CO2 and Ventilation

Atmospheric CO2 Levels from Mauna Loa

Today I was teaching a Certified Indoor Environmentalist course in Connecticut when we measured carbon dioxide (CO2) levels in the meeting room.   When I read 2,850 parts per million (ppm) on my meter, I immediately assumed something was wrong with the device.   That was upsetting because it had just come back from factory calibration.  In such a well conditioned, modern meeting room, we couldn’t really have CO2 levels that high, could we? A typical meeting room ranges from say 800-1,200 ppm.

When taking the device outdoors, we were getting readings around 380 ppm as one would expect (see chart).  In other areas of the hotel, levels were more in line with expectations (around 1,000 ppm).  Our high levels were not a computer glitch, but an accurate reading of the CO2 levels in the space.

Here is a key point learned by the class today: ventilation may be lacking even in a room that feels cool & crisp and is well maintained.  It’s not just hot and stuffy rooms that may be under-ventilated.  This reiterates the importance of measuring ventilation either directly or approximating it with the use of a surrogate such as CO2.

In a future blog post I’ll describe the advantages and disadvantages of using CO2 to approximate ventilation.  It’s a good screening tool that incorporates mechanical and natural ventilation (not to mention air infiltration as well).

By Ian Cull

I'm I.A.N. the Indoor Air Nerd. I'm a speaker and consultant on indoor air quality issues. To learn more about me, click "about" at the top of this page.

9 replies on “CO2 and Ventilation”

Measuring Carbon Dioxide levels is really only a measure of how many people are in the room. Since CO2 is easy to track, it has been used in the Standards for years, and the standards are much lower than need be, as evidenced by your reading in the Connecticut classroom – no one was uncomfortable even at that level. I would argue that increasing ventilation rates in order to reduce CO2 levels unnecessarily is an expensive solution as that additional outside air volume must be heated or cooled! And the allergens, etc. that might be brought in add to discomfort. Purifying the air as it enters, or returns, into the air handlers is the ideal solution, reducing the % of outside air needed by cleaning up not only CO2, but more objectionable VOCs allergens, molds, and bacteria!

As I’m sure you know, ASHRAE allows the “IAQ Procedure” which substitutes outdoor air ventilation with advanced air cleaning. I agree that there are many scenarios where advanced air cleaning is preferable. When I was consulting in India, outdoor air ventilation actually made matters worse!


I do a lot of CO2-based ventilation studies and am finding a systematic under ventilation condition in many conference rooms. What is happening is that these spaces begin by being overcooled and so the thermostat takes a while before it experiences a “rise in temperature” sufficient to send a signal to the local VAV mixing box to increase supply air volume. Therefore the space seems cool but unfortunately is significantly under-ventilated where the actual ventilation rate might only by 5 cfm of OA per person, and so is limited in its ability to dilute and remove air contaminants and any viruses if they are being shed by any of the occupants will persist long enough in the air to perhaps expose other occupants.

I find in-space CO2 monitoring in conference rooms to be a very important part of ventilation assessment efforts.


I agree that many of these rooms are pre-cooled in the morning. If it’s a variable air volume (VAV) system, this will lead to minimal ventilation until the body heat (and other sources) can affect the thermostat. Things can get even worse with the small residential-type systems sometimes used by hotel meeting rooms. These units will turn completely off when the thermostat is satisfied, proving zero mechanical ventilation!

I think CO2 or any other population based system (demand controlled ventilation) is the way to go!

Thanks for the comment!

Thanks for the link David. I’ll add that to my IAQ library!

A few years ago we had a discussion about RFID chips on security badges being used for Demand Controlled Ventilation. Things have to get much worse before security badges are standard issue in most office buildings. Until then, CO2 seems like the best bet.

So: Could one use the CO2 gradient i.e. CO2 inside classroom – Co2 outside as a proxy for ventilation in a classroom please?

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