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Preventing the Spread of COVID-19 By Circulating Air in Schools and Other Buildings

How COVID-19 Spreads Through the Air

stop the spread

The virus that causes COVID-19 can spread from one person to another in tiny particles of water and virus called aerosols. We make these aerosols when we breathe, and we make more of them when we talk, yell, or sing. Aerosols are different than larger droplets that spread COVID-19. Larger droplets fall to the ground quickly, three to six feet from the person who makes them. Aerosols can stay floating in the air for hours and can travel long distances. Aerosols have less virus in them than the larger droplets, so you have to inhale more aerosols to get sick. Aerosols can build up if the air inside is not circulated the right way.

Preventing the Spread of COVID-19 in the Air

control of airborne transmission

In addition to wearing face masks and staying at least six feet away from other people, good air circulation inside buildings, schools, and homes, (ventilation) will reduce the spread of COVID-19 in aerosols.

One way to measure ventilation is to figure out how often the air in a space is completely replaced. This is called Air Changes per Hour (ACH). In a 30-foot by 30-foot classroom that has 25 students in it, the air should be replaced at least every 15 minutes, which equals an ACH of 4. If the air is replaced at least every 10 minutes, there is an ACH of 6, which is better. There is not a standard for ACH, but we do know that a higher ACH lowers the risk of disease spreading through the air.

Ventilation with clean outdoor air is healthier because it removes viruses and other particles, and it also removes gasses, like the carbon dioxide that everyone makes when they breathe out.  However, outdoor air is hard to heat or cool, so most ventilation systems use at least some recirculated air.

Ventilation with recirculated air will not reduce the risk of COVID-19 unless that recirculated air goes through a filter that is made to get rid of tiny particles. A filter’s Minimum Efficiency Reporting Values (MERV) rating describes how well it removes different sized particles in the air. A MERV rating of 13 or higher (MERV 13+) means that the filter gets rid of at least 90% of the particles the size of virus-containing aerosols. High Efficiency Particulate Air (HEPA) filters are designed to exceed the highest MERV rating. A HEPA filter gets rid of at least 99.97% of particles that are even smaller than aerosols.

Ways to Increase Ventilation

  • Open the windows
    Opening the windows is an easy and effective way to improve ventilation.  Researchers at Harvard University found that if you open the windows in a room just six inches you can get 5 or more ACH with clean, outdoor air. This approach is best when there is also another open window or door to allow a breeze to go in one opening and out another.
  • Open windows and put a box fan in the window.
    Using a box fan or other machine to get rid of inside air through open windows gives you steady ventilation with outdoor air. If you put a typical 20-inch box fan on high, it gives you an ACH of more than 12 in a room that is 30 feet by 30 feet (about the same size as an average classroom). While air can flow into the room around the sides of the windows or through other openings, the effectiveness of this approach can be improved when there is also another open window or door to allow a breeze to go in one opening and out another.
  • Use a dedicated exhaust fan.
    Install a vent and exhaust fan in a room to push the indoor air outside. Most homes and buildings have exhaust fans in the bathroom and sometimes in the kitchen. They can also be added to any room. Exhaust fans in bathrooms in schools or other busy buildings should always be on.
  • Use a portable air purifier.
    A portable air purifier with a HEPA filter can be used in rooms when you cannot open windows or use fans. If you use an air purifier, follow these recommendations:
    • Get a purifier that is the right size for the room where you are going to use it. A typical classroom (30 feet by 30 feet) would need two purifiers that provide 300 cfm (510 m3/h) of HEPA-filtered air to get to 4 ACH. If there is already an HVAC system providing outdoor or filtered air to the room, one air purifier might be enough additional clean air in that room. The volume of these systems is measured with a clean air delivery rate (CADR). If the air purifier’s CADR is listed in cubic meters per hour, multiply the CADR by 0.589 to get the CADR in cfm.
    • Follow the manufacturer’s directions for maintenance, including how often the machine’s filter should be replaced.
    • Do not buy an air purifier with extra features, such as ozonation. There is no evidence that these extra features make the air purifier remove more particles, and they can produce gasses that can cause lung irritation.
    • Put the air purifier as close to the center of the space as possible. Try to keep the sides and top of the machine at least two feet from furniture and other objects so the air flow is not blocked.
  • Upgrade the HVAC system that is already being used.
    HVAC systems that provide forced air can be modified to make them better at removing droplets and aerosols. Try any or all of these approaches:
    • Adjust vents to let as much outdoor air and as little recirculated air as feasible to maintain a comfortable temperature. Make sure vents are not blocked on either end. Make sure there are no objects on top of indoor vents, and make sure there are no plants growing around outdoor vents. Vents work best when there is plenty of space around them.  If the HVAC system does not have 13+ MERV filtration, it is important to use as much outdoor air as the system can handle.
    • Upgrade filtration on the unit to MERV 13 or higher. This will not be possible on some systems without reducing the amount of air flow, and it could damage a system that can’t handle it. Never put a filter with a higher MERV than the HVAC system is designed for.
    • Make sure the system’s fans are on whenever people are in the building, for two hours before people are in the building, and for one hour after all people leave the building.
    • If the calculated amount of outdoor or MERV 13+ filtered air provided by the system’s fans is not enough to give at least 4 ACH in a room, add other methods with the HVAC system.
  • Do not use window air conditioners.
    Most window air conditioners do not use outdoor air and do not do a good job of filtering aerosols in recirculated air.
  • Limit the number of people that can be in the room if you cannot get the ventilation rate you want.

Is My Ventilation Method Working?

There are ways to measure or calculate if the ventilation methods you are using in a room are working the right way.

  • In a room that uses outdoor air for ventilation, compare the expected levels of carbon dioxide to the observed levels, use a carbon dioxide detector and this CO2 Steady State Calculation tool. Most carbon dioxide detectors cost about $200.
  • For rooms with MERV 13+ or HEPA filtration, calculate the predicted ACH using the system’s designed CADR in cfm and the volume of the room: ACH = (cfm x 60)/(length x width x height).
  • There are other more complex approaches to measure airflow rates of outdoor air and/or filtered air you can use to see if a system is working the way it should.