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Why can measuring CO2 levels help us fight COVID-19 ?

Text updated on 2021-05-03


Ventilation is one of the major solutions to combat aerosol-borne diseases such as COVID-19. But how do you know if a room is sufficiently ventilated? The level of CO2 in a room, a gas produced when we breathe, gives us information about the levels of air exchange in the room. Its measurement can be used as an indicator of ventilation!

Despite all the efforts made, the COVID-19 epidemic continues and the phases of containment - decontainment - recontainment follow one another. The coronavirus is transmitted mainly by droplets and aerosols containing particles of SARS-CoV-2 (see the question How is COVID-19 caught?). Aerosols are micro-droplets produced when people breathe. In enclosed spaces, they become airborne up to several meters from the person who produced them and can remain there for several hours.

The mask filters out some of the aerosols, but it becomes less effective if the quantity of aerosols is too large. Another very effective measure to limit the transmission of aerosol-borne viruses is to reduce the concentration of aerosols by diluting them. To reduce the concentration of aerosols in a room, simply ventilate the room with outside air! The windows should be opened wide to completely change the air in the room.

Is the air in enclosed spaces sufficiently renewed?

Whether in schools, in places of business, or in public transport, it seems that the current ventilation protocols are often insufficient. It is important to open the windows in closed places, for a few minutes every 30 minutes, and continuously in public transports (subway, RER, bus and tramway trains) to dilute the quantity of aerosols and limit the propagation of the coronavirus.

How does the CO2 level tell us about the amount of aerosols?

CO2 is produced by human respiration along with aerosols. The more we breathe, the more CO2 and aerosols accumulate in the air of an enclosed space. The CO2 level therefore tells us about the concentration of aerosols. Unlike viral aerosols, the advantage of CO2 is that it is very easy to measure its concentration in real time with sensors!

What is the ideal CO2 level?

Since CO2 is produced with each exhalation, it tells us how much air has already passed through human lungs. The normal level of CO2 in outdoor air is 400 ppm (parts per million). An increase of 400 ppm compared to the base rate (i.e. a value of 800 ppm) corresponds to air of which 1% has already passed through human lungs. In France, the High Council of Public Health recommends not to exceed 800 ppm in shops. If the air in a room is regularly renewed, the CO2 concentration will remain below 800 ppm and the concentration of aerosols will be low. As part of the fight against COVID-19Measuring the CO2 level is one way of checking whether the air is sufficiently renewed.


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Sources

Modeling study that shows that ventilation of a room combined with the wearing of a mask by all individuals in the room can reduce the risk of infection by 5 to 10.

Lelieveld, J., Helleis, F., Borrmann, S., Cheng, Y., Drewnick, F., Haug, G., ... & Pöschl, U. (2020). Model Calculations of Aerosol Transmission and Infection Risk of COVID-19 in Indoor Environments. International Journal of Environmental Research and Public Health, 17(21), 8114.

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Ventilation of public transport is already recommended in several countries: in the United States

Buses will run with windows opened (weather permitting).

Ventilation of public transport is already recommended in several countries: in Japan.

Passengers who will use Tokyo Metro lines during rush hour are asked to be understanding and cooperative with regard to opening the windows of train cars in order to ventilate the interior.

Ventilation of public transport is already recommended in several countries: in Korea.

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Article in French that explains how to air rooms properly and how to use the CO2 concentration figures. The CO2 concentration of exhaled air is about 4%, or 40,000 ppm. Diluting the exhaled air by a factor of 50 (2% of the air in the room is exhaled air) adds a CO2 concentration of 40,000/50=800 ppm. The concentration read on the sensor is the sum of the outside concentration and this difference, i.e. 400+ 800=1,200 ppm. Similarly, a total concentration of 800 ppm corresponds to a difference of 400 ppm with the outside air, i.e., a dilution of 400/40 000=0.01=1%.

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This study conducted at a university in Taipei, Taiwan, demonstrates the role of ventilation in reducing the transmission of tuberculosis, a respiratory disease that is transmitted by aerosols. CO2 concentration was used to measure ventilation. This study showed that ventilation that allows a CO2 concentration below 1000 ppm leads to a 97% decrease in the incidence of tuberculosis in contact cases.

Du, C. R., Wang, S. C., Yu, M. C., Chiu, T. F., Wang, J. Y., Chuang, P. C., ... & Fang, C. T. (2020). Effect of ventilation improvement during a tuberculosis outbreak in underventilated university buildings. Indoor air, 30(3), 422-432.

A comparative study in a student residence between a well-ventilated building (recently renovated with a ventilation system supplied 100% by outside air) and a poorly ventilated building (the air is recycled without any outside air supply). The average CO2 level is 1,230 ppm in the well-ventilated building and 1,492 ppm in the poorly ventilated building. The ventilation rate per room averages 11.8 L/s (litres per second) in the well-ventilated building and 4 L/s in the poorly ventilated building. The acute respiratory infection (ARI) rate averaged 2.1% in the well-ventilated building and 97.9% in the poorly ventilated building.

Zhu, S., Jenkins, S., Addo, K., Heidarinejad, M., Romo, S. A., Layne, A., ... & Srebric, J. (2020). Ventilation and laboratory confirmed acute respiratory infection (ARI) rates in college residence halls in College Park, Maryland. Environment International, 137, 105537.

In one church, a choir member who had the COVID-19 and sang repeatedly infected 12 people. Video recordings of the services showed that the people who were infected were sitting in the same aisle, up to 15 m from the chorister, without close physical contact, suggesting airborne transmission.

Katelaris, A. L., Wells, J., Clark, P., Norton, S., Rockett, R., Arnott, A., ... & Bag, S. K. (2021). Epidemiologic Evidence for Airborne Transmission of SARS-CoV-2 during Church Singing, Australia, 2020. Emerging Infectious Diseases, 27(6).

Further reading

Is the SARS-CoV-2 coronavirus transmitted by aerosols?

Can we catch COVID-19 without direct interaction with a contagious person?

How can we reduce the contagion in enclosed spaces: schools, colleges, places of business, and public transport?