It doesn’t take long for airborne coronavirus particles to make their way into a room. At first, only people sitting near an infected speaker are at high risk, but as the meeting or class continues, the tiny aerosols can spread.
This does not mean, however, that everyone faces the same level of risk.
As an engineer, I conducted experiments on how aerosols move, including those whose size can carry viruses.
What I have found is important to understand as more and more people return to universities, offices and restaurants and more and more meetings move indoors when the temperatures drop. It indicates the most risky areas in rooms and explains why adequate ventilation is crucial.
As we saw last week with President Donald Trump and others in Washington, the coronavirus can spread quickly up close if precautions are not taken. College campuses have also been grappling with COVID-19. Cases among 18 to 22 year olds more than doubled in the Midwest and Northeast after schools reopened in August.
As the number of cases increases, so does the risk for anyone spending time in these rooms.
Most current models describing the role of ventilation in the fate of airborne microbes in a room assume that the air is well mixed, with a uniform particle concentration throughout. In a poorly ventilated room or in a small space, this is probably true. In these scenarios, the entire room is a high risk region.
However, in large spaces, such as classrooms, good ventilation reduces the risk, but probably not uniformly. My research shows that the level of risk depends a lot on the ventilation.
To understand how the coronavirus can spread, we injected aerosol particles similar in size to humans into a room, and then monitored them with sensors. We used a 30ft by 26ft college classroom designed to accommodate 30 students who had a ventilation system that met recommended standards.
When we released particles at the front of the classroom, they hit the back of the room in 10 to 15 minutes. However, due to the active ventilation in the room, the concentrations in the back, about 20 feet from the source, were about a tenth of the concentrations near the source.
This suggests that with proper ventilation, the highest risk of contracting COVID-19 could be limited to a small number of people near the infected speaker. As the time spent indoors with an infected speaker increases, the risk spreads throughout the room, even with good ventilation.
In the past, the transmission of respiratory disease has focused on the role of the larger particles that are generated when we sneeze and cough. These droplets quickly fall to the ground, and social distancing and wearing a mask can largely prevent their infection.
The biggest concern now is the role of the tiny particles called aerosols that are generated when we speak, sing or even breathe. These particles, often smaller than 5 microns, can escape from fabric masks and remain in the air for up to about 12 hours. The Centers for Disease Control and Prevention finally recognized this risk on October 5 after Trump was hospitalized and several other people in or near the administration tested positive for COVID-19.
Although these smaller particles carry on average less virus than the larger particles 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 the transmission of airborne diseases.
To minimize the transmission of COVID-19 indoors, the CDC’s main recommendation is to eliminate the source of the infection. Distance learning has actually done this on many campuses. For face-to-face teaching, technical measures such as ventilation, separation screens and filtration units can directly remove particles from the air.
Of all engineering controls, ventilation is probably the most effective tool in minimizing the spread of infection.
Understanding how ventilation reduces your risk of contracting COVID-19 starts with air change rates. An air exchange of one per hour means that the air supplied to the room for one hour is equal to the volume of air in the room. The air change rate ranges from less than one for homes to around 15-25 for hospital operating rooms.
For classrooms, the current regulation of primary air flow corresponds to an air renewal of about six per hour. This means that every 10 minutes the amount of air brought into the room is equal to the volume of the room.
The height of the concentration depends in part on the number of people in the room, their emission and the rate of air exchange. With social distancing halving the classroom population and everyone wearing masks, the air in many indoor spaces is actually cleaner now than it was before the pandemic.
It is important to remember that not all parts of a room are at the same risk.
The corners of the room will likely have less air exchange – so particles can stay there longer.
Being near an air outlet vent could mean that airborne particles from the rest of the room could overwhelm you. A study of ventilation airflow at a restaurant in China has traced its role in several COVID-19 illnesses among customers.
About 95% of the particles in the room will be removed by a properly functioning ventilation system within 30 minutes, but an infected person in the room means these particles are also being emitted continuously. The rate of particulate removal can be accelerated by increasing the rate of air exchange or adding other engineering controls such as filter units. Opening windows will often increase the effective air change rate as well.
As schools, restaurants, malls, and other common spaces begin to accommodate more people indoors, understanding the risks and following CDC recommendations can help minimize the spread of infection.
Suresh Dhaniyala is the Bayard D. Clarkson Distinguished Professor of Mechanical and Aeronautical Engineering at Clarkson University
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