How Coronavirus Really Spreads Indoors

Researchers studied how transmission occurs in three scenarios — a living room, a restaurant and a classroom.

By Caitlin Kilgore
covid article

COVID-19 cases continue to rise across the United States, with many states seeing record numbers of hospitalizations. As the holidays approach and cold weather drives people inside, experts say we’re likely to see even bigger spikes in coming weeks. All that said, there's never been a better time than now to understand how, exactly, transmission of coronavirus occurs indoors.

An article in El País, a Spanish daily newspaper, calculated the risk of infection from COVID-19 in several indoor scenarios using the COVID-19 Airborne Transmission Estimator — a tool developed by José Luis Jiménez, an atmospheric chemist at the University of Colorado and an expert in the chemistry and dynamics of air particles. (Check out the interactive visuals here!

Previously, it was thought that droplets (particles larger than 300 micrometers) from an infected person contributed to the majority of coronavirus transmission, but evidence now points to aerosols (respiratory droplets that are less than 100 micrometers in diameter) as the main contributor of transmission. This is because droplets fall to the floor whereas aerosols can remain suspended in the air for hours — behaving similarly to smoke filling a room. 

Here, three indoor scenarios and how safety measures factor in. (Note: As the El País authors write, these simulations are not exhaustive and do not account for the many variables that can affect transmission.)

1

Social Gathering in a Living Room

The scenario: Six people get together in a private home and one person is infected.

Without masks and ventilationRegardless of whether the six people maintain safe distances, if they spend four hours together talking loudly without wearing face masks in a room with no ventilation, the infected person will likely spread it to the five other people.

With masks: Even if the six people wear masks, after four hours of talking in a room without ventilation, four of them are still at risk of infection. Masks alone cannot prevent infection if exposed to the virus for long periods of time.

With masks, ventilation and reduced time: When all six people wear face masks, shorten the length of time that they are gathering by half and increase ventilation in the space (by opening windows, for example), the risk of infection drops to one or below.

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2

A Bar or Restaurant

The scenario: Fifteen patrons and three staff members in a bar with 50 percent reduced capacity.

Without masks and ventilation: If no safety measures are taken, in the worst case scenario all 14 of the other customers will be infected after four hours. 

With masks: If customers and staff wear masks, eight of the people there will become infected after four hours. 

With masks, ventilation and reduced time: If the bar or restaurant is well-ventilated (with either air filtration or air flow from open windows) and the time customers spend there is cut in half, the risk of infection falls to one person.

3

A Classroom

The scenario: The teacher is infected in a classroom of 24 students. Note: With the classroom model, the aerosol transmissions change whether the infected person is a teacher or a student. Teachers talk more than students and raise their voices, increasing the amount of aerosols expelled, but an infected student may only speak occasionally.

With an infected teacher, without masks or ventilation: By far the riskiest scenario, after two hours in the classroom with an infected teacher and no safety measurements, 12 students could become infected.

With masks: If everyone wears a face mask, the number of students infected drops to five. The model also points out that in real outbreaks, any of the students could become infected regardless of their seat in the classroom or proximity to the teacher because aerosols spread randomly throughout an unventilated room.

With masks, ventilation and breaks: When the classroom is ventilated (from either open windows or air conditioning) and the class takes a break after an hour to help refresh air flow, the risk drops to one student.

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