MIT Develops a Method to Assess COVID-19 Transmission Risks Indoors

By David L. Chandler, Massachusetts Institute of Technology, April 17, 2021

Credit: Two MIT professors have proposed a new approach to estimate the risks of exposure to Covid-19 under different indoor environments based on the number of people, the size of the room, the type of activity, whether or not masks are worn and the ventilation. and filtration rates. Credit: Jose-Luis Olivares, MIT; images from iStockphoto

MIT researchers have developed a publicly available model based on physics and data from previous dissemination events.

Two MIT professors have proposed a new approach to estimate the risks of exposure to Covid-19 under different indoor environments. The guideline they developed suggests a limit for the exposure time, based on the number of people, the size of the room, the type of activity, whether masks are worn, and the ventilation and filtration rates. Their model provides a detailed physics-based guideline for policy makers, companies, schools and individuals trying to estimate their own risks.

The guideline, published this week in the journal PNAS, was developed by Martin Z. Bazant, professor of chemical engineering and applied mathematics, and John WM Bush, professor of applied mathematics. They emphasize that an important feature of their model, which has received less attention in existing public health policies, is a specific limit on the amount of time someone spends in a particular environment.

Their analysis is based on the fact that in enclosed spaces, small airborne pathogen-bearing droplets emitted by humans while talking, coughing, sneezing, singing or eating tend to float in the air for long periods of time and mixed well. space by air currents. There is now overwhelming evidence, they say, that such air transmission plays an important role in the spread of Covid-19. Bush says the inquiry early last year was prompted by concerns that many policy decisions were primarily guided by the “6-foot rule,” which does not adequately address airborne transmission in indoor spaces.

Using a strictly quantitative approach based on the best available data, the model produces an estimate of how long, on average, it would take one person to become infected with the SARS-CoV-2 virus if an infected person entered the room , based on the main set of variables that determine a particular indoor situation. Rather than a simple yes or no answer as to whether a particular environment or activity is safe, it provides a guide to how long someone can safely expect to participate in that activity, be it a few minutes in a store, or an hour. in a restaurant, or several hours a day in an office or classroom, for example.

“As scientists, we’ve tried to be very considerate and only go with what we see as hard data,” Bazant says. “We’ve really tried to stick to things that we can carefully justify. We think our study is the most rigorous study of this type to date. Although new data is emerging every day and there are still many uncertainties about the transmission of the SARS-CoV-2 virus, he says, “We are confident that we have made conservative choices on every point.”

Bush adds: “It is a fast moving field. We submit a paper and the next day a dozen relevant papers come out, so we make every effort to include them. It’s like shooting a moving target. Although their model was initially based on the transmissibility of the original strain of SARS-CoV-2 from epidemiological data on the best-characterized early dissemination events, they have since added a transmissibility parameter, which can be adjusted to take into account the higher spread rates of the new emerging variants.This adjustment is based on how the transferability of a new strain relates to the original strain; for example, for the British strain, which is estimated to be 60 percent more transferable than the original, this parameter would be set to 1, 6.

One thing that is clear, they say, is that simple rules, based on distance or capacity limits for certain types of businesses, do not represent the full picture of risk in a particular environment. In some cases, that risk may be greater than those simple rules indicate; in others it may be lower. To help people, whether policymakers or individuals, create more comprehensive assessments, the researchers teamed up with app developer Kasim Khan to put together an open-access mobile app and website that provides users with specific details about a the situation – size of the room, number of people, type of ventilation, type of activity, wearing a mask and the transferability factor for the prevailing load in the area at that time – and get an estimate of how long it would take under those circumstances for a new person contracts the virus when an infected person enters the room.

The calculations were based on inferences from various mass dispersion events, where detailed data was available on the number of people and their age range, the size of the enclosed spaces, types of activities (singing, eating, sports, etc.), ventilation systems, wearing a mask, the amount of time spent and the resulting number of infections. Events they studied included the Skagit Valley Chorale in Washington State, where 86 percent of the seniors in attendance became infected during a two-hour choral practice.

Although their guideline is based on well-mixed air in a particular room, the risk would be higher if someone were directly in a focused jet of particles ejected by, say, a sneeze or a scream. But overall, the assumption of well-mixed indoor air appears to be consistent with the data from actual diffusion events, they say.

“If you look at this guideline for limiting cumulative exposure time, it includes all the parameters that you think should be there: the number of people, the time spent in the room, the volume of the room, the speed. air conditioning and so on, ”says Bush. “All of these things are quite intuitive, but it’s nice to see them appear in one equation.”

While the data on the critical importance of airborne transmission has now become clear, Bazant says, public health organizations initially put much more emphasis on hand washing and surface cleaning. Early in the pandemic, there was less appreciation for the importance of ventilation systems and the use of face masks, which can dramatically affect safe occupancy rates, he says.

“I would like to use this work to establish the science of air transmission specific to Covid-19, by only considering all the factors, the available data and the distribution of droplets for different types of activities,” says Bazant. He hopes the information will help people make informed decisions for their own lives: “Understanding science can help you do things differently in your own home and in your own company and at your own school.”

Bush gives an example: “My mother is over 90 and lives in an aged care facility. Our model makes it clear that it is convenient to wear a mask and open a window – this is what you have under control. He was shocked that his mother was planning to attend a rehearsal class at the facility because he thought it would be good because the people would be six feet apart. As the new study shows, that would be because of it. number of people and activity level are actually a very risky activity, he says.

Since they made the app available in October, Bazant says, they’ve had about half a million users. Their feedback helped the researchers further refine the model, he says. And it has already helped influence some business reopening decisions, he adds. For example, the owner of an indoor tennis facility in Washington State that was closed due to Covid restrictions says he may reopen in January, along with certain other low-occupancy sports facilities, based on an appeal he made in large part over this one. guideline and information from his participation in Bazant’s online course on the physics of Covid-19 transmission.

Bazant says the new tools not only recommend guidelines for specific spaces, but also provide a way to assess the relative merits of different intervention strategies. For example, they found that while improved ventilation systems and the use of face masks make a big difference, air filter systems have a relatively smaller effect on the spread of disease. And their research can provide guidance on how much ventilation is required to achieve a certain level of safety, he says.

“Bazant and Bush have provided a valuable tool for estimating (among other things) the upper limit of time spent sharing airspace with others,” said Howard Stone, a professor of mechanical and aerospace engineering at Princeton. University that was not affiliated with this. work. While such an analysis can only provide a rough estimate, he says the authors “describe these types of estimates as a means of helping others assess the situation they are in and how to minimize their risk. This is especially useful because a detailed calculation for every possible space and set of parameters is not possible. “

Reference: “A Guideline to Limit Indoor Air Transmission of COVID-19” by Martin Z. Bazant and John WM Bush, April 15, 2021, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2018995118