The amount of carbon dioxide (CO2) in a poorly ventilated indoor space depends on the number of people who exhale their breath in that space. And aerosol virus contained in exhaled breath is a common way that COVID-19 germs are spread. Therefore, might the CO2 level in a given space serve as an approximation of the likelihood of coronavirus spread?

Broadly speaking, poor ventilation of indoor spaces can increase the likelihood of aerosol transmission, and a higher CO2 level is an indicator of insufficient ventilation. In indoor spaces, the CO2 concentration should not exceed 1,000 parts per million (ppm), which is higher than the 400 ppm concentration in the fresh air.

Deployment Of CO2 Sensors

Measuring aerosols in the air is complex and expensive. However, measuring CO2 is inexpensive. Using low-cost and compact CO2 sensor devices can help to warn against high concentrations of CO2 in the air, and by extension, the likely higher levels of aerosols and greater virus spread.

Seeking to provide a warning of high CO2 levels (and thus a likely higher concentration of aerosols) points to a need to increase deployment of inexpensive CO2 sensors in a variety of applications. The simplest implementation might be a CO2 traffic light that signals air quality changes with red, yellow, and green lights.

Data Analysis, Remote Access, & Monitoring

CO2 sensors tied to the cloud for data analysis, remote access, and monitoring and linked to other IoT devices CO2 sensors can also be tied to the cloud (along with other sensors) for data analysis, remote access, and monitoring. They might be linked to other Internet of Things (IoT) devices such as air purifiers and thermostats.

CO2 sensors that are the most accurate tend to be bulky and expensive. However, using a microelectromechanical system (MEMS) technology, a CO2 sensor based on photoacoustic spectroscopy (PAS) can analyze CO2 concentration in parts per million. It provides precise results in a small format.

CO2 Levels Proxy For COVID-19 Transmission

Each person in a building will exhale approximately 8 liters of air per minute. The air has been in contact with lung tissue and contains tiny liquid droplets (aerosols) that can float in the air. Exhalation air also has a concentration of about 40,000 ppm of carbon dioxide, which increases the CO2 levels in a room. In effect, CO2 levels can be used as a proxy for COVID-19 transmission risk.

Evidence shows that long-range aerosol-based transmission routes play an important role in transmitting the SARS-CoV-2 coronavirus. Therefore, analyzing aerosol concentration provides an indicator of the safety of an indoor space. Because of the close link between indoor CO2 concentration and aerosol density, using CO2 sensors to monitor an indoor environment can help to monitor possible indoor spread.

CO2 Monitoring

The Federation of European Heating, Ventilation, and Air Conditioning Associations (REHVA) in Germany has published guidelines for schools to use CO2 monitoring, including the use of a traffic light indicator. In addition to CO2 levels, other factors should also be considered when determining infection risks, such as the number of infected people in a region, activity type, air filtration, and mask-wearing.

Activities such as talking, singing, or shouting can increase risk. Ventilation using outdoor air dilutes both CO2 and virus concentrations. Germany’s Federal Environment Agency’s general guidelines for health assessment of carbon dioxide in indoor air include advice relevant to COVID-19. The guideline classifies any CO2 concentration between 1,000 and 2,000 ppm as questionable; anything above 2,000 is unacceptable.

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Lessons From The Past: The Value Of Ventilation In A Pandemic
Lessons From The Past: The Value Of Ventilation In A Pandemic

If history truly repeats itself, might we learn lessons from the past – even lessons about managing a novel coronavirus that upends our way of life and changes the world forever? The most commonly cited parallel to the COVID-19 pandemic is the Spanish flu pandemic of 1918. Both diseases are caused by viruses that had not been seen before. In both cases, no one had immunity to a highly infectious germ that was spread through respiratory droplets. Both outbreaks occurred in multiple waves over several years. Furthermore, in both cases, it became clear that ventilation, fresh air, open spaces and sunlight are useful factors in promoting good health. Fresh Air Movement During the time of the Spanish flu, there were signs posted in buses and throughout New York that advised: "Keep your bedroom windows open [to] prevent influenza, pneumonia [and] tuberculosis." There was even a national campaign known as the “Fresh Air Movement,” calling for people to be outside more, and urging greater ventilation indoors. The movement included a kind of traveling show that spread the word about the “national poison,” which was the result of people breathing stale air inside closed rooms. These concerns predated by decades our enthusiasm for “indoor air quality.” In became common after 1918 to position radiators providing steam heat under open windows to combine warmth with fresh air, even on the coldest of days.   The Open-Air Treatment of Pandemic Influenza It was also common practice by 1918 to place the sick outside in tents or in specially designed open wards But the advantages of fresh air go back even further, as described in a 2009 article in the American Journal of Public Health (AJPH) titled “The Open-Air Treatment of Pandemic Influenza.” During the 1918 pandemic, as today, many cities banned public assembly, closed schools, isolated those infected and mandated the wearing of face masks. It was also common practice by 1918 to place the sick outside in tents or in specially designed open wards, according to the AJPH article. The practice dates back to English physician John Coakley Lettsom (1744-1815), who was among the first advocates of the “open-air method.” The 1800s saw emergence of tuberculosis sanitoriums, which treated the lung disease with a combination of fresh air, gentle exercise in the open, nutrition, and a minimum of medicines. Lack of ventilation Spending time in well-ventilated houses in the country became seen as superior to patients being confined to warm, badly ventilated rooms to protect them from the supposedly harmful effects of cold air. Lack of ventilation forced patients to breathe foul air, contaminated with germs, over and over. Research later confirmed the importance of measures to prevent influenza virus from spreading through buildings. Improvements in air-handling equipment, portable filtration units, and introduction of physical barriers and other partitions or doors also provided protection. These lessons were clear long before the advent of the novel coronavirus that causes COVID-19. Their successful deployment during the pandemic have further supported their value. importance of HVAC Although the COVID-19 pandemic caught the world off-guard, there were plenty of historical precedents However, lockdowns during the pandemic have also tended to keep the population closed up in buildings, sometimes with less-than-adequate ventilation and access to fresh air. In retrospect, some of those decisions seem regrettable.  Although the COVID-19 pandemic caught the world off-guard, there were plenty of historical precedents. Copious research over the years supported the best approaches to stemming the spread of the virus, although it took time for historical insights to work their way into the general practice implemented in the current pandemic. There is also historical precedent for the importance of HVAC in the current pandemic. Ventilation and fresh air have become higher priorities, as has the HVAC market’s role in providing a safer indoor climate with minimal disease spread.

Pandemic Spotlights Need To Balance Costs While Improving Air Quality In Schools
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Attitudes about indoor air quality need to change, especially given the current pandemic that forces people to spend most of their time indoors. But addressing the pandemic through increased ventilation and better indoor air quality can be expensive. For example, the Los Angeles Unified School District, the second-largest in the nation, has spent $6 million on HVAC upgrades and new air filters in response to the pandemic and expects to pay about $1.7 million a month for ongoing inspections and filter replacements. Updating & Improving HVAC Systems Updating HVAC systems to minimize virus spread has been an expensive proposition all around. Some school districts in California report the costs are insurmountable. Sometimes seeking to replace or update an HVAC system opens a can of worms: Electrical systems must be rewired, asbestos must be removed, and/or an expensive roof needs to be replaced. Schools in low-income areas are especially likely to be in poor condition, and unable to afford improvements. Some school districts have used money from the federal CARES Act – a $2 trillion federal economic package passed in March – to make ventilation improvements. Hope remains that additional state and/or federal money will be available, but funding is still likely to be inadequate. Airborne Transmission Study showed that some classrooms had air change rates below 0.5 changes per hour The airborne transmission was initially underplayed as a means of spreading the novel coronavirus. There was more emphasis on the dangers of touch during the early days of the pandemic. However, the airborne (aerosol) spread is now believed to make up about 75% of transmissions. A group of 239 scientists from around the world advocated more action to address aerosol spread in a July 2020 open letter to the World Health Organization (WHO). The concern is a global challenge. For example, a survey of 20 classrooms in the United Kingdom, carried out by National Air Quality Testing Services (NAQTS), revealed very low air change rates that could increase the risk of virus transmission. The study showed that some classrooms had air change rates below 0.5 changes per hour (3 to 5 changes per hour would be desirable). Even small increases in flow rate could reduce the risk of infection significantly. Raising airflows from zero to 100 cu m/hour cuts the risk by up to a third, according to NAQTS. Fresh Air Ventilation & Filtration The Scientific Advisory Group for Emergencies (SAGE) advised the UK Government last fall of a need to ensure undisrupted education for children of all ages. A critical part of keeping children in school is clear guidance and support packages, including better ventilation and air filtration, particularly through winter. The German government advises schools to open their windows for at least five minutes every hour Other countries can learn a lot about the value of opening windows to allow in more fresh air from the Germans. For years, Germans have habitually opened their windows twice a day, even in winter. In fact, “lüften,” or airing a room, is among the cheapest and most effective ways of decreasing the spread of the coronavirus. The German government advises schools to open their windows for at least five minutes every hour; for example, when classes are changing. Improving Indoor Air Quality Airing of rooms is a likely factor in the lower number of coronavirus cases reported in Germany compared to, say, the United Kingdom. In the end, improving indoor air quality involves some combination of letting in more fresh air, upgrading air filtration systems, and installing technologies such as UV light to kill pathogens. However, implementing these measures only mitigates the likelihood of contracting COVID-19. Some risk remains.

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