Indicating Coronavirus Infection Risk by Monitoring CO2 Concentration

Seven million people die globally each year as a result of exposure to polluted air. Indoor air quality is currently one of the top five environmental risks to public health, with up to five times more air pollution being found indoors than outdoors. It can impact people’s energy efficiency, well-being, performance, and health.

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Carbon dioxide (CO2) is one aspect that negatively affects indoor air quality. When metabolizing carbohydrates to generate energy, humans naturally produce CO2 and they exhale it through respiration.

If the concentration of CO2 in the air increases, the inherent CO2 concentration gradient between the lungs and inhaled air is lowered and the body is less able to expel this gas. This results in a higher level of CO2 in the blood, which can affect the crucial functions of the body.

Coronavirus May be Spread Via Microdroplets Which Can Travel Tens of Meters

A number of other components negatively affect indoor air quality in addition to CO2: animal hair, paints and solvents, and – perhaps most pertinent in today’s climate – viruses and bacteria. The current coronavirus-19 (COVID-19) pandemic has infected more than 38 million worldwide as of October 16th, 2020, and the death toll is now over 1 million.1

Like many other viruses, COVID-19 is spread via respiratory droplets emitted from infected people through coughing, breathing, and sneezing, and is also transmitted via direct, shared contact with contaminated surfaces.

During colder months these infectious diseases usually become more prevalent because people typically spend more time indoors where the air quality is poorer.2,3

In order to limit the spread of COVID-19 via large respiratory droplets (which fall close to where they are expired, usually within 1–2 meters) and to reduce contact with contaminated surfaces, increased handwashing and social distancing have been introduced.3

Yet, small airborne microdroplets (≤5 µm), which can travel tens of meters, traveling across a room easily, is the third mode of transmission of many viruses. In Hong Kong, a team of scientists analyzed a community outbreak of severe acute respiratory syndrome coronavirus (SARS-CoV)-1 virus in 2004, at a housing complex.

By utilizing computational airflow dynamics, they discovered that a large number of cases were because of the spread of small airborne microdroplets.4

Multiple recent studies on the transmission of the current coronavirus (SARS-CoV-2) have exhibited that it is transmitted in the same way, showing beyond a reasonable doubt that small airborne microdroplets released during breathing and talking can lead to the spread of infection in enclosed environments.

As a result of these findings, scientists are now calling for national and international bodies to recognize this transmission mode and instigate control measures to minimize this route of infection.2

By Increasing Ventilation Viral Transmission Can be Reduced

Fortunately, like CO2 concentration, this route of transmission can be reduced easily by increasing ventilation. Some buildings are fitted with ventilating, heating, and air conditioning (HVAC) systems which can perform mechanical ventilation.

For buildings which do not have mechanical ventilation, like restaurants and homes, natural ventilation via the opening of windows and doors can also be effective, although the airflow in this situation is more dependent on the position of windows and doors, how far windows can open, and outside weather conditions.2,3

Whilst, unfortunately, the level of viral particles in the air is not detectable, using a CO2 sensor, it is fairly simple to measure the CO2 concentration. So, using CO2 levels as a surrogate to monitor the level of infectious material in the air can be effective.

The level of CO2 in the air can be thought of as a ‘traffic light’ system: Green is the CO2 concentration found in outdoor air and is between 400–1000 ppm; yellow is where 80 % of people are satisfied with perceived air quality and is between 1000–1600 ppm; red is where there are detectable negative impacts on human health and well-being and is ≥1600 ppm. The air quality is considered poor at this level and the risk of viral transmission is increased.

Sensirion CO2 Sensors Can be Used as a Surrogate to Measure the Level of Infectious Material

Sensirion is the world’s leading manufacturer of digital microsensors. They provide two CO2 sensors: the SCD30 and SCD40, both of which are fitted with CMOSens® technology for extremely accurate CO2 measurement via infrared detection (±30 ppm and ±50 ppm, respectively).

These sensors are small, especially the SCD40, as it fits in the space of one cubic centimeter. This is to enable easy integration into HVAC systems at a relatively low cost.4,5 When the CO2 levels reach 1000 ppm, these systems can then be programmed to increase ventilation.2,3

As people are increasingly spending time indoors due to colder weather and now that the global coronavirus pandemic is now in its second wave in many parts of the world, increasing ventilation through natural or mechanical ventilation could help to minimize the spread of COVID-19 infections and consequently save lives.2,3

References and Further Reading

  1. (2020). COVID-19 pandemic: Tracking the Global Coronavirus Outbreak.
  2. Morawska L., et al. (2020). It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19). Clinical Infectious Diseases. DOI: 10.1093/cid/ciaa939.
  3. Covaci A. (2020). How Can Airborne Transmission of COVID-19 Indoors be Minimised? Environmental International.
  4. Yu I.T.S., et al. (2004). Evidence of Airborne Transmission of the Severe Acute Respiratory Syndrome Virus. New England Journal of Medicine. DOI: 10.1056/NEJMoa032867
  5. (2020). CO2 and RH/T Sensor Module.
  6. (2020). CO2 Sensor SCD40 (coming soon).


This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.

For more information on this source, please visit Sensirion Inc.


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