CAMBRIDGE, Mass. — COVID-19 has left many businesses across the world struggling to stay afloat. With public safety rules limiting indoor capacity and mandating customers stay six feet apart, some shops literally don’t have the space to make ends meet. Now, a new study is challenging these public health guidelines, saying a one-size-fits-all policy doesn’t work. Researchers from the Massachusetts Institute of Technology find the popular “six-foot rule” for indoor spaces doesn’t actually fix the problem of airborne transmission.
The team goes on to say the CDC and World Health Organization overlook how much time people are spending indoors. Their study argues certain locations, like restaurants, have been closed down when they really don’t need to be.
To this point, most social distancing mandates have been based on studies of people sneezing and coughing. That science points to infectious air droplets traveling around six feet, leading to many blanket rules on how many people can fill certain spaces.
The report finds these pathogen-bearing droplets people expel when they eat, talk, cough, and sneeze tend to float through the air for long periods of time. They end up mixing in throughout the entire space by way of air currents in the room. With this in mind, MIT researcher Martin Bazant says indoor customers are really no safer at six feet than they are at 60 feet.
“As scientists, we’ve tried to be very thoughtful and only go with what we see as hard data,” the professor of chemical engineering and applied mathematics says in a university release. “We’ve really tried to just stick to things we can carefully justify. We think our study is the most rigorous study of this type to date.”
What really matters when it comes to staying safe indoors?
Bazant and professor of applied mathematics John Bush developed a new way to calculate COVID exposure risk in indoor environments. Their guideline suggests infection risk depends more on exposure time, the number of people indoors, size of the space, the activities taking place, if people are wearing masks, and the ventilation in the room.
Researchers say the model offers a physics-based guideline for businesses, schools, and even consumers who want to gauge their own risk of COVID exposure. Previous studies have already prompted some school districts to cut their social distancing guidelines to just three feet.
The new model estimates how long it would take for one person to contract COVID-19 once an infected person enters the same space. Instead of a simple yes or no answer, the model reveals an average of how long a person could safely engage in certain activities indoors. Study authors find the estimates can range from a few minutes in a store, to an hour in a restaurant, to several hours within an office or classroom.
“When you look at this guideline for limiting cumulative exposure time, it takes in all of the parameters that you think should be there — the number of people, the time spent in the space, the volume of the space, the air conditioning rate and so on,” Bush says. “All of these things are kind of intuitive, but it’s nice to see them appear in a single equation.”
So is there an app for that too?
The MIT team partnered with app developer Kasim Khan to provide consumers with open access to their model. Users can enter specific details about certain situations to measure the risk of COVID infection for themselves. These factors include the room size, what activity they’ll be performing, and even the transmission rate of the dominant coronavirus strain in an area.
Accounting for all those variables, the app can tell people, on average, how long they should stay inside that space. Researchers based their calculations on information from various mass-spreading events, where they had access to all this relevant data. This includes the Skagit Valley Chorale in Washington state, where 86 percent of the seniors at a two-hour choir practice contracted COVID-19.
“I’d like to use this work to establish the science of airborne transmission specifically for COVID-19, by just taking into account all factors, the available data, and the distribution of droplets for different kinds of activities,” Bazant adds. “If you understand the science, you can do things differently in your own home and your own business and your own school.”
“My mother is over 90 and lives in an elder care facility,” Bush explains. “Our model makes it clear that it’s useful to wear a mask and open a window — this is what you have in your control.”
Bush also notes his mother was planning to attend an exercise class, thinking the six-foot rule would make things safe. However, the team’s new model reveals that, because of the number of people there and the activity they’re performing, the risk of exposure is actually extremely high.
Using science to reopen the economy
Bazant reports that, since becoming available in October, half a million people are using the app version of the model. User feedback is also helping the team to refine their calculator further. Researchers add that these new guidelines are already influencing business owners to reopen their shops.
In January, an indoor tennis facility in Washington state and other stores forced to shut down over COVID restrictions were allowed to reopen. Their appeal was based largely on the data coming from the MIT exposure model and the tennis facility owner’s participation in Bazant’s online course on the physics of COVID transmission.
The study appears in the journal PNAS.