Recommendations for use of CO2 sensors to control room air quality during the COVID-19 pandemic

6 min readAug 28, 2020

This blog is updated regularly

Basics

For COVID-19, it is now considered certain that airborne transmission is a major pathway of infection. See the RKI situation report. Since August 18, the chapter transmission has been written:

A consequent measure is the improvement and control of the room ventilation. Air enriched with infectious aerosols is replaced by outside air.

CO2 as an indicator for room ventilation

CO2 is released into the air when people breathe. The amount of CO2 released is related to the number of people and their activity (the amount of energy consumed).

Every person has a certain chance of emitting infectious aerosols. This amount is related to the number of people, the activity (especially whether people speak, sing or laugh) and the respiratory protection measures used.

Ventilation removes both CO2 and aerosols from a room. The concentration of CO2 can therefore be taken as a measure of the amount of contagious aerosols to be expected.

Recommendations for CO2 air quality monitoring

The REHVA (The Federation of European Heating, Ventilation and Air Conditioning associations) recommends on its information page on COVID-19, especially for schools, that a CO2 monitor should emit yellow light from 800 ppm and red light from 1000 ppm. The CO2 monitor should be placed in a visible place in the classroom away from fresh air openings. This recommendation applies to classrooms in which ventilation depends on the opening of windows or the like.
The Federal Ministry of Labor and Social Affairs recommends in the SARS-CoV-2 occupational safety regulations to check the quality of the ventilation by means of a CO2 measurement. A CO2 concentration of 1000 ppm is still considered acceptable. This value should be undercut if possible.

Scientific Background

A recent preprint by Zhe Peng and Jose L Jimenez “Exhaled CO2 as COVID-19 infection risk proxy for different indoor environments and activities” examined the complicated correlation between CO2 concentration and infection risk. See also the explaining tweed by Jose L Jimenez.

Figure 1 — Excess CO2 volume mixing ratio (ppm) that an uninfected individual inhales for 1 h for a probability of infection of 0.01% and probability of infection per ppm excess CO2 inhaled for 1 hr for (A) variants of the university class case (see Table S1 for the case details), (B) various activities , and (C) several indoor environments (From Peng and Jimenez 2020).

VOC and eCO2

The quality of indoor air can also be assessed by measuring the VOC value (VOC stands for Volatile Organic Components). Such devices are often cheaper than CO2 measuring devices. Many of these devices convert the VOC internally into a so-called eCO2 (Estimated CO2, CO2 equivalent value).

I am not aware of any recommendations for VOC values in connection with COVID-19. Accordingly, I cannot make any statements about the recommended eCO2 values.

Some devices on the market which measure VOC (or TVOC) state that they can also measure CO2. This information is incorrect as these devices only display the eCO2 value.

In order to see whether a purchased device calculates an eCO2 value from the VOC value instead of measuring CO2, proceed as follows.

A glass with fresh sparkling mineral water is placed in a small bag together with the measuring device. Several thousand ppm of CO2 are expected after a few minutes. A self-made CO2 measuring device (simple design) shows 5000 ppm after 20 minutes in the experiment shown (because the measuring range was exceeded)

The JSM-131 air quality meter claims that it is measuring CO2. If it is also placed in a bag with mineral water, the displayed eCO2 value does not change significantly.

Buyable CO2 measuring devices

Real CO2 measuring devices are available from around 100 euros. Cheaper devices often say they measure CO2. Instead, they indicate the eCO2 value (see above). I expect that most cheap device that pretends to measure CO2 always outputs eCO2 when it is stated that VOC or TVOC is measured.

Pure sensors are cheaper, some of them can be used as a measuring device with little effort (see below).

Individual devices are tested in Dirk Paessler’s blog on CO2 measurements at Corona.

Do-it-yourself CO2 measuring devices

A cheap CO2 sensor like the MH-Z19B can be bought for around 20 euros. The sensor has a voltage output. With an additional voltage measuring device, a CO2 measuring device can be set up quickly and easily without soldering.

A more detailed description can be found on Github in the CO2-Measurement-simple repository.
More do-it-yourself instructions can be found on the Internet. If necessary, the limit values for the traffic lights should be adjusted.

CO2 sensors

Also in the case of sensors, it seems sometimes to be the case that it is stated that a VOC sensor also detects CO2. That seems e.g. to be the case with the MQ-135 gas sensor. The data sheet only states in the Application section that the sensor is suitable for detecting CO2. However, CO2 is no longer explicitly discussed further below.

The following list is subjective and is limited to inexpensive sensors, as I expect these to be available in large numbers.

A widely used, inexpensive CO2 sensor is the MH-Z19B, which can easily be expanded to a CO2 measuring device as described above.

The MH-Z14A also has a voltage output and should therefore be easy to read out without an additional µC.

The Telaire T6703 CO2 has no voltage output, so it needs a µC to read it out.

Unlike the above sensors, the MG811 does not use infrared radiation but a “solid electrolyte cell principle” and has a voltage output.

And so on …

Experience with CO2 measurements

I noticed the following (see also CO2-Measurement-simple):

Breathe against the device. Sometimes a CO2 measuring device shows higher values for a while. This happens when the it is close to the exhaled air. Since the exhaled air contains approx. 40,000 ppm CO2, it can quickly happen that the housing or the measuring chamber is filled with CO2-enriched air.
It is therefore advisable to position a CO2 measuring device accordingly
Background CO2 concentration. Depending on weather and environment I see sometimes an outdoor CO2 concentration as high as 500 to 600 ppm. You may search for “CO2 dome” in the internet or start with http://www.co2science.org/subject/u/urbanco2dome.php.
This should be regarded when (a) a calibration of the sensor is done and (b) concentrations of CO2 in a room are evaluated.

Related blogs

Dirk Paessler’s blog on CO2 measurements at Corona helped me a lot when creating this document.

I look forward to criticism and suggestions for improvement in the comments.