How to quantify the ventilation rate of an indoor space using an affordable CO2 monitor

Last update: 4-Aug-2020, added intercomparison w/ LICOR, dry ice method

COVID-19 and the need to quantify ventilation rate of indoor spaces

There is increasing evidence that COVID-19 is a disease with a substantial, and potentially dominant, fraction of aerosol transmission. There is significant evidence about this route being likely important, including that the patterns of transmission are most consistent with aerosols. The arguments against aerosol transmission do not hold water.

Defending against aerosol transmission requires many of the things that we have been doing already, such as social distancing, but also puts a new emphasis on the importance of ventilation. Here, ventilation means the replacement of indoor air with outdoor air. We need to know the ventilation rate of an indoor space in order to be able to estimate the infection rate through aerosols in that location (e.g. classroom, shop, office), for example with my public aerosol transmission estimator spreadsheet. You would enter the ventilation rate at the location indicated by the red arrow.

Screenshot of the COVID-19 Aerosol Transmission Estimator, showing the location where the ventilation rate of the space needs to be entered.

The ventilation rate can change enormously between different spaces, and these changes are very important. We use units of “air changes per hour” (ACH, in units of h-1) to express this rate. If a certain room has a ventilation rate with outdoor air of 1 h-1, that means that in 1 hour, 63% of the indoor air has been replaced by outdoor air. In 2 hours, that’s 86%, and in 3 hours 95%. If the ventilation rate is 6 h-1, then in 1 hour 97% of the air is replaced by outdoor air. Clearly the ventilation rate is very important to determine how long virus-laden aerosols may stay around in a given space.

However, determining a realistic ventilation rate for a space can be difficult. Even for some commercial buildings that have building proctors and maintenance personnel, people trying to use the estimator for those buildings are telling us that such personnel does not have information on the appropriate ventilation rates, and does not know how to obtain that information.

Can the ventilation rate be estimated with a portable CO2 meter?

In research, we measure the ventilation rate using a “tracer release” experiment. We release a puff of an inert, non-sticky gas that we can measure with high time resolution. For interested researchers, this paper is one of the most advanced applications of this method. We then see measure that tracer decays, and the rate of decay is the ventilation rate. CO2 is a useful tracer, as are various volatile organic compounds. CO2 has a big advantage in that humans are a ubiquitous, free source of it .

The problem is how to do this “at home.” The research CO2 analyzers that I have used to do this can do this cost several thousand dollars, which is prohibitive for most situations. However, I have bought and tested a $159 CO2 monitor (US, see this link for Europe), recommended by our European colleagues at REHVA, and it seems to work well (see Appendices 1 and 2 at the bottom for results of different tests). Below is a picture of this monitor.

Aranet4 CO2 analyzer, with my hand for scale. This is the manual

There are many other monitors that can probably do the same thing. Look for NDIR technology. I chose this one because it does have a display, traffic light feature, and Bluetooth connectivity, in addition to the endorsement from REHVA.

How to estimate the ventilation rate in practice

The experiment is simple. But do take good notes of what you did when, or it is easy to get confused later when looking at the data, since you will likely try multiple things etc.

(1) Leave the CO2 monitor outside for at least 5 minutes to record the background concentration.

(2) Stay in the location of interest (with the CO2 monitor there) for a while, so that CO2 builds up (see Appendix 3 for ideas if that is not enough CO2 for the experiment, like for a large room or with very good ventilation). Then leave quickly, and let the analyzer record CO2 for several hours.

(3) Best to leave a fan(s) on the whole time, to mix the air in the space. This leads to a smoother decay, and makes the data easier to interpret, and it should not affect the ventilation rate. Data without a fan will look more “blobby” than the data I show here.

(4) Look at the data through the App of the CO2 monitor (you can also download it into a computer and graph it in Excel etc., but that is not needed).

An example experiment for our home studio

An example for my first experiment is below. You need to determine how long it takes for the “excess” CO2 (above the ambient level) to decay to 36% of its peak.

For the example below, the outdoor level was 545 ppm (the monitor has an offset, which can be adjusted, but that is not important for this measurement). The CO2 peak was 1473 ppm when the person left the space at 6:43 pm, so the excess CO2 above ambient was 928 ppm. We need to look at the time in which it decays to 879 ppm (= 545 ppm + 37% * (1473–545)). CO2 was still 1066 ppm when I removed the analyzer from the space at 7:09 am. From the rate of decay (40 ppm in the final hour), it would have taken another 4.5 hours to reach 886 ppm approximately. So the decay time is ~15.5 h, and the ventilation rate is 1 / 16 = 0.06 h-1. There is a little spreadsheet calculator on the ventilation section of the estimator, where you can enter the numbers and it does the calculations for you:

Calculation of ventilation rate from the CO2 experiment in the Readme page of the COVID-19 Aerosol Transmission Estimator

This is a new space with low infiltration (although I am surprised that it is so low). For most locations I would expect results of 0.5–3 h-1. I am going to take more data with a window slightly open to illustrate another example, but in the interest of time I am posting this initial experiment, since it is good enough to illustrate the procedure.

My first experiment. First (left) the monitor was outdoors. Then I brought it into a studio (small separate building) where my wife had been working. She left shortly afterwards, at which point the CO2 peaks and stops decreasing. Then CO2 is slowly removed by infiltration of outdoor air. Ad the end of the experiment I took the monitor into the house, which had lower CO2 concentration.

A second attempt with a window partially open

To illustrate what I expect is a more typical result, I redid the experiment with a window partially open. In this case the CO2 decay was much faster, and I estimate 1.2 h-1 under this condition.

Results of the second experiment, with a window partially open. Low values before the peak are outdoors, then move analyzers into the studio, and let it be, with the fan on. Cursor is at point where excess CO2 had decreased 63% from the peak.

Caveats of the ventilation rate determined in this way

First of all, this method will only quantify the ventilation rate with outdoor air. In many spaces, air is recirculated and filtered, and many virus-containing aerosols will be removed by the filter. But CO2 will be unaffected by the filters. The effect of the filtered recirculated flow goes elsewhere in the spreadsheet (“Additional Control Measures”), see the Readme page in the estimator for some discussion of the details.

Second, the air change rate is not constant, and can change in time. For example if you have the windows open, it will vary a lot with changes in outdoor winds. It can also change indoors depending on the dynamics of the HVAC system and other factors. Even if the windows are closed and there is no HVAC system, wind and other factors will affect the exchange rate as they can drive infiltration. For this reason, and if this is a space where you are going to spend a lot of time, it is a good idea to repeat this experiment a few times, and see that you are getting consistent results. See this scientific study and this other one that show examples of the variability.

Third, in large buildings where air can flow between spaces, either through corridors, or through the HVAC system, this can be more difficult. Experts call this interzonal transport. E.g. people on a different room may breathe CO2, that is then reach the room of interest, confusing the measurement.

The summary is that this method will give you an ballpark estimate of the outdoor air change rate, but that value can change in time and reality may be more complex in larger buildings. It is still much more information than you have before doing the experiment though, in most cases.

Appendix 1: checking the response time of the CO2 analyzer

The first test of the quality of a gas analyzer is to see whether it responds quickly to changes in concentration. If it responds too slowly, then that would limit the measurement of fast ventilation rates. We do this by leaving the monitor in a location with high CO2, and then by quickly moving it outside.

Response of the Aranet4 analyzer to a rapid change in CO2 concentration

The graph below is a screenshot of the CO2 monitor app in my phone. The CO2 level responded very quickly, with most of the response within the 1 minute time response of the analyzer. This is more than adequate to measure ventilation rates in any indoor environment.

Appendix 2: intercomparison of Aranet CO2 sensor with a research-grade LI-COR analyzer

Dr. Demetrios Pagonis in my research group performed a quick experiment comparing the Aranet sensor with a research grade LI-COR sensor. The results are below. The LI-COR sensor provides data every 1 s, while the Aranet does so every 1 min. Overall I would call the results very good, for an affordable sensor like this one. Some deviations are clearly due to time response, since the Aranet does not have a sample flow and has to rely on the CO2 diffusing into the sensor volume. There is also a positive offset in this particular sensor which I have not managed to remove yet (maybe I don’t quite understand the manual on how to do this), but that is not a concern for the time response calculations. And it can be subtracted in your head, after measuring the air outdoors.

Time series comparison of Aranet4 sensor with a research-grade LI-COR sensor
Scatter plot and orthogonal distance regression linear fit between the Aranet4 and LI-COR CO2 measurements

Appendix 3: what to do if your breathing is not enough CO2

This can be a problem in large and/or well-ventilated spaces. The breathing of a single person or a few people may not increase CO2 enough, and then it may be hard to extract the information from the experiment, given the limited precision of the analyzer. At least during the pandemic you do not want to get a lot of people together just for this experiment. The ideas below have been suggested by others. Be careful, and make sure that you have permission from whoever manages the space before you try these.

Dr. Demetrios Pagonis suggests a common technique used by indoor air quality researchers, which to buy dry ice and put it in water. A 10-lb block from the grocery store (~$10) is equivalent to ~100 person-hours of breath CO2. This should be good enough for even the biggest spaces. This seems the safest of all the suggestions.

In that case one suggestion from Peter Alstone in Twitter was to mix baking soda and vinegar in the space, which produces CO2. This YouTube video demonstrates the reaction. How much vinegar and baking soda you need will depend on the size of the room. I would try with a modest amount, see if you can see the CO2 increase enough, and go from there.

Another suggestion was to borrow a CO2 tank, such as those used for carbonated beverages in bars and restaurants. This starts to make me nervous in terms of safety. If you were to exhaust a large amount of CO2 in a closed space by accident, it could be fatal. So I would be very careful if you try this, especially if you don’t have experience working with gas cylinders. Try first in a smaller space with the door open. Or have someone wait outside while you talk on the phone with them, or inject the CO2 by pointing the output of the tank towards the space, from right outside the space, or some other such method. If you are not sure about safety, ask for help.

You could also use a cooking device, like a natural gas or propane stove, if it is already present in the space. I would not bring in a camping stove or similar, due to fire safety concerns.

Acknowledgements

I am grateful for input and suggestions on this topic from Andy Persily, Shelly Miller, Dustin Poppendiek, Ty Newell, Rich Corsi, Bill Bahnfleth, and Jeff Siegel.

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