Monitoring CO₂ concentration in closed spaces

Author: Sofia Deloudi, Sensor Expert
 

Every year 7 million people die worldwide as a result of exposure to polluted air. With up to 5 times more air pollution being found indoors than outdoors, indoor air quality is currently one of the top five environmental risks to public health as it can impact people’s energy efficiency, performance, well-being, and health.
 

One aspect that negatively affects indoor air quality is carbon dioxide (CO₂). Humans naturally produce CO₂ when metabolizing carbohydrates to generate energy, and exhale CO₂ through respiration. However, if the concentration of CO₂ in the air increases, the inherent CO₂ concentration gradient that exists between the lungs and inhaled air is reduced and the body is less able to expel this gas. This then leads to a higher level of CO₂ in the blood, which can impact the body’s crucial functions.
 

Coronavirus may be spread via microdroplets that can travel tens of meters

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

COVID-19, like many other viruses, is spread via respiratory droplets that are emitted from infected people through breathing, coughing, and sneezing, and is transmitted as well from direct, shared contact with contaminated surfaces. These infectious diseases often become more prevalent during colder months as people tend to spend more time indoors, where the air quality is poorer.
 

Social distancing and increased handwashing have been introduced to limit the spread of COVID-19 via large respiratory droplets (which fall close to where they are expired, typically within 1–2 meters) and to reduce contact with contaminated surfaces. However, the third mode of transmission of many viruses is through small airborne microdroplets (≤5 µm), which can travel tens of meters, easily traveling across a room.
 

In 2004, a team of scientists analyzed a community outbreak of severe acute respiratory syndrome coronavirus (SARS-CoV)-1 virus at a housing complex in Hong Kong. Using computational airflow dynamics they found that a large number of cases were due to the spread of small airborne microdroplets. Recently, multiple studies on the transmission of the current coronavirus (SARS-CoV-2) have shown it is transmitted in the same way, demonstrating beyond a reasonable doubt that small airborne microdroplets released during breathing and talking can result in the spread of infection in enclosed environments. Scientists are now calling for national and international bodies to recognize this mode of transmission and instigate control measures to minimize this route of infection.
 

Viral transmission can be reduced by increasing ventilation

Fortunately, this route of transmission, like CO₂ concentration, can be easily be reduced by increasing ventilation. Some buildings are fitted with heating, ventilating, and air conditioning (HVAC) systems that can perform mechanical ventilation. For buildings without mechanical ventilation, such as homes and restaurants, natural ventilation via the opening of doors and windows can also be effective, although the airflow in this situation is more dependent on how far windows can open, the position of windows and doors, and outside weather conditions.

While the level of viral particles in the air is unfortunately not detectable, it is however relatively easy to measure the CO₂ concentration using a CO₂ sensor. Therefore, 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 between 400–1'000 ppm and is the CO₂ concentration found in outdoor air; yellow is between 1'000–1'600 ppm and is where 80% of people are satisfied with perceived air quality; red is ≥1'600 ppm and is where there are detectable negative impacts on human health and well-being. At this level, the air quality is considered poor 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 offer two CO₂ sensors: the SCD30 and SCD4x, which are fitted with CMOSens® technology for highly accurate CO₂ measurement using infrared detection or PASens Technology based on a photoacoustic measurement principle (±30 ppm and ±50 ppm, respectively). These sensors are small – the SCD4x especially as it fits in a space of one cubic centimeter – to allow easy integration into HVAC systems at a relatively low cost.4,5 These systems can then be programmed to increase ventilation when the CO₂ levels reach 1,000 ppm.

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