It doesn’t take long for airborne coronavirus particles to make their way through a room. At first, only people sitting near an infected speaker are at high risk, but as the meeting or class goes on, the tiny aerosols can spread. However, that doesn’t mean everyone faces the same level of risk.
With unlock, more and more people are returning to universities and offices. As the case numbers rise, the risk to anyone who spends time in those rooms rises as well. This is proper ventilation is crucial.
Most current models describing the role of ventilation on the fate of airborne microbes in a room assume the air is well mixed. In a poorly ventilated room or small space, this means the entire room is high-risk. However, in larger spaces, such as classrooms, good ventilation reduces risk, but likely not uniformly.
In the past, the transmission of respiratory diseases has focused on the role of larger particles generated when we sneeze and cough. These droplets fall quickly to the ground, and social distancing and mask wearing can largely prevent infection from them.
The bigger concern now is the role of tiny particles known as aerosols that are generated when we talk, sing or even just breathe. These particles, often smaller than 5 micrometers, can escape from cloth face masks and linger in air for up to about 12 hours. The CDC finally acknowledged that risk on October 5 after Trump was hospitalized and several other people in or close to the administration tested positive for Covid.
While these smaller particles, on average, carry less virus than larger particles that people emit when they cough or sneeze, the high infectivity of SARS-CoV-2 combined with the high viral load before symptoms appear makes these particles important for airborne disease transmission.
How much ventilation is enough?
To minimize Covid transmission indoors, the CDC’s top recommendation is to eliminate the source of infection. Remote learning has effectively done this on many campuses. For face-to-face teaching, engineering measures such as ventilation, partition shields and filtration units can directly remove particles from the air.
Of all the engineering controls, ventilation is probably the most effective tool to minimize infection spread.
Understanding how ventilation lowers your risks of getting Covid-19 starts with air exchange rates. An air exchange of one per hour means that the air supplied to the room over one hour equals the volume of air in the room. Air exchange rate ranges from less than one for homes to around 15-25 for hospital operating rooms.
For classrooms, the current regulations of primary air flow correspond to an air exchange of about six per hour. That means that every 10 minutes, the amount of air brought into the room equals that of the volume of the room. How high the concentration gets depends in part on the number of people in the room, how much they emit and the air exchange rate.
Parts of the room to avoid
• It’s important to remember that not all parts of a room are at equal risk.
• The corners of the room will likely have a lower air exchange – so particles can linger there longer.
• Being close to an air exit vent could mean that airborne particles from the rest of the room could wash over you. A study of ventilation airflow in a restaurant in China traced its role in several Covid illnesses among the patrons there.
• About 95% of particles in the room will be removed by a properly functioning ventilation system in 30 minutes, but an infected person in the room means those particles are also continuously emitted. The pace of particle removal can be accelerated by increasing the air exchange rate or adding other engineering controls such as filtration units. Opening windows will also often increase the effective air exchange rate.
• As schools, restaurants, malls and other communal spaces start accommodating more people indoors, understanding the risks and following the CDC’s recommendations can help minimize spread