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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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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.

Particles of SARS-CoV-2 can remain stable (same size, morphology and appearance) in aerosols for up to 16 hours, suggesting that these particles remain infectious after a long time in suspension.

Fears, A. C., Klimstra, W. B., Duprex, P., Hartman, A., Weaver, S. C., Plante, K. S., ... & Roy, C. J. (2020). Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions. Emerging infectious diseases, 26(9), 2168.

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.

Air-conditioned buses in Seoul to run with windows open amid virus woes.

Quebec recommendations for ventilation in schools and health care facilities. Ventilation and transmission of COVID-19 in schools and health care settings.

Report of the Scientific and Technical Expert Group (January 2021). Last accessed on 14/04/21

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%.

B. Semin, E. Kierlik, J-M. Courty. (2000). How to air rooms properly. Pour la science, 518.

Covid-19 Opinion on the proposed reinforced health protocol for shops, Haut Conseil de la santé publique

HCSP, 22/11/2020

Study to measure the efficiency of different masks.

Konda, A., Prakash, A., Moss, G. A., Schmoldt, M., Grant, G. D., & Guha, S. (2020). Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano.

A child was infected after a bus ride that also included two people from Wuhan who were found to have the disease. COVID-19.

Jiehao, C., Jin, X., Daojiong, L., Zhi, Y., Lei, X., Zhenghai, Q., ... & Mei, Z. (2020). A case series of children with 2019 novel coronavirus infection: clinical and epidemiological features. Clinical Infectious Diseases, 71(6), 1547-1551.

Several people were infected after a 1 hour and 40 minute bus ride when the contagious individual with COVID-19 was more than 6 meters away.

Shen, Y., Li, C., Dong, H., Wang, Z., Martinez, L., Sun, Z., ... & Xu, G. (2020). Airborne transmission of COVID-19: epidemiologic evidence from two outbreak investigations.

A detailed study of people who travelled by train between December 19, 2019 and March 6, 2020 in China revealed that 234 passengers were likely contaminated on the train. In at least 13 cases, the contamination occurred while the contagious individual was off the train. Those who were in the same row as the contagious individual were 10 times more likely to get the COVID-19 than those placed one or two rows down.

Hu, M., Lin, H., Wang, J., Xu, C., Tatem, A. J., Meng, B., ... & Lai, S. (2020). The risk of COVID-19 transmission in train passengers: an epidemiological and modelling study. Clinical Infectious Diseases.

A study of 103 cases of COVID-19 contamination possibly work-related in Asia up to April 2020 shows that the occupations most at risk of catching the coronavirus SARS-CoV-2 are, after care givers, bus and taxi drivers and workers in the transport sector.

Lan, F. Y., Wei, C. F., Hsu, Y. T., Christiani, D. C., & Kales, S. N. (2020). Work-related COVID-19 transmission in six Asian countries/areas: A follow-up study. PloS one, 15(5), e0233588.

An analysis of 318 cases of "cluster" infection at the beginning of the epidemic (when people were not wearing masks) showed that 34% of cases occurred on public transport, with an average of 3.8 people infected per trip.

Qian, H., Miao, T., Liu, L., Zheng, X., Luo, D., & Li, Y. (2020). Indoor transmission of SARS-CoV-2. Indoor air.

Study on an animal model (hamster) which shows that SARS-CoV-2 is transmitted by aerosols without direct contact between hamsters.

Sia, S. F., Yan, L. M., Chin, A. W., Fung, K., Choy, K. T., Wong, A. Y., ... & Yen, H. L. (2020). Pathogenesis and transmission of SARS-CoV-2 in golden hamsters. Nature, 583(7818), 834-838.

Study on an animal model (ferrets) shows that SARS-CoV-2 is transmitted by aerosols without direct contact between ferrets placed in different cages but under the same flow of air.

Richard, M., Kok, A., de Meulder, D., Bestebroer, T. M., Lamers, M. M., Okba, N. M., ... & Herfst, S. (2020). SARS-CoV-2 is transmitted via contact and via the air between ferrets. Nature communications, 11(1), 1-6.

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?