CO2 Sensors provide a low cost, real time measurement of indoor air quality. They can be used to indicate potential trouble spots in classrooms as children return to school.
For years, carbon dioxide sensors have been used to monitor CO2 levels in offices, schools and classrooms. Research has shown there is a direct correlation between elevated CO2 levels and poor air exchange in building air exchange systems.
CO2 as an Indicator of Fresh Air
Carbon dioxide gas levels in a room can be used as an indicator of fresh air. Too many students in a classroom or too little air exchange will result in a build-up of CO2 levels over time.
Fresh air contains approximately 0.04% (400 parts per million, or ppm) of CO2 molecules. In an occupied, well-ventilated room, it is not uncommon for the CO2 level to rise over 1,000 ppm. ASHRAE recommends an 800-1,000 ppm limit for classrooms to ensure overall student health and performance.
Note that it isn’t the carbon dioxide itself that is the problem. Humans can still breathe in enclosed areas with CO2 levels approaching 3%. Instead, elevated CO2 levels are an indicator of poor air exchange. High CO2 levels in the air correlate with high levels of airborne dust, dander, mold, particulate matter, chemicals and aerosolized liquids that can carry the coronavirus.
Instead of subjective measurements like “the room feels stuffy,” monitoring CO2 levels give a more accurate description of indoor air quality. This information can be used to either remotely control the air handling machinery or provide a warning where increased fresh air is required.
Fresh Air and Covid-19 Risk
Just as the risk of transmission of coronavirus is lowered outdoors, the risk is lowered with increased fresh air in classrooms. The best evidence is that Covid-19 transmission is airborne.
There are three likely routes to coronavirus infection:
• First is the risk of touching contaminated surfaces. This risk can be mitigated by frequently washing hands and disinfecting hard surfaces.
• The second risk factor is being directly exposed to particles from an infected person coughing nearby. This risk is mitigated by the 6-foot “rule” and through the use of masks. Masks don’t protect the wearer. Instead, they limit the number of particles an infected person can spread.
• The third risk factor is airborne coronavirus carried by aerosolized liquids. The virus can travel on the aerosol lingering in the air for several minutes after an infected person coughs or sneezes. Outdoors, the aerosol is quickly carried away in the wind. Indoors, this aerosol can linger and be potentially breathed in by another student.
In order to mitigate the risk factor of airborne virus transmission, the solution is to clean the air.
Cleaning Classroom Air
There are two primary ways to clean air in an enclosed area: air filtration or fresh air exchange.
Air filtration through the use of High Efficiency Particulate Air (HEPA) filters has been shown to reduce the number of particles in the air. HEPA filters do not kill viruses. Instead, they capture them in the filter fibers where the virus dries out and soon dies.
Research shows that the particle size of the Covid virus is around 0.1 micrometer (µm), too small to be captured by HEPA filtration. But the virus doesn’t travel by itself. Instead, it is carried on respiratory droplets around 5 µm in size. Since a HEPA filter is at least 99.97% efficient at capturing particles 0.3 µm in size it is 99.97% efficient at capturing human-generated viral particles.
The challenge with air filtration is the physical size of the filter. Many portable HEPA air filtration units do not have the ability to circulate enough air. In addition, because they are floor standing, they may not provide an adequate job of capturing particles high in the air.
There are several manufacturers currently promoting HEPA filtration units as suitable for cleaning classroom air. While they can be helpful, tests show that common home HEPA filter air cleaners are not sufficient in a classroom setting. High air-flow, wall or ceiling mounted units, or multiple smaller units in the same room are required.
Fresh Air Exchange
As an alternative to putting HEPA filtration units into each classroom, the same effect can be achieved by replacing the room air with fresh, uncontaminated air. Air exchange in a classroom is controlled by the HVAC system which is calibrated to provide a constant flow of fresh air. Both CDC and ASHRAE have recommended that schools operate ventilation systems at maximum airflow to constantly change the air.
The challenge in older buildings is that there are “pockets” of interior space or rooms that cannot achieve a good fresh air flow. Therefore, the CDC also suggests hand-held, portable CO2 detectors to test different areas in the building where proper airflow does not occur.
For classrooms a wall-mounted CO2 monitor can also be used to alert staff of low air exchange by showing the CO2 level in real time.
With the coming school year, parents and school boards will face the challenge of sending children back to school in a safe manner. Through a combination of hygiene, masks, HEPA filtration and verified fresh air exchange, classrooms can be made safe for children.
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