Social Distancing Indoors: One Size Does Not Fit All

April 26, 2021
Frank Diamond

MIT professors argue that many variables should be included when determining just how much social distancing is needed in different indoor settings.

Not all indoor settings are the same: ventilation varies, size varies, occupancy varies. Not only that, the activities people perform indoors also vary a lot. So, the 6-foot social distancing protocol—allocated by both the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO)—doesn’t always apply, according to a study in the journal PNAS (Proceedings of the National Academy of Science of the United States of America).

Investigators with the Massachusetts Institute of Technology (MIT)—Martin Z. Bazant, PhD, a professor of chemical engineering and applied mathematics, and John W.M. Bush, PhD, who teaches applied mathematics—have offered what they hope will be a guide for schools, businesses, policymakers, and individuals to better access the risk of contracting COVID-19 indoors. They argue that their data suggest that things like school closings and occupancy limits may not be necessary in many instances.

Which is not to say that social distancing indoors is not important, Bush tells Infection Control Today®. “Social distancing is a valuable safety measure in that it protects against large-drop transmission (as arises when one coughs or sneezes) and transmission by respiratory jets (as accompany normal breathing),” Bush tells ICT® in an email. “The pathogen concentration of such exhaled air decreases with distance; thus, as far as infection by these respiratory flows is concerned, the farther you are way from an infected person, the better.”

Social distancing rules should be pliable: Sometimes you’ll need more, sometimes less will do. “Provided masks are worn, so that the risk of short-range respiratory flows is greatly reduced, the greatest danger indoors may be posed by airborne transmission,” Bush tells ICT®. “In such circumstances (when masks are worn), it is possible that the social distancing guidelines are overly restrictive; however, it is also possible that they are not restrictive enough. Our study allows one to make a quantitative assessment of which settings are safe and which are not, and when the social distancing guidelines are sufficient or inadequate.”

In an MIT press release, Bush offers an example from his own experience. “My mother is over 90 and lives in an elder care facility. 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 said that his mother felt safe attending an exercise class at the facility because the participants would be 6 feet apart. However, the space of the exercise room and the number of participants would actually make that a high-risk activity.

In the same press release, Bazant said that “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 tells ICT® in an email exchange that it’s important to “note some more of the practical applications of our work. Our safety guideline may be applied to different indoor spaces using a user-friendly online app, which has been translated into many languages. The safety guideline can also be expressed as a limit on the exhaled carbon dioxide concentration, a convenient proxy for indoor airborne transmission risk, which can be monitored in real time using low-cost sensors.”

Bush and Bazant gathered data using an app and website developed by Kasim Khan in which specific details of what’s going on in an indoor setting are entered—including the variables mentioned above. Then they estimated how long it would take under the various circumstances for the virus to spread from 1 person infected with COVID-19.

Bush said that the study “indicates that one should limit the time spent in an indoor space, to an extent determined by the parameters defining the space, including the number of occupants, the ventilation and filtration rates, the level of human activity and mask use.”

That last, mask use, is possibly the most important variable.

“Provided masks are worn, so that the risk of short-range respiratory flows is greatly reduced, the greatest danger indoors may be posed by airborne transmission,” Bush says. “In such circumstances (when masks are worn), it is possible that the social distancing guidelines are overly restrictive; however, it is also possible that they are not restrictive enough. Our study allows one to make a quantitative assessment of which settings are safe and which are not, and when the social distancing guidelines are sufficient or inadequate.”

They used real-world events, such as the Skagit Valley Chorale in Washington state, where the Skagit Valley Chorale in Washington state, where 86% of the elderly participants became infected by COVID-19 during a 2-hour choir practice.

“Similarly, when 23 of 68 passengers were infected on a 2-hour bus journey in Ningbo, China, their seated locations were uncorrelated with distance to the index case,” the study states. “Airborne transmission was also implicated in the COVID-19 outbreak between residents of a Korean high-rise building whose apartments were linked via air ducts. Studies have also confirmed the presence of infectious SARS-CoV-2 virions in respiratory aerosols suspended in air samples collected at distances as large as 16 ft from infected patients in a hospital room.”