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.
Airborne transmission of viruses increases during the winter months, because people spend more time indoors and it is usually too cold to keep windows open. In winter, the air is drier, especially in heated indoor spaces. Dry air damages the linings of the respiratory tract and can make it easier for virus to get into the respiratory tract. It also means smaller aerosols float in the air for longer periods of time. Therefore, airborne transmission of COVID-19 is expected to be more common during the winter months.
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.
Places of worship and other locations where people gather pose a particularly high risk for spreading COVID-19. People from many different households gather in one place, and this means you are exposed to other people, including older adults and people with chronic health conditions, who don’t live with you.
Talking and singing increase the risk of airborne transmission of COVID-19. When you talk or sing, you breath out more air and the vibrations of the vocal cords during singing greatly increases the amount of aerosols generated. Some large outbreaks have occurred with church choirs, even when the choir members followed social distancing guidelines. Any singing should be limited in volume and duration as much as possible. If singing must occur, people should be 12 feet away from other people, wear masks, and improve indoor air ventilation.
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.
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.
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.
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 ozone generation. 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.
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:
Most window air conditioners do not use outdoor air and do not do a good job of filtering aerosols in recirculated air.
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.
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