How safe are indoor gatherings? The COVID risk at home, in a restaurant, at school

The Spanish newspaper El País recently gauged the risk of aerosol-based coronavirus infection in a variety of scenarios.


Say you’ve gone home for the holidays despite the pandemic.

You’re in the living room of your parents’ house with them, your sister, uncle and aunt. Your folks have served drinks and everyone is catching up with what’s new in each other’s lives.

People are talking loudly because of physical distancing (and the booze), but not wearing masks.

Unbeknownst to you or anyone else, one person is infected with the SARS CoV-2 virus. Your Aunt Gladys caught the bug at a book club meeting, but she’s asymptomatic.

The room is not ventilated — it’s late fall and cold outside, so naturally all the windows are closed.

The gathering continues for four hours. When it ends, Aunt Gladys won’t be the only one with the virus.

Using the COVID Airborne Transmission Estimator, a tool developed by University of Colorado, Boulder, atmospheric chemist José Luis Jiménez — an expert in the chemistry and dynamics of air particles — the Spanish newspaper El País recently gauged the risk of aerosol-based coronavirus infection in a variety of scenarios.

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In this family gathering simulation, the odds are Aunt Gladys has infected you and everyone else in the room.

What if you all wear masks? That would keep everyone safe from Gladys, right?

Nope. Even if everyone has a cloth face covering and kept 6 feet apart, all but one person would contract the virus.

The outcome changes markedly when everyone wears a mask, opens a door and a window to create a draft in the space used, and limits the gathering to under two hours. In that scenario, the risk of infection drops below one.

The modeling tool is not perfect, and it makes several assumptions, including that people would adhere to the 6-foot rule so that droplet transmission is not be a factor; that no one is immune; and that infected individuals are talking in a loud voice, thereby expelling more aerosols. A default value is applied for mask efficacy (50% for cloth masks) and for the viral load emitted by the infected person.

Noting the tendency of nightclubs to host superspreader outbreaks, El País also ran a simulation in which one infected person spends four hours in a bar or restaurant with capacity reduced by 50 percent. Besides the infected individual, there are 14 other patrons and three staff members. Masks are not worn consistently, and there is no ventilation. Under those conditions, “patient zero” could be expected to infect 14 of the 17 people in the bar.

As in the family living room scenario, the cases would drop if masks are worn conscientiously. But even so, eight new infections would occur. If the time spent there is reduced to two hours, if people wear face coverings and if the bar is ventilated — by a high-quality air-conditioning unit, for example — infection risk is limited to one person.

In a third simulation, El País investigated what could happen if a teacher is the patient zero in a classroom with 24 students. The time frame was shorter — the lesson lasted only two hours — but the results were similar:

  • There's a risk of up to 12 students being infected if no other measures are taken.
  • If masks are worn by everyone, the risk drops to five students.
  • If masks are worn, if the room is ventilated either mechanically or by fresh air, and if the lesson time is cut in half to one hour, the chance of infection is lowered dramatically, with only one student facing a reduced possibility of infection.

The estimator’s calculations were based on analysis of aerosol transmission in COVID-19 outbreaks, including a well-publicized case in March of indoor transmission during a choir rehearsal in Washington state.

Social distancing was observed, but the lack of masks, poor ventilation and loud singing over a prolonged period proved to be a triple whammy. A single superspreader in the first row passed the virus on to 53 fellow singers. Two died.

Droplet transmission was ruled out, because most of those infected were not in range (6 feet or less) of the infected individual.

To see interactive infographics of the above scenarios, check out the work of El País’ artists here.