of the exhaled air is:
- 5.0–6.3% water vapor
- 79% Nitrogen
- 13.6–16.0% Oxygen
- 3.8–5.3% Carbon Dioxide
- 0.93% Argon
- ppm of Hydrogen, from the metabolic activity of microorganisms in the large intestine.
- ppm of Carbon Monoxide from degradation of heme proteins.
- 1 ppm of Ammonia.
- trace many hundreds of VOCs especially isoprene and acetone.
Conclusion
All in all, CO2 monitors are great tools that can help us mitigate the spread of an airborne virus but also ensure good ventilation rates and good air quality indoors. Unfortunately, many CO2 monitors don’t indicate that users should keep them at a distance while they are measuring the indoor concentrations.
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See The Air 
[…] The device can be placed on any surfaces (table, desk, etc) but it has a hole on the back in order to hang it on the wall, where it supposes to be in my opinion. When we keep CO2 monitors close to our face/mouth by placing them in front of us, the sensors tend to measure high concentrations because each time we exhale air, CO2 concentrations can be around 38,000ppm, read more here. […]
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It depends? I have a TACX bicycle setup in a room in our loft. For a while I had three CO2 monitors set up. One (IQAir) on the small low table about 1m in front of my bike so maybe 1.5m in front of my face. Another (Gaslab 501) on the wall next to me on my left which was about 1m away from my face. Another (uRAD A3) on the opposite wall about 3m to my right.
With no ventilation the two closest to me both rose to 4,000 ppm over about a 45 minute period while the one further away never got above 900. With a small fan on the floor blowing at me the two closest rose slower but still hit 2400 while the one further away got to 1700. With the fan plus a window open (and this is upper level, well above the equal pressure plane, so a lot of outflow) the two closest stayed below 800 while further away was below 500.
In this case the two closer monitors indicated what I was actually breathing and were the more important measures. My performance correlated to the closer monitors, not to the one further away.
A lot depends on how much mixing of air is happening in the space and what the sources of CO2 laden air and fresh air supply/exhaust are.
What I experienced can also happen in an office or bedroom whether a single person or multiple. With poor mixing a portion of the room could have very good ventilation and very low CO2 and other gases while others parts of the room might have very high CO2 and other gases.
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